Synthesis and characterization of Cu ZrCl : a thermochromic, van …user.physics.unc.edu/~ftsui/jac_338_173.pdf · 2002. 5. 22. · nitrogen. Data were then collected on an Enraf-Nonius

LJournal of Alloys and Compounds 338 (2002) 173–184www.elsevier.com/ locate / jallcom

Synthesis and characterization of Cu ZrCl : a thermochromic,2 6

van Vleck paramagneta b b a ,*Andrew M. Dattelbaum , Liang He , Frank Tsui , James D. Martin

aDepartment of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USAbDepartment of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA

Received 14 November 2001; accepted 29 January 2002

Abstract

The solid state reaction of CuCl and ZrCl yields the pale yellow crystalline solid of composition Cu ZrCl . This material crystallizes4 2 6¯in the trigonal space group P3m1 ([164) at room temperature with lattice constants a510.9092(4) and c56.1517, Z53. The structure is

shown to be related to ZrCl , where every other Zr site has been replaced with Cu atoms that partially occupy the six tetrahedral3

interstices surrounding the now vacant octahedral interstice. At room temperature, two copper atoms are disordered about threecrystallographically distinct sites. Two thermochromic transitions are observed for this material at 120 and 673 K, which appear to berelated to the movement of copper from the center of a tetrahedral interstice to a trigonal planar face. A second-order Jahn–Tellerdistortion accounts for this structural variation. This along with the insertion of the empty Zr-4d orbitals into the already small copper (I)based HOMO–LUMO gap afford significant mixing between ground and excited states resulting in notable van Vleck paramagnetism. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Metal halide compounds; Crystal structure; X-ray diffraction; Neutron diffraction; Magnetic measurements

321. Introduction frameworks from the phosphate-like [CuCl ] building4

blocks with the Lewis acidic ZrCl [3,5]. Unlike our prior4

The chemistry of copper (I) is unique among transition work with the group 13 Lewis acids (Al and Ga), themetals because of its distinctive electronic configuration. introduction of Zr(IV) into the cuprous halide lattice isWith respect to reactivity, the high lying and filled 3d- expected to further narrow the band gap of these wideorbitals allow reactivity with electrophiles; at the same band gap semiconductors by inserting the t -type zir-2g

time the low lying and empty 4s- and 4p-orbitals provide a conium 4d-orbitals into the already narrow gap betweensite for nucleophillic attack. This relatively small HOMO- the Cu 3d- and 4s,p-orbitals. In prior reports we haveLUMO gap is also responsible for the wide range of described the synthesis and characterization ofcoordination geometries observed for Cu(I) [1]. Though ((C H )Cu) ZrCl [6] and [H NMe ]CuZrCl [7]. We here6 6 2 6 2 2 6

10this d -metal may have a crystal field preference for a describe the synthesis and characterization of the parenttetrahedral geometry, a second-order Jahn–Teller mixing compound Cu ZrCl . We further demonstrate that the2 6

10 0between the filled d- and empty s- and p-orbitals stabilizes interaction between the d -Cu(I) and d -Zr(IV) is respon-two and three coordination [2]. In addition to unique sible for the observed thermochromism, and provides astructure and reactivity, the electronic configuration of system in which to probe the nature of van Vleck paramag-Cu(I) has profound implications with respect to optical and netism.luminescent properties [3], as well as with respect tomobility in ionic conductors [4]. In the course of ourcontinuing investigation of the structure and reactivity of 2. ExperimentalCu(I) centers in Lewis acidic metal halide lattices, we havebeen interested in the possible construction of open 2.1. General procedures

All reactions were performed under an inert atmosphere*Corresponding author.E-mail address: [email protected] (J.D. Martin). of dry N in a glove box or using Schlenk line techniques.2

0925-8388/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0925-8388( 02 )00230-X

174 A.M. Dattelbaum et al. / Journal of Alloys and Compounds 338 (2002) 173 –184

ZrCl (99.99%) was used as received from Aldrich. The was subsequently confirmed by the structural solution4

CuCl was prepared from Cu metal and CuCl (Aldrich) refined using the NRCVAX suite of programs [10]. All2

according to literature methods and further purified by zirconium and chlorine atoms were located by directsublimation [8]. Powder X-ray diffraction measurements methods using the SIR92 program [11]. One copper sitewere obtained using an Enraf-Nonius Guinier camera and (Cu(1)) was also observed in the initial direct methodswere indexed with respect to a silicon standard. Diffuse solution. Subsequent difference maps identified two addi-reflectance spectra were taken using a Cary 3e UV–Vis tional sites that are partially occupied by copper atoms. Allspectrometer equipped with an integrating sphere and atoms were refined anisotropically. A full matrix leastmeasured with respect to a pressed polytetrafluoroethylene squares calculation using the 452 unique reflections (I.

powder standard. Reflectance spectra are plotted as the 2.5s(I)) gave a final refinement with R factors of R50.0532remission function F(R )5(12R ) /2R , based on the and R 50.087.` ` ` w

Kubelka–Munk theory of diffuse reflectance [9]. Differen-tial scanning calorimetry (DSC) data were collected on a

2.4. X-ray structure determination of Cu ZrCl at 157 K2 6Perkin-Elmer DSC 7 at a heat /cool rate of 108 /min.Samples were placed into stainless steel, high-pressure

The crystal used for the room temperature data collec-pans and hermetically sealed with gold foil caps. Magnetic

tion was removed from the capillary under a flow of argon,susceptibility measurements were performed on a Quantum

covered in silicone grease, placed on the end of a glassDesign MPMS-5S SQUID magnetometer with a 5 T

fiber, and immediately transferred to the diffractometermagnet in fused silica magnetic susceptibility containers

where it was cooled to 157 K under a stream of dryand in DELRIN sample holders.

nitrogen. Data were then collected on an Enraf-NoniusCAD4-Mach diffractometer with monochromated Mo(Ka)

2.2. Preparation of Cu ZrCl radiation. Lattice constants for the trigonal cell, a52 6˚ ˚10.906(2) A, c512.234(1) A, were determined from a

A sample of 0.200 g of ZrCl (0.858 mmols) and 0.140 symmetry-constrained fit of 24 well-centered reflections4

g of CuCl (1.72 mmols) was ground together in a mortar between 338,2u ,368 and their Friedel pairs. A uniqueand pestle, then transferred into a fused silica tube (6 quadrant h, k, 6l, was collected with 2429 independentmm3 8 cm I.D.). The reaction vessel was evacuated and reflections by v-scans for 08,2u ,608. These data weresealed with a torch. The reaction was heated to 450 8C in a scaled to three intensity check reflections using a five-pointbox furnace for 1 day, and then cooled at 1.0 8 /min to smoothing routine. An empirical absorption correction wasroom temperature. A small temperature gradient in the applied using psi scan data. Systematic absences were

¯furnace caused the Cu ZrCl to crystallize on the cool end found to be consistent with the space group P3c1, which2 6

of the tube. The bulk material was characterized by powder was subsequently confirmed by the structural solutionX-ray diffraction, DSC, magnetic susceptibility and diffuse refined using the NRCVAX suit of programs. In spite ofreflectance spectroscopy. the doubled c-axis and change of space group, refinement

of the low temperature data yielded essentially the samestructure solution as that found for the room temperature2.3. X-ray structure determination of Cu ZrCl at 298 K2 6 data, with a similar distribution of the disordered copperatoms but a small variation in certain atom positions. AllA pale yellow single crystal (0.2430.2230.12 mm)atoms were refined anisotropically. A full matrix leastcovered in silicone grease was mounted in a Pyrexsquares calculation on 877 unique reflections [I . 1.5s(I)]capillary and sealed under a N atmosphere. Diffraction2 gave a final refinement of R511.1 and R 510.8.wdata were collected on an Enraf-Nonius CAD4 diffrac-

˚tometer at 298 K using MoK radiation (l50.71073 A).a

˚Lattice constants for the trigonal cell, a510.9892(4) A, 2.5. Neutron powder diffraction˚c56.1517(2) A, were determined from a symmetry con-

strained fit of 24 well centered reflections (338,2u ,358) Cu ZrCl (6.0 g, 0.014 mol) was sealed in a vanadium2 6

and their Friedel pairs. A zero-layer Weissenberg photo- container of length 50 mm and diameter 6.0 mm (I.D.)graph and axial photographs taken on the CAD-4 con- inside a dry, He-filled glove box. Data were collected atfirmed these lattice constants and gave no indication of any room temperature, and at 100 K using a closed-cycle Hesuperstructure. A hemisphere of data, 6h, k, 6l, with 2878 refrigerator for temperature control. Neutron powder dif-independent reflections was then collected by the u /2u fraction data were collected using the BT-1 32 detectorscan mode for 08,2u ,548. Data were scaled to the neutron powder diffractometer at the NIST Center forintensity check reflections using a five-point smoothing Neutron Research reactor, NBSR. A Cu(311) mono-

˚curve fitting routine. An empirical absorption correction chromator with a 908 take-off angle, l51.5402(2) A, andwas applied using psi scan data. Systematic absences were in-pile collimation of 15 min of arc were used. Data were

¯found to be consistent with the space group P3m1, which collected over the range of 3–1688 2u with a step size of

A.M. Dattelbaum et al. / Journal of Alloys and Compounds 338 (2002) 173 –184 175

0.058. The instrument is described in the NCNR web site(http: / /www.ncnr.nist.gov).

The structure was also evaluated by the Rietveld refine-ment of the room temperature neutron diffraction datausing GSAS [12]. Lattice constants and atom positionsobtained from the CAD-4 data were utilized as startingpositions for the refinement. Doubling of the c-axis wasalso considered, but it did not improve the overall fit of thedata. The full profile refinement parameters include thezero point, 12 background parameters, three profile param-eters plus the structural parameters. The refined structuralparameters include two lattice parameters, the 15 atomicposition parameters not dictated by special positions, thethree isotropic thermal parameters for the chlorides, and

Fig. 1. Final Rietveld refinement plot for room temperature neutronthe three fractional occupancies for the copper positions. powder data for Cu ZrCl . The experimental data are represented by2 6Pseudo-translational symmetry led to a high correlation of crosses, while the calculated pattern is shown by the solid line. The lowerthe metal thermal parameters, and thus, they were all fixed trace is the difference curve between observed and calculated patterns.

The Bragg reflections are shown by the vertical bars.to U 50.03. The final agreement factors are R 56.04%,iso p

R 57.07% for the full profile.wp

positions and selected bond distances and bond anglesfrom the room temperature single crystal refinement aregiven in Tables 2–4, respectively. (The corresponding data3. Resultsfor the low temperature and neutron refinements areavailable from the author.) ORTEP drawings describing3.1. Solid-state structurethe local structure around each of the three copper sites aregiven in Fig. 2. The structure of Cu ZrCl consists of aThe solid-state structure of Cu ZrCl was determined by 2 62 6

complex network based on a hexagonal close-packedsingle crystal X-ray diffraction at room temperature and atchloride sublattice. Two crystallographically distinct zir-157 K, and by Rietveld refinement of room temperature

¯conium cations reside on 3m and 3m sites, respectively,neutron powder diffraction data. A summary of thesetogether filling 1/6 of the octahedral interstices. The Zr–Clcrystallographic data is presented in Table 1 and the

˚distances around the 3m site, Zr(1)–Cl(1)52.481(3) A33difference function of the Rietveld refinement of the¯˚neutron powder diffraction data is given in Fig. 1. Atomic and Zr(1)–Cl(3)52.478(3) A33, and the 3m site, Zr(2)–

Table 1Crystallographic data for Cu ZrCl at 298 K and 157 K (single crystal X-ray), and 293 K (powder neutron)2 6

Formula Cu ZrCl2 6

Formula weight (g /mol) 431.22Crystal dimensions (mm) 0.2430.2230.12 PowderCrystal system TrigonalTemperature (K) 298 157 293

¯ ¯ ¯Space group (No.) P3m1 (164) P3c1 (165) P3m1 (164)˚a (A) 10.9892(4) 10.906(2) 10.9648(3)˚c (A) 6.1517(2) 12.234(1) 6.1411(4

3˚V (A ) 643.37(3) 1260.2(2) 639.41(4)Z 3 6 3R-merge 0.020 0.053

23r (mg cm ) 3.338 3.386 3.358calcd

˚l(Mo Ka), A 0.71073 0.71073 1.5405 (neutron)21

m (cm ) 78.8 80.4No. reflns measured 2878 2429No. reflns unique 537 1218

a dR 0.053 0.111 R 0.061pb eR 0.087 0.108 wR 0.071w p

cGOF 2.87 2.09a R 5 o(F 2 F ) /F .0 c 0b 2 2 1 / 2R 5 [o(w(F 2 F ) ) /wF )] .w 0 c 0c 2 1 / 2GOF5[o(w(F 2 F ) ) /(No. of reflns. 2 No. of parameters)] .0 cd R 5 ouI 2 I u /oI .p o c oe 2wR 5 œ(M /oI ).p o


Table 2Atomic coordinates and isotropic displacement parameters for Cu ZrCl (1) at 298 K2 6

Occupancy x y z Biso

Zr(1) 1.0 1/3 2/3 0.0244(2) 1.64(5)Zr(2) 1.0 0 0 1/2 1.52(6)Cu(1) 0.52(5) 0.2279(3) 0.7721(3) 0.4344(6) 6.2(2)Cu(2) 0.32(1) 0.1083(4) 0.8917(4) 0.9122(1) 6.3(3)Cu(3) 0.17(1) 0.4242(8) 0.5758(8) 0.609(2) 5.8(5)Cl(1) 1.0 0.4405(1) 0.5595(1) 0.2539(4) 2.31(9)Cl(2) 1.0 0.1070(1) 0.8930(1) 0.2716(4) 2.35(8)Cl(3) 1.0 0.2261(1) 0.7739(1) 0.7958(4) 2.56(8)

Table 3˚Bond distances (A) for the room temperature structure of Cu ZrCl2 6

Zr(1)–Cl(1) 2.481(3)33 Cu(2)–Cl(2) 2.340(7)32Zr(1)–Cl(3) 2.478(3)33 Cu(2)–Cl(2) 2.211(6)

Cu(2)–Cl(3) 2.354(7)Zr(2)–Cl(2) 2.474(2)36

Cu(3)–Cl(1) 2.204(9)Cu(1)–Cl(1) 2.308(3)32 Cu(3)–Cl(1) 2.71(2)Cu(1)–Cl(2) 2.511(6) Cu(3)–Cl(3) 2.226(7)32Cu(1)–Cl(3) 2.223(4)

˚Cl(2)52.474(2) A36 are essentially equivalent. Thecopper cations reside in slightly distorted tetrahedral

Fig. 2. ORTEP drawings of the chain-like units running along the c-axisinterstices and are disordered over three crystallographical-demonstrating the local coordination with occupancy of the three respec-

ly distinct sites with occupancies of Cu(1)552(5)%, tive copper sites. (a) Cu(1), (b) Cu(2) and (c) Cu(3).Cu(2)532(1)%, and Cu(3)517(1)% (Occupancies of46%, 29%, and 16% were found for Cu(1), Cu(2), andCu(3), respectively, when the thermal parameters were cated the presence of a non-merohedral twin. Flack’sfixed at B 50.08.). (Refinement of the neutron powder method of left coset decomposition was utilized to de-iso

data yielded occupancies of Cu(1)541(2)%, Cu(2)5 termine the possible twin laws for a merohedral twin [13].24(2)%, and Cu(3)521(2)%.) Copper atoms 1 and 3 in Information about each possible twin law for the Laue

¯particular are distorted toward a trigonal face of the group 3m1 [14] was incorporated into the structure model˚interstice with three short Cu–Cl distances of 2.2–2.3 A using the twin refinement options provided by SHELXL97

˚and one long distance, Cu(1)–Cl(2)52.511(6) A and [15]. None of the possible twin laws led to an improve-˚Cu(3)–Cl(1)52.71 A. The bond distances around Cu(2) ment of the structure refinement.

˚are more regular, ranging from 2.21(1) to 2.35(1) A. All Low-temperature single crystal X-ray diffraction (157attempts to refine the structure in lower symmetry space K) and X-ray and neutron powder diffraction (100 K) datagroups in order to accommodate a stoichiometric oc- were obtained to examine the temperature dependence ofcupancy of the copper sites resulted in the same structural the copper site occupancy. The 157 K single crystal X-raysolution with the symmetry of the original space group data gave evidence for a doubling of the c lattice constant

¯P3m1. Neither Weissenberg nor axial photographs indi- (and thus a change in the space group), however, the

Table 4Selected bond angles (deg.) for Cu ZrCl at 25 8C2 6

Cl(1)–Zr(1)–Cl(1) 90.81(8)33 Cl(2)–Cu(2)–Cl(2) 118.6(2)32Cl(1)–Zr(1)–Cl(3) 179.87(8)33 Cl(2)–Cu(2)–Cl(2) 97.8(3)Cl(1)–Zr(1)–Cl(3) 89.10(7)36 Cl(2)–Cu(2)–Cl(3) 108.3(4)Cl(3)–Zr(1)–Cl(3) 90.99(8)33 Cl(2)–Cu(2)–Cl(3) 106.1(2)32Cl(2)–Zr(2)–Cl(2) 180.0(1)33 Cl(1)–Cu(3)–Cl(1) 100.1(4)Cl(2)–Zr(2)–Cl(2) 90.92(8)36 Cl(1)–Cu(3)–Cl(3) 123.9(2)32Cl(2)–Zr(2)–Cl(2) 89.08(8)36 Cl(1)–Cu(3)–Cl(3) 98.1(5)32

Cl(3)–Cu(3)–Cl(3) 105.2(3)Cl(1)–Cu(1)–Cl(1) 99.9(1)Cl(1)–Cu(1)–Cl(2) 101.5(2)32Cl(1)–Cu(1)–Cl(3) 119.2(1)32Cl(2)–Cu(1)–Cl(3) 112.6(2)


of about 2.9 eV. A second broad maximum in the absorp-21tion spectrum is observed at around 35 000 cm . By way

of comparison, the diffuse reflectance spectrum of thecolorless [H NMe ]CuZrCl , and CuCl, which has a band2 2 6

gap of 3.2 eV [16], are also given in Fig. 4b and c,respectively. The other starting material, ZrCl , has a band4

gap in the vacuum ultraviolet; beyond the limits of ourinstrumentation. These spectra further provide no evidencefor the presence of any copper (II) chloride (onset of

21absorption at 16 000 cm ). Due to our instrumentallimitations it has not been possible to obtain spectroscopicdata for either the high-temperature or low-temperaturephases.

The thermochromic and structural phase transitions wereFig. 3. Synchrotron powder X-ray diffraction data for Cu ZrCl obtained further quantified by differential scanning calorimetry2 6

at (a) 298 K and (b) 100 K. (DSC) measurements shown in Fig. 5. Two sharp endo-therms are observed at 673 and 735 K upon heating asample of Cu ZrCl from room temperature to 773 K at a2 6structural solution was otherwise equivalent to that de-rate of 10 8 /min. These correspond to the temperatures ofscribed above for the room temperature refinement withthe high-temperature thermochromic (and structural) phaseonly small changes in atom positions and copper sitetransition, and the melting point, respectively. Cooling atoccupancies observed. The 100 K X-ray and neutronthe same rate finds that both transitions supercool by aboutpowder diffraction data indicate a slightly more pro-20 8. These same transitions were observed on multiplenounced structural change as seen by the splitting ofheat–cool cycles confirming that no decomposition takesseveral of the diffraction peaks (X-ray data are shown inplace under these conditions. However, the material willFig. 3). We have not yet solved the structure of thesublime at the elevated temperatures, thus requiring the uselow-temperature phase, and an attempt to obtain 100 Kof hermetically sealed, high-pressure DSC pans. Lowsingle crystal X-ray diffraction data was prevented bytemperature DSC measurements were made from roomfracture of the crystal upon cooling.temperature to 93 K at a cooling rate of 10 8 /min.However, these data are essentially featureless, given the3.2. Color and thermochromismsensitivity of our instrument.

At room temperature, Cu ZrCl exhibits a pale yellow2 63.3. Magnetic susceptibilitycolor. Upon cooling the sample in liquid nitrogen (77 K) it

becomes colorless, but returns to its original color uponA series of magnetic measurements were made towarming. Upon heating to 673 K the sample exhibits a

confirm that the yellow color in Cu ZrCl was not due to a2 6sharp thermochromic transition to a burnt red color beforeCu(II) impurity. In the absence of a paramagnetic impuri-it is observed to melt at 735 K. The room temperaturety, we further wanted to investigate the van Vleckdiffuse reflectance spectrum of Cu ZrCl , shown in Fig. 4,2 6

21 paramagnetism of this material with a relatively smallwith the onset of absorption at approximately 21 000 cmis consistent with the yellow color and an optical band gap

Fig. 5. DSC data obtained upon (a) heating Cu ZrCl from 25 to 500 8C2 6

Fig. 4. Reflectance data for (a) Cu ZrCl , (b) [H NMe ]CuZrCl and (c) and (b) cooling to room temperature (cooling data is offset by 230 mW2 6 2 2 62CuCl, plotted as the remission function, F(R ) 5 (1 2 R ) /2R . for clarity) at 10 8 /min.` ` `


10HOMO-LUMO gap between the high-lying d -orbitals of0Cu(I) and the low-lying d -orbitals of Zr(IV). Measure-

ment of the magnetization was made both as a function oftemperature and of the applied field. Because any magneticresponse from this material would be expected to be quitesmall, measurements were made in both fused silica andDELRIN sample holders to verify that the observedresponses were not artifacts of the sample holder. Parallelmeasurements were also made on materials from differentsynthetic preparations to control for possible impuritiesintroduced in the synthesis.

A plot of the magnetic susceptibility, x, as a function ofFig. 7. Plot of the magnetization of a 0.041 g sample of Cu ZrCl as a2 6temperature at applied field strengths of 0.05, 0.5, 1.0 and function of the applied field at 250 K. The inset shows the low field M vs.

5.0 T is given in Fig. 6. A diamagnetic core correction of H data obtained at 300, 180, 100, 20, and 5 K demonstrating the24 211.9310 emu mol , taken from tabulated values [17], temperature dependence of this data.

was applied to this data. A weak and nearly temperatureindependent susceptibility is observed, albeit surprisinglydependent upon the magnitude of the applied field. Apronounced step is observed in the magnetic susceptibility at 120 K was observed in the [H NMe ]CuZrCl measure-2 2 6

data at around 120 K, which apparently corresponds to the ments, and the known hydrogen-bond induced structurallow-temperature thermochromic transition. This transition phase transition at 210 K [7] is not apparent in theis most pronounced in the low-field measurements, and magnetic data. Although about half the magnitude, the Mtends to be masked at higher applied fields. A plot of the vs. H data for [H NMe ]CuZrCl similarly exhibit a2 2 6

magnetization as a function of the applied field is given in paramagnetism at low applied field that saturates above anFig. 7. These data demonstrate a paramagnetic suscep- applied field of about 0.1 T. Unlike Cu ZrCl the y2 6

tibility at low applied fields. However at an applied field of intercept of the slope of the high field data is temperaturegreater than about 0.1 T the paramagnetism saturates and independent for [H NMe ]CuZrCl suggesting that both2 2 6

the diamagnetic susceptibility dominates. While the slope paramagnetic and diamagnetic components of the laterof the high field response of the M vs. H curve (the material are temperature independent.diamagnetic susceptibility) is approximately constant overthe temperature range examined, the y intercept increaseswith decreasing temperature due to a temperature depen- 4. Discussiondence of the paramagnetic contribution. A parallel set ofmeasurements was made on the related compound 4.1. Understanding the structure of Cu ZrCl2 6

[H NMe ]CuZrCl , which was made from the same CuCl2 2 6

and ZrCl starting materials. No step in the susceptibility The complex network structure of Cu ZrCl can most4 2 6

readily be understood by a comparison to the structure ofZrCl . The c lattice constant in these materials is virtually3

˚ ˚identical; 6.13 A for ZrCl and 6.15 A for Cu ZrCl ,3 2 6

consistent with the hcp chloride sublattice common to bothmaterials. In the a–b plane, the unit cell of Cu ZrCl2 6] ]Œ Œrepresents a 3 3 3 expansion of the ZrCl unit cell as3

shown in Fig. 8a and b. In ZrCl , the face-sharing3

octahedral chains, Fig. 8c, reside on the 6 -axis, which3

also correspond to the edges of the unit cell. In Cu ZrCl ,2 6

half of the Zr atoms are removed and are replaced by Cuatoms that are distributed throughout tetrahedral intersticessurrounding the now vacant octahedral interstice. If thezirconium atom at z50.5 is removed, the symmetry of this

¯broken chain is lowered to 3m and all six tetrahedralinterstices surrounding this site are equivalent; this is thesite of Cu(2) in Cu ZrCl . By contrast if the zirconium2 6

atom at z50.0 is removed, the symmetry of the brokenchain is lowered to 3m. Now two crystallographicallydistinct sets of tetrahedral interstices surround the vacantFig. 6. Plot of molar susceptibility, x vs. T data obtained at applied field

strengths of 0.05–5.0 T, respectively. octahedral interstice providing the sites for Cu(1) and


Fig. 9. Ball and stick representation of the Cu ZrCl networks formed by2 6

complete occupancy of the (a) Cu(1), (b) Cu(2) and (c) Cu(3) sites,respectively.

Fig. 8. A structural comparison of (a) ZrCl and (b) Cu ZrCl . In (c) the3 2 6 links six different chains through long, apical Cu(1)–¯lowering of symmetry from 6 in the ZrCl chains to 3 and 3,3 3 ˚Cl(2), 2.511(6) A, bonds. Full occupancy of the Cu(3) site,

respectively, depending on which Zr atoms are removed is demonstrated.Fig. 9c, results in analogous chains that are oriented in the2c direction and are connected to each other by thebridging Cl(1), which is apical to the adamantane-type

˚Cu(3) in Cu ZrCl . Independently, the Cu(1) and Cu(3) building block of one chain, Cl(1)–Cu(3)52.71(2) A, and2 6

sites form rings of three corner shared tetrahedra which in inner to a similar building block of a neighboring chain,˚turn are capped by an octahedral zirconium chloride unit Cl(1)–Cu(3)52.20(1) A. While adjacent chains are con-

creating the Cu ZrCl adamantane-type building blocks. nected by bridging halides, in this lattice model the3 12

These adamantane-type building blocks are then stacked Zr(2)Cl octahedra are completely isolated in channels6

into chains pointing in the 1c direction for occupation of constructed from the network of chains. By contrast, whenthe Cu(1) site and in the 2c direction for occupation of only the Cu(2) sites are fully occupied, as shown in Fig.the Cu(3) site, Fig. 2a and c, respectively. By contrast, the 9b, three chains of bicapped trigonal anti-prisms are linked

¯Cu(2) sites surrounding a 3m octahedral interstice form to Zr(1)Cl octahedra through chloride bridges, Cu(2)–6˚chains of bicapped trigonal antiprisms equivalent to the Cl(3)52.35(1) A. This connectivity also leaves three

superposition of the two adamantane-type chains formed terminal Zr(1)–Cl(1) contacts in the network.by Cu(1) and Cu(3). Unlike the structure of ZrCl where The structural solution described above suggests the3

the chains are isolated by van der Waals contacts, occupa- possibility of a variable copper stoichiometry, up totion of the tetrahedral copper sites link these chains into Cu ZrCl . However, off composition loadings exhibit no6 6

three dimensional networks through inter-chain Cu–Cl discernable change in lattice constants, thus providing nobonds. These external Cu–Cl bonds that are elongated in evidence of any significant variation in stoichiometry.Cu ZrCl , likely the result of a second-order Jahn–Teller Copper rich compositions (Cu ZrCl ), or even occupa-2 6 21x 6

trigonal pyramidal distortion commonly observed in cupr- tion of only the Cu(2) site by greater than 0.5 would˚ous halides [2]. require quite close copper–copper contacts, |2.3 A. While

Full occupancy of any one copper site would result in such edge shared tetrahedral linkages are known forthe formation of one of the three different lattices shown in cuprous chlorides, connectivity through sharing tetrahedralFig. 9a–c; each with the same Cu ZrCl stoichiometry. corners is much more common. The requirement of such2 6

The set of zirconium and chloride positions are equivalent close copper–copper contacts in the lattice resulting fromin each figure. When the Cu(1) site is fully occupied, Fig. the full occupation of the Cu(2) site leads us to suspect9a, the lattice consists of chains of Cu ZrCl building that this structure is energetically unfavorable. Neverthe-3 12

blocks running along the 1c direction, which are con- less, we find no reason why one of the ordered latticesnected laterally along a and b by chloride bridges to resulting from the complete occupancy of either Cu(1) orisolated Zr(2)Cl octahedra. Each Zr(2)Cl octahedron Cu(3) is not energetically favored over the observed6 6


disordered structure. Careful examinations of the diffrac- 4.2. Color and thermochromismtion data provide no evidence for the twinning of crystals

The pale yellow color observed for Cu ZrCl at roomwith different copper site occupancies. To consider the 2 6

temperature is a result of the onset of absorption at aroundpossibility of the in-registry growth of crystallite domains,2120 000 cm (500 nm) as seen in Fig. 4a. This absorptioneach with a unique copper site occupation, the calculated

originates from the insertion of the empty zirconium d-powder patterns for the three lattices obtained by the10 0orbitals into the copper centered (Cu-d to Cu-s ) bandoccupation of individual copper sites (Fig. 10b–d) were

gap of cuprous chloride. We previously demonstrated thatcompared with the observed powder diffraction (Fig. 3).the electronic localization caused by alternation of Al (III)For each of these calculated patterns the (100) reflection isand Cu (I) in CuAlCl flattens the valence and conductionobserved to exhibit significant intensity. But this reflection 4

bands and shifts the absorption deeper into the UV than isis absent in the observed room temperature powder andobserved for the binary CuCl [3]. The lowest energysingle crystal data. Notably, the (100) reflection is alsoabsorption in CuAlCl was also clearly shown to be due toabsent from the diffraction pattern calculated for the model 4

10 9 1based on the fractional occupancy of all three copper the Cu-d to Cu-d s transition. Here we assign the broad21positions (Fig. 10a). By contrast, the 100 K X-ray powder absorption maximum at 35 000 cm to the copper cen-

diffraction data (Fig. 3b) exhibits a reflection at Q50.59 tered transition in Cu ZrCl . (The Cl-lone pair to Zr-4d2 621

A that may be related to a (100)-type reflection. This transition in ZrCl is in the vacuum UV.) Unlike Al(III),4

reflection is calculated to be less intense in the neutron however, the octahedral Zr(IV) introduces the low-lyingdiffraction given the difference between X-ray and neutron and empty t -type non-bonding orbitals to the system.2g

scattering factors, and as a result is not observed in the low Under the C symmetry of the Cu ZrCl adamantane3v 3 12

temperature neutron data. The appearance of this low Q cage-type units the zirconium nonbonding orbitals trans-reflection along with the splitting of the other reflections is form with a and e symmetry, and, the set of copper1

suggestive of a lower symmetry model in which the Cu d-based Cu–Cl s* orbitals transform as two orbitals of a1

atoms are possibly localized in certain of the tetrahedral symmetry, one a orbital and three sets of orbitals with e210interstices at low temperature. Nevertheless, we conclude symmetry. Several transitions between the Cu-d to the

0that at room temperature any ordering of copper atoms into Zr-d are allowed, and sufficient orbital overlap is possible˚domains with specific site occupancies does not occur over given the Cu–Zr distance of only 3.2 A, such that this

distances of larger than at most a few unit cells. Given the metal-to-metal charge transfer is responsible for the lowapparently insignificant difference in energetic preference energy band edge, and thus the color at room temperature.for the three copper site occupancies, we speculate that the These assignments are further supported by Extended

¨copper atoms have significant mobility throughout this Huckel calculations, in which the projected DOS clearlylattice. By contrast, at low temperature, the copper atoms shows a majority of Cu and Cl character with a smallare likely localized in specific sites. amount of Zr in the valence band, whereas the much

narrower conduction band is localized on the non-bondingZr d-orbitals. [18]

The abrupt color changes over small ranges in tempera-ture at around 120 and 673 K, respectively, are characteris-tic of discontinuous thermochromic transitions and suggestthat the thermochromism of Cu ZrCl is due to either first-2 6

or second-order structural phase transitions. The lowtemperature DSC measurements provide no evidence forthe phase transition, consistent with a very small andreasonably a continuous change in enthalpy across thephase transition. Thus the 20–30 8 temperature range overwhich the structural transformation appears to occursuggests that the low temperature transition is a secondorder phase transition. In addition to the proposed localiza-tion of the copper atoms into discrete sites, based on thelow temperature diffraction data, we suspect that thecopper atoms adopt a more ideal tetrahedral geometry atlow temperature, analogous to the phase transition be-havior we have previously reported for ((bz)Cu) ZrCl [6]2 6Fig. 10. Calculated powder X-ray diffraction patterns for the lattices ofand [H NMe ]CuZrCl [7]. The movement of copper2 2 6Cu ZrCl (a) with a fractional occupancy of all three copper sites, (b)2 6away from a tetrahedral face into the center of thewith complete occupancy of Cu(1), (c) with complete occupancy of Cu(2)

and (d) with complete occupancy of Cu(3). interstice will result in a slight lowering of the Cu–Cl s*


the t -type Cu–Cl s* orbitals of the tetrahedral interstice2

are split such that an a orbital is stabilized and an e set of1

orbitals are destabilized as described in Scheme 1. Such adestabilization of the HOMO by placing the copper into asite of trigonal coordination above the high temperaturephase transition could account for the decrease in the bandgap, and thus the yellow to burnt-orange thermochromism.

4.3. van Vleck paramagnetism

The magnetic susceptibility of materials with closedshell ground states (i.e. with no permanent magneticmoment) yet exhibit a temperature independent or vanVleck paramagnetism is a relatively understudied phenom-enon. Initially described by van Vleck this temperatureindependent paramagnetism observed for close shell ma-terials originates from the coupling of ground and excitedScheme 1.

states through a magnetic field if there is spin–orbitalcoupling [20]. As such, the magnitude of this effect isgreatest for systems that exhibit low-lying excited states.

2 22HOMO (Scheme 1) thus shifting the absorption edge into The [MnO ] and [CrO ] anions are archetypal exam-4 4

the UV to give a colorless material. ples of van Vleck paramagnetism, although this phenom-A sharp endotherm at 673 K characteristic of a first- ena has been observed for numerous closed shell transition

order phase transition is observed by DSC upon heating metal compounds [21]. Normally resulting from in-Cu ZrCl prior to the melting endotherm at 735 K, as tramolecular interactions, van Vleck paramagnetism has2 6

shown in Fig. 5. At this phase transition an abrupt color also been measured for a series of charge transfer salts inchange from yellow to burnt-orange is observed. This which the magnitude of the paramagnetic susceptibility isphase transition is reversible as seen by the reverse color inversely correlated to the energy of the charge transferchange and the exotherm in the DSC observed upon bands [22]. Recently, controversy has surfaced with regardcooling. All attempts to obtain structural data for this high to the possible van Vleck paramagnetism in In ZrBr ,2 6

temperature phase have been complicated by the sublima- which was proposed to originate from an indirect elec-tion of the material out of the X-ray beam when at elevated tronic coupling between occupied In 5s- and empty Zrtemperatures. Undoubtedly at temperatures above this 4d-orbitals through the bromide bridges [23,24]. Thephase transition there will be increased disorder and/or challenge facing all of these studies is the deconvolution ofmobility of the copper atoms. This high temperature phase the contribution of the diamagnetic core from any vantransition may be related to the thermochromism observed Vleck paramagnetic susceptibility. In most cases, ain Ag HgI and Cu HgI [19]. In these materials the room diamagnetic susceptibility is measured that is smaller than2 4 2 4

temperature color is assigned to iodine-to-mercury charge that expected, based on Pascal’s constants for the diamag-transfer. The thermochromism has been assigned to an netic core [17]. Paramagnetism is thus implied afterincreased width of the iodine p-bands due to metal–iodine subtraction of the diamagnetic core correction from theinteractions above the phase transition temperature when measured susceptibility. However, measurement of thecopper or silver atoms are disordered over multiple tetra- magnetization as a function of the applied field andhedral sites. In Cu ZrCl , the HOMO (valence band) is not temperature allow us to separate the diamagnetic and2 6

halide centered because the chlorine is much more elec- paramagnetic components of the susceptibility.tronegative than iodine. However, the Cu–Cl s* orbitals Given the small magnitude of the observed magnetic(bands) have a substantial mix of both copper and chlorine response for Cu ZrCl , it is difficult to unequivocally2 6

character and thus will similarly be influenced by the distinguish an intrinsic effect of a material from a smallcopper distribution and site symmetry. As the copper magnetic impurity, although, the temperature independenceatoms traverse between tetrahedral interstices, they must of van Vleck paramagnetism is notably distinct from thepass through either a trigonal face or two coordinate edge behavior of a simple paramagnetic impurity. Thus, weof the tetrahedron. It has been shown that such copper ion must preface this part of the discussion with the possibilitymobility is facilitated by a second-order Jahn–Teller that the observed effects are due to some kind of magneticdistortion whereby the low-lying and empty copper 4s- and impurity. Nevertheless, based on the following evidence4p-orbitals mix into the copper d-orbitals [2]. By moving we have attempted to rule out the likelihood that thethe copper atom into a trigonal face between interstices, observed effects are due to magnetic impurities: (1)


Equivalent data are obtained for samples originating from perature independent. Thus we believe that the observeddifferent sample preparations. (2) Equivalent data are response is intrinsic to these materials and not an impurity.obtained from samples sealed in fused silica and DELRIN The negative linear slope at high fields of the M vs. H datasample holders, respectively. (3) Samples of Cu ZrCl are is equal to the diamagnetic susceptibility, x . The2 6 dia

EPR silent at room- and low-temperature. (4) Distinct, but diamagnetic susceptibility of Cu ZrCl , extracted from the2 6

related behavior is observed for multiple preparations of average of the linear fits of the 2 and 10.4 to 5.0 T datathe related compound [H NMe ]CuZrCl that was pre- obtained at temperatures between 5 and 300 K, are plotted2 2 6

pared from the same CuCl and ZrCl starting materials. (5) in Fig. 11, and are observed to be essentially temperature4

The temperature effect on the M vs. H measurements are independent. The slight up-turn in this x below 100 Kdia

distinctly different for Cu ZrCl and [H NMe ]CuZrCl . may be a result of a small paramagnetic impurity. Similar2 6 2 2 6

The molar magnetic susceptibility data for Cu ZrCl , at behavior is observed for [H NMe ]CuZrCl . The mag-2 6 2 2 624 21applied field strengths of 0.05–5.0 T (Fig. 6), are essential- nitude of the measured x , 21.6310 emu mol fordia

24 21ly temperature independent. However, when plotted on the Cu ZrCl , and 21.7310 emu mol for2 6

expanded scale of Fig. 6 (two orders of magnitude less [H NMe ]CuZrCl , are consistent with the values obtained2 2 624than a S51/2 system), subtle temperature and field effects from the sum of Pascal’s constants, 21.9310 emu

21 24 21are observed. Immediately apparent is the field dependence mol and 21.8310 emu mol , respectively. Thisof this data, and the jump in the susceptibility data at about diamagnetism can then be subtracted from the M vs. H120 K. We note that at applied fields above about 1 T, the data to yield Fig. 12, for example, which is presumably a‘paramagnetic susceptibility’ of Cu ZrCl is only a result picture of the field dependence of the van Vleck paramag-2 6

of the measured diamagnetic susceptibility being smaller netism.than the diamagnetic core correction. However, a para- The saturation magnetization of this van Vleck paramag-magnetic susceptibility is measured directly under applied netism, M , can be readily determined from the y-sat vV

fields of less than about 1 T. Data obtained on the related intercept of the high field x curve of Fig. 7, and isdia

compound [H NMe ]CuZrCl exhibit a similar field de- plotted as a function of temperature in Fig. 13. Only a2 2 6

pendence, however, no transition at 120 K is observed. small decrease in this M is observed forsat vV

Thus, we believe that jump in the susceptibility of [H NMe ]CuZrCl upon decreasing temperature from 3002 2 6

Cu ZrCl is a manifestation of the low temperature to 20 K. A slight decrease in the y-intercept would be the2 6

thermochromic, and structural phase transition. A similar expected effect of a small paramagnetic impurity (i.e. theconclusion was proposed by Dronskowski [23] to explain measured slope, x 1x , would be less negativedia impurity

the observed magnetic response in In ZrBr . His conclu- resulting in a smaller y-intercept). By contrast the M2 6 sat vV

sion was later contested by Jansen et al. [24]. However, of Cu ZrCl is about twice the magnitude of that observed2 6

when the data from the latter report are digitally scanned for [H NMe ]CuZrCl and exhibits a remarkable tempera-2 2 6

and expanded to the same scale as that reported in theformer (as well as subtracting the diamagnetic core correc-tion), the two sets of data are seen to be very similar,further supporting the idea that the structural phase transi-tions may be manifest in the van Vleck paramagnetism.Nevertheless, the impact of this phase transition on thesusceptibility appears to be masked at higher applied fieldstrengths, suggestive of a magnetic saturation.

To further probe the field dependence of this magneticresponse, we have measured the magnetization as afunction of applied field at several temperatures between 5and 300 K for both Cu ZrCl (shown in Fig. 7) and2 6

[H NMe ]CuZrCl . These data exhibit a maximum in the2 2 6

magnetization at applied field strengths of about 0.1 T. Amixture of a ferromagnetic impurity and a diamagneticcompound might account for such data. However, while

27only 4.7310 g of iron (out of the total sample of 0.04124g) could account for a molar magnetization of 1310

21emu mol , it seems unlikely that multiple preparationsFig. 11. Lower plot: Diamagnetic susceptibility, x , extracted as thewould yield the same very small amount of an iron dia

slope of the M vs. H data between 0.4 and 5.0 T, as a function ofimpurity. Furthermore, while the M vs. H data fortemperature. Upper plot: van Vleck paramagnetic susceptibility, x ,para vVCu ZrCl exhibit a notable temperature dependence, inset2 6 extracted as the slope of the M vs. H data between 6200 T, as a function

of Fig. 7 (itself inconsistent with a ferromagnetic impuri- of temperature. (Note the order of magnitude difference in the scales ofty), the equivalent data for [H NMe ]CuZrCl are tem- these respective plots.).2 2 6


paramagnetic susceptibility, x , from the slope of thepara vV

low field M vs. H data (60.02 T), although there isconsiderably more scatter to this data, thus any conclusionsfrom it must be made cautiously. Nevertheless, these data,

23shown in Fig. 11, suggest a x of about 1.2310para vV21emu mol above the thermochromic transition and a

23 21x of about 1.6310 emu mol below the phasepara vV

23 21transition. A value of x ¯2310 emu mol ispara vV

obtained for [H NMe ]CuZrCl , although the magneti-2 2 6

zation begins to saturate and thus deviate from a linearresponse even by 0.01 T. Interestingly, both the colorless,low-temperature phase of Cu ZrCl and the colorless2 6

[H NMe ]CuZrCl exhibit a larger x than is ob-2 2 6 para vVFig. 12. Saturation of the van Vleck molar magnetization of Cu ZrCl at2 6 served for the room-temperature phase of Cu ZrCl that2 6250 K as a function of the applied field.has the smaller band gap. These data are consistent withthe above hypothesis that the low temperature phase ofture dependence increasing approximately linearly to aboutCu ZrCl exhibits a more ideal local tetrahedral coordina-2 6120 K then reaching a plateau between about 100 and 40tion around Cu, analogous to that observed inK. The gradual increase in the M upon decreasingsat vV [H NMe ]CuZrCl . However, this trend in the x is2 2 6 para vVfrom room temperature to 120 K is consistent with ainverse to that anticipated based on the band gap (i.e.gradual lattice contraction, as seen in the room temperaturex is anticipated to be inversely proportional to thepara vVand 157 K lattice constants (a volume contraction of aboutband gap). While the optical band gap due to the Cu→Zr2%), which induces increased orbital overlap. At 120 Ktransition increases as the copper moves into the center ofthe structure changes in such a way that the band gap isa tetrahedral interstice, this motion also significantlyincreased in energy out of the visible spectrum as evi-reduces the second-order Jahn–Teller mixing of the Cu-3ddenced by the thermochromic transition. The increasedand Cu-4s,p orbitals, which lowers the energy of the emptyband gap thus results in a diminished mixing between theCu-4s,p orbitals. Thus, while the Cu→Zr band gap in-ground and excited states, and thus the change in thecreases on going to the tetrahedral geometry, the Cu-temperature response of the M . However, the plateau,sat vV 3d→Cu-4s band gap may decrease. Therefore, a combina-instead of a decrease in the M , may indicate that thesat vV tion of the copper centered and copper→zirconium cen-structural change causing an increased band gap alsotered charge transfer likely account for the increase in theresults in a bonding configuration with greater coupling ofx and M of Cu ZrCl at low temperature.para vV sat vV 2 6the ground and excited states. The origin of the steeper

up-turn in the M is not clear, but may be related to thesat vV

localization of the copper sites below the second order5. Summaryphase transition.

It is possible to extract a value for the van VleckThe solid state reaction of CuCl and ZrCl was shown to4

yield Cu ZrCl . The structure of this compound is most2 6

reasonably understood as a derivative of the ZrCl struc-3

ture-type. Here, every other metal site in a ZrCl face-3

sharing octahedral chain is removed, and is replaced bycopper(I) atoms, which partially occupy the six tetrahedralinterstices surrounding the now vacant octahedral inter-stice. The multiple tetrahedral interstices provide threecrystallographically equivalent sites over which the copperatoms are disordered at room temperature. The compoundCu ZrCl is observed to undergo two thermochromic and2 6

structural phase transitions at 120 and 473 K, respectively.Copper-to-zirconium charge transfer, the energy of whichis determined by the geometry of the copper coordinationsphere, best accounts for the color and thermochromism.As the copper atoms move out of a more ideal tetrahedral

Fig. 13. Plot of the y-intercept of the high field M vs. H curve (equal to interstice at low temperature toward a site in the trigonalM ) as a function of temperature for Cu ZrCl andsat vV 2 6 face of the interstice at high temperature, the band gap is[H NMe ]CuZrCl . The line through the [H NMe ]CuZrCl is the least2 2 6 2 2 6

decreased and thus the absorption edge is red shifted intosquares linear fit. The line through the Cu ZrCl is simply to highlight the2 6

change as a function of temperature. the visible range.


Coupling between the high-lying filled Cu–Cls* orbitals DOE BES contract (DE-AC02-98CH10886). This workand the low lying and empty Zr 4d-orbitals (or to the low was supported by the National Science Foundation (DMR-lying Cu 4s- and 4p-orbitals) provides a stage on which to 9501370, DMR-0072828, DMR-9703419 and DMR-investigate the temperature independent paramagnetism 0108605) and instrumentation grants (CHE-9509532 andsaid to result from the field induced mixing of the ground DMR-9601825). J.D. Martin is a Cottrell Scholar of theand low-lying excited states described by van Vleck [20]. Research Corporation.While one can never completely exclude the possibilitythat a weak magnetic response is due to impurities, themagnetic behavior of the closed shell materials Cu ZrCl References2 6

and [H NMe ]CuZrCl seem to provide new insights into2 2 6

the nature of van Vleck paramagnetism. As classically [1] L. Subramanian, R. Hoffmann, Inorg. Chem. 31 (1992) 1021.[2] J.K. Burdett, O. Eisenstein, Inorg. Chem. 31 (1992) 1758.understood, the magnitude of the van Vleck paramagnetism[3] R.M. Sullivan, J.D. Martin, J. Am. Chem. Soc. 121 (1999) 10092.is clearly dependent on the structure of the material as well[4] R.A. Huggins, in: A.S. Nowick, J.J. Burton (Eds.), Diffusion in

as its band gap (or HOMO-LUMO gap). However, it is Solids, Academic Press, New York, 1975, p. 445.reasonable to assume that like spin-only magnetization, [5] J.D. Martin, A.M. Dattelbaum, R.M. Sullivan, T.A. Thornton, J.one must also consider saturation effects. The van Vleck Wang, M.T. Peachey, Chem. Mater. 10 (1998) 2699.

[6] A.M. Dattelbaum, J.D. Martin, Inorg. Chem. 38 (1999) 6200.paramagnetism exhibits saturation effects similar to that[7] A.M. Dattelbaum, J.D. Martin, Inorg. Chem. 38 (1999) 2669.described by the Brillion function for classic paramagnetic[8] G.B. Kauffman, L.Y. Fang, Inorg. Syn. 22 (1983) 101.

systems, though here saturation occurs at very low applied [9] W.W. Wendlandt, H.G. Hecht, Reflectance Spectroscopy, Inter-fields. As a result, most literature reported values for van science, New York, 1966, Chapter 3.Vleck paramagnetic susceptibility reflect the saturation [10] E.J. Gabe, Y. Le Page, J.-P. Charland, F.L. Lee, P.S. White, J. Appl.

Crystallogr. 22 (1989) 384.magnetization. That the colorless (i.e. larger band gap)[11] A. Altomare, M.C. Burla, G. Camulli, G. Cascarano, C. Giacovazzo,[H NMe ]CuZrCl and low temperature form of Cu ZrCl2 2 6 2 6 A. Guagliardi, G. Polidori, J. Appl. Cryst. 27 (1994) 435.

exhibit the larger values of x than the yellow roompara vV [12] A.C. Larson, R.B. Von Dreele, GSAS: General Structure Analysistemperature phase of Cu ZrCl , can be accounted for if the System Manual, Los Alamos Report LAUR 86-748, Los Alamos2 6

less distorted tetrahedral Cu site provides more effective National Laboratory, Los Alamos, NM, 1986.[13] H.D. Flack, Acta Cryst. A43 (1987) 564.overlap between the ground and excited states, relative to[14] Y. Le Page, J.D.H. Donnay, G. Donnay, Acta Cryst. A40 (1984)the second-order Jahn–Teller distorted trigonal prismatic

679.geometry. By contrast the M is apparently doubled forsat vV [15] R. Herbst-Irmer, G.M. Sheldrick, Acta Cryst. B54 (1998) 443.Cu ZrCl with respect to [H NMe ]CuZrCl because it [16] M. Ferhat, A. Zaoui, M. Certier, B. Khelifa, Phys. Lett. A 2162 6 2 2 6

has twice the number of copper atoms per formula unit. It (1996) 187.[17] L.N. Mulay, E.A. Boudreaux (Eds.), Theory and Applications ofis necessary to now reexamine a variety of other materials

Molecular Diamagnetism, Wiley, New York, 1976, pp. 494–495.that are reported to exhibit van Vleck paramagnetism to[18] A.M. Dattelbaum, Thesis, North Carolina State University, NC,

firmly establish the saturation effects, and then develop a USA, 2000.model consistent with the phenomenon. [19] H.-R.C. Jaw, M.A. Mooney, T. Novinson, W.C. Kaska, J.I. Zink,

Inorg. Chem. 26 (1987) 1387.[20] J.H. van Vleck, The Theory of Electric and Magnetic Sus-

ceptibilities, Oxford, London, 1932.Acknowledgements[21] (a) P.W. Fowler, E. Steiner, J. Chem. Phys. 97 (1992) 4215;

(b) P.W. Fowler, E. Steiner, J. Chem. Soc. Faraday Trans 89 (1993)The authors acknowledge Dr Paul Boyle for collecting 1915;

the single crystallographic data, Dr Barbara Reisner, Dr (c) D.P. Raychaudhuri, P.N. Sengupta, Indian J. Phys. 10 (1936)245;Brian Toby, and Roger Sullivan for collecting and analyz-(d) D.P. Raychaudhuri, P.N. Sengupta, Indian J. Phys. 10 (1936)ing the neutron powder diffraction data. Neutron powder253.

data were obtained with support from the National Institute [22] M. Ghosh, R. Basu, Naturwissenshaften 62 (1975) 392.of Standards general user program (award [ NIST-1965), [23] R. Dronskowski, J. Am. Chem. Soc. 117 (1995) 1991.and synchrotron powder X-ray diffraction data were [24] M. Jansen, N. Wagner, M. Becker, U. Wedig, J. Am. Chem. Soc. 122

(2000) 808.obtained at the National Synchrotron Light Source via the

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Synthesis and characterization of Cu ZrCl : a thermochromic, van …user.physics.unc.edu/~ftsui/jac_338_173.pdf · 2002. 5. 22. · nitrogen. Data were then collected on an Enraf-Nonius