OFFICE OF NAVAL RESEARCH

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00

00o TECHNICAL REPORT NO. 21

PREPARATION AND CHARACTERIZATION OF SEVERAL

CONDUCTION TRANSITION METAL OXIDES

BY

AARON NOLD AND KIRBY DWIGHT

PREPARED FOR SYMPOSIUM M

MATERIALS RESEARCH SOCIETY

SAN DIEGO, APRIL 1989

OTtOEIECTED

JUN0 5 1989

May 24, 1989 S,

Brown UniversityDepartment of Chemistry

Providence, RI 02912

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11 TITLE (IncluOe Security Classification)* PREPARATION AND CHARACTERIZATION OF SEVERAL CONDUCTION TRANSITION METAL OXIDES

12. PERSONAL AUTHOR(S)

AARON WOLD AND KIRBY DWIGHT

1 3a. TYPE OF REPORT -T3. TIME COVERED 14 DA -E OF REPORT (Year, Month. Day) 1S PAGE COUNTTECHNICAL FROM _ TO MAY 24, 1989 11

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TO BE PUBLISHED WITH MRS SYMPOSIUM M, APRIL 1989 PROCEEDINGS.

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'FIELD GROUP SUB.GROuP

ABSTRACT (Continue on reverse if necessary and identify by block number) ....

he structure-property relationships of several conducting transition metal oxides, as wellas their preparative methods, are presented in this paper. The importance of preparing

homogenous phases with precisely Known stoichiometry is emphasized. A comparison is alsomade of the various techniques used to prepare both polycrystalline and single crystal'samples. For transition metal oxides, the metallic properties are discussed either interms of metal-metal distances which are short enough to result in metallic behavior, or interms of the formation of a l conduction band resulting from covalent metal-oxygeninteractions. Metallic behavior is observed when the conduction bands are populated witheither electrons or holes. The concentration of these carriers can be affected by eithercation or anion substitutions. The discussion in this presentation will be limited to theelements Re, Ti, V, Cr, Mo and Cu.

20 O1STR'BUIION 'iAVAILABILITY OF ABSTRACT 21 ABSTRACT SECURITY CLASSIFICATIONCUNCLASIPfIEOUNLMITED 0 SAME AS PPT C DTIC USrRS UNCLASSIFIED

22a NAME OF KIESPOtIBLE INDIVIU)UAL 22b ILLPHONE(include AeaICoae , 0 ,¢ ( YMBOL

DD FORNi 14

73, 84 %, A R 'A A'U ... ' " . .l. , i'iaiAI:O S(CU ITY (L I I(AIIO J OW Ti4IS PAGE

All Othe, ed-ltoni a'e ob'oIete

,~~ENE

I

I

PREP.R..VTION AND Ch1lARACTERIZATIONOF SEVERAL CONDUCTING TRANSITION METAL OXID'S

AARON WOLD AND KIRBY DWIGHTDepartment of Chemistry, Brown UniversityProvidence, Rhode Island 02912

ABSTRACT

The structure-property relationships of several conducting transitionmetal oxides, as well as their preparative methods, are presented in this paper.The importance of preparing homogeneous phases with precisely knownstoichiometry is emphasized. A comparison is also made of the varioustechniques used to prepare both polycrystalline and single crystal samples. Fortransition metal oxides, the metallic properties are discussed either in terms ofmetal-metal distances which are short enough to result in metallic behavior, orin terms of tht Ioriiation of a n' conduction band resulting from covalentmetal-oxygen interactions. Metallic behavior is observed when the conductionbands are populated with either electrons or holes. The concentration of thesecarriers can be affected by either cation or anion substitutions. The discussionin this presentation will be limited to the elements Re, Ti, V, Cr, Mo, and Cu.

INTRODUCTION

It is not possible in this paper to discuss all of the known conductingoxides. This treatment will be limited to a number of binary and complexoxides which have been carefully prepared and characterized. It is hoped thatthe review of these representative conducting transition metal oxides will enablethe reader to appreciate the importance of careful chemistry in theunderstanding of their properties. The structure and electronic properlies oftransition metal oxides depend on the stoichiumetry and itiogeiieity uf theprepared phases. Poorly prepared compounds cannot be effectively characterized.Consequently, an understanding of the variables which influence homogeneityand stoichiornetry is important if progress is to be made in developing newoxides which are both of fundamental interest and useful for practicalapplications. In this paper the discussion will be limite'l to three structuretypes, namely ReO 3 , rutile and CuO. The discussion of eacn :,tructure type willbe illustrated with specific compounds and will encompass problems encounteredin their preparation and characterization. The relationship between siructure,stoichiometry and conductivity will be emphasized, and correlations will he madein terms of the one-electron energy diagrams proposed and used so successfullyby J. B. Goodenough.

Rheniumn(VI) Oxide

Biltz and his co-workers firist repoLed Ihe pro liai':Iion of ReO 3 II "Improved methods for its preparation ore described by Ncct an kin et al. 1:11.The oxide is red and reported to be di airgrincilic 141. Ferretti, Rogers andGoodenough 151 indicated that Ren:, exisls with . considerable range ofcomposition. Since the (ectrical properilios are Ip e nrent on ,he a,:uipie

st-'jA .,uvmc!":,', li r ,nUciiometric single crystals wore grown I by chemical v:i por'transport of ReO:3 obtained by the reduclion of Red2) 7 with CO. Non-stoichiometric R20.1 was produlced by i he redtilt i proess, bll. purified whentransported in ,I two-zone furnirce with iodiine ns Hie iranport agent. Tietransporled, piurified crystals were found to be sloichiotnctric ilhiii tihe limilsof the an :i1ylical procedure used (tO.2e) (61. Resistivity mfeasterileitVIIs imad(de onsi ,,le crystals of stoiclioiw(tric ReO:, 151 indicaled 1hai11 y exhibited itallicbelrivi or.

In the ReO 3 structure (Fig. 1), the oxygen atoms occupy three-quarters ofthe positions in a cubic close-packed type lattice. The rhenium atoms occupyone-quarter of the available octahedral sites. The unit of structure, asrepresented in Fig. 1, is a simple cube of rhenium atoms with oxygen atoms onall of the twelve edge centers. The cubic ReO 3 structure is the simplest three-dimensional structure formed from vertex sharing of octahedral (ReO 6 ) groups.Each rhenium, therefore, has six oxygen nearest neighbors and each oxygen atomhas two rhenium neighbors arranged linearly.

According to Morin 17] and Goodenough 181, for ReO 3 where rhenium is inthe +6 oxidation state, the crystal environment permits the formation of aconduction band by n overlapping of the metal "d" orbitals with the pH orbitalof the oxygen. For the ReO 3 structure (Fig. 2), each anion contributes two oorbitals (sp) and two n orbitals pn 2 . Each octahedrally coordinated cationcontributes six o orbitals (eg 2Sp3 ) and three n orbitals (t0 ). A large overlapof the metal t 3 g and anion P n orbitals can result in the Tormation of bondingand antibonding 11 bands (Fig. 2) 181. In Fig. 2, the o and n bands arecomposed of bonding orbitals and the o' and n' bands of antibonding orbitals.The pn* energy level is a non-bonding state and is composed of anion P.orbitals. The number of states is designated by [n] and refers to the spin andorbital degeneracies per molecule. ReO 3 has 25 outer electrons per molecule andhence can fill up to one-sixth of the total number of n' states. Hence, thismodel is consistent with the observed metallic behavior of Re0 3 .

This model was confirmed by Ferretti, Rogers and Goodenough 151 whocompared the electrical conductivity of single crystals of ReO 3 with that ofsintered bars of polycrystalline Sr2MgReO 6 . The latter compound is an orderedperovskite and was first prepared by Longo and Ward (91. In this compound, therhenium and magnesium atoms are at the centers of oxygen octahedra which arelinked by vertex sharing. Sr2MgReO 6 differs from ReO 3 in that Mg,2 and Re6

+

are in a nearly perfect ordered arrangement; the rhenium atoms cannot approacheach other through oxygen more closely than about 7.6A. However, in Re0 3 , theRe atoms are about 3.8A apart.

Ferretti et al. 151 carried out an investigation to distinguish betweenmodels for the conduction band involving either direct overlap of t,-, orbitalsacross the face diagonal of the unit cell or indirect overlap (n bondng) of thetg metal orbitals and the oxygen Pn orbital. The former model would allowboth compounds to be metallic, but the latter predicts metallic behavior(delocalized d-electrons) for ReO 3 and semiconducting behavior, at most, forSr 2 MgReO 6 . Sr 9 MgReO 6 was found to be a semiconductor which is consistent .with metal-oxygen-metal overlap. Rhenium trioxide single crystals were foundto be metallic conductors with conductivities approaching that of metallic silver. -

The compound was found to be diamagnetic, indicating that the weak Pauli -

paramagnetic moment c;Xpected from the conduction electrons is insufficient loovercome the diamagnetism of the electrons in closed shells.

PiO2 and Transition Metal Dioxides witht-hlL -Rut-i-le Structure

One of the polymorphs of Tie.), futile, crystallizes with the structureshown in Pig. 'I (space group P4,)/mnjm). The metal coordiniates are: Ti: (,0,0),(1/2,1/2,1/2) and 0:±(x,x,O).(1/2+x,1/2-x,1/2). As caI be seen from Fill. 3, thestructure cons;ists of chains of TieU octahedra and e:tch pair share opposilecd e'w". Each tlalitim alOm is surrounded octalhedrally by six oxygen atoats,whereas each oxygent is sutrrotn(hde by three I itaijun atoms arraned is cornersof an equilaleral triing.lc. 'Th, t;ll'CttrO, th(c'lcf,,r, has :1 6:,1 coordination.

Rhenium 0 Oxygen

Fig. 1. Structure of ReO 3

Re(VI) 3 02-

p 3 L12Ic-*

e 2-6g EF

t 3

__ .I ,eI6' r1 SW ' -, --,

- - -. ch mai .nr ..~id /Ltiu~ ofor i'cO. " ie i i , o e

2A i" dW 1p'* P " Tr

'KTI IIRk

: .- ... IDrTIC TABL-

Fig. 2. Schematic E.nergy ]AJ~l I--Avtrlbutv (,oe

for R c0 3 . A..... -I-lw o

1) t~a t Seia

I" U 1 181 II

.. ... .... .. ..

MetalOxygen

Fig. 3. Rutile Structure

2 Ti(IV) 4 2-

P 3 (2410 II __ I

e 2

3.. ',.WI:.

-L C

" - ">- " - >7 " P Tr'

Fig. ,A. Sclicinatic Encrg;y Rhmlsfror TiO,

t t t{l l I I I

A number of transition Metal dioxides crystallize with rutile-relatedstructures. Magneli et al. [10-13i, as well as Goodenough 18,141, have relatedcertain properties of various transition metal dioxides to the number oftransition metal d-electrons. Goodenough 181 has proposed, arid Rogers 1151 hasapplied, a one-electron energy diagram for TiO (rutile) and related compounds(Fig. 4). In this diagram the metal e0

2sp3 wave functions of a doubled formulaunit Ti 2 0 4 are mixed with hybridize a sp 2 wave functions of oxygen to formbonding and antibonding o states. As a result of the long range ordering of thestructure, the states are broadened to form bands with finite widths.Interaction between two t.-g cation orbitals (t,,,±L) with the remaining oxygenPn gives rise to bonding and antibonding n bands. "IC l') c orbitals of thecations are directed along the c axis and may form a a band ]f the oetal-metalseparation is small enough for significant overlap to occur. The allowablenumber of electrons that can be accommodated by any band is given in brackets.and this number is equal to the product of the number of atoms per formulaunit, the orbital degeneracy per atom and the spin degeneracy per atom. Therutile TiO., has an electronic configuration of 3d for Ti(IV). The Ti-O o and nbands are completely filled so that the Fermi level is located at the middle ofthe energy gap between these bands and the antibonding ir and ration-cationd-bands. Hence. TiO.) (rutile) should be a semiconductor and become metallicwhen the Fermi level -is raised as the TiO.- is reduced to form some Ti(lII) 3u'states.

Vanadium (IV) Oxides

'The vanadium oxides with compositions between V2 0:3 and VO., are formedfrom slabs of a rutile-like structure sharing faces. Each of these phases showstransitions from the semiconducting to metallic state. Many of their propertiescan also be explained by considering structural changes which occur at thetransition and utilizing the concepts of Goodenough 181 in which strong covalentmixing with oxygen p. orbitals to form a n' band plays a central role (Fig. 5a).For VO,, above the transition temperature of 67°C, these bands overlap and arepartially filled, giving rise to metallic conductivity. The compound is tetragonal(P42/mnm) and all the V-V distances are equal (:2.87A). Below the transitiontemperatlre, the formation of V-V pairs along the letragonal c-axis causes adoubling of the crystallographic unit cell 1161, and a splitting of the tlc orbitalin two (Fig. 5b). The resulting lower o level lies below the bottom i the ICband, becomes filled and lowcrs the Fermi energy below the boltom of lhe tband. The W band, arising from covalent mixing, is now emply 181. E'xcitationof electrons from the filled o level to the bottom of the u band g;ives rise tothe semiconducting behavior, -

Other Transit ion Metal Dioxides Crvys alliZin wit _h it he Rutil Structure

The simple model proposed by Goodenough 1,,141 has beeni applied byRogers 11,51 to account for the electronic behavior of I he t ransition metaldioxides with rutile-like structures. Molybdenim(IV) oxide has 2 unpairedelectrons for each molybdenuin ;{toiu. Therefore, one olectron is available forMo-:Mo o bonding and the scrotd electron call pa rtially fill ile Mo-o W" band(Fig.6). However, the short mo-Mo separation is not fully conisislenl \it h asingle o bond. Hence, so eli of tho electrons from lhe m let :l oxyteii ii b:inid :airpromoted to a mnetal-metal i1 ban( which is located at 1 lie same iery level a.Sthe metal-metal 0 slale 11-51.

ror n7o.), Io,,er; 11!,q huts ilidic:ted that there( is a sh'aip ki c eIase illdirecl catioln-catlioll han[ldil,>. eli k', th e r 1 I do [lot eIter ilto metal-meta o

bonds, but rem:iin :it di:;'rele cat i i Sites In 1115i. ceimpon d (Fig. ,'). Theexi.lener' of lhl..se Ilill-bollo illi', sitc., .kilhill ti1e IS" ba:nld is collisSt('Til zilh !he

2 V(IV) 4 o2 -

P3 [241 C-*

Si ----- - -

2 -

ai 31 TT*'mc 1 | 141 01-- - E F

Fig 5.S. a Enrg Bands

Fig;. 5a. Schematic Energy Bands

for Tetragonal V0 2

2 VQV) 4 02 -

p 3 i1241I-

SI Irr*,cIlS l :.

2 2 F 1"-7-

32 ~ --- ------------------------------- E

121 Cr-

131-IT

. .. tLtt. 2 1 - .,q

Fig. 5b. Scliematic Energy Bandsfor N'oiiocli11ic VO.,

2 MO(IV) 4 02-

p 3 (2410- ISI-

e 2 J

PTT

f24~

Fig. 6. Schemnat ic Energy Bandsfor .M,'oO,

2 Cr(IV) 4 0 2

p 3 1241 c-*

Si -

4-

.L C

-TT

s 2

- 12410N,

Fig. 7. Sclicrnutic Energy Bawlsfor C'r()

observed ferromagnetic metallic properties of CrO... \ number of the )latinummetal dioxides, e.g. Ru, Ir, Os and possibly Rh, shiow metallic behavior which isconsistent with the partial filling of the metal-oxygen W" band. For fltO) wherethe n' band is filled, the Fermi level is raised to lie between the n-and o"bands and the compound should behave as a semiconductor. Rogers 1151 hasreported powder samples of PtO., to be semiconducting.

Copper Oxides

Copper(ll) oxide occurs in nature as the mineral tenorite. The structure117) shows Cu(II) with square planar coordination of oxygen around the copper.The space group is C2,'c with unit cell dimensions a = 4.6837(5)A, b =3.4226(5)A, c = 5.1288(6)A and (3 = 99.54(1)1. Fig. 8 shows the chains ofoxygen coordination parallelograms formed by sharing edges. The structure is adistorted PdO type with two O-Cu-O angles of 84.5* and two of 95.5'. Each

copper has 4 0' neighbors at 1.96A and two next nearest 0" neighbors at 2.78A.The line O"-Cu-O" is inclined at It to the normal to the Cu(O')., plane and theshortest Cu-Cu distance is 2.90A.

.-\nother oxide of copper, Cu]60 1 4 has been reported 1181. It is the raremineral paramelaconite and crystallizes with the space group 14 1iamd, with unitcell dimensions a = 5.817A, c = 9.803A. The structure is related to tenoritebut the oxygen vacancies result in a rearrangement of the monoclinic tenoritestructure to a tetragonal one.

In Cu(II) oxide, the Jahn-Teller distortion due to the stabilization of asingle d-hole per atom, Cu(Il)3d 9, results in the observed difference of itsstructure with the monoxides of the other first row transition metals. For CoO,the equitorial oxygen p0 overlaps strongly with the d(x 2 -\ 2 ) orbital to um a

bands, but little, if any, with the d(z2). The copper d' , az, dxz orbitalscontribute to the formation of n bands. 0' Keefe and Stonfe 11*91, Roden et al.

[201, as well as Yang et al. 1211, have indicated that CuO is ant iferromagnut icwith an observed anomaly in the magnetic susceptibility at 230 K. TheGoodenough band model 122,23 indicates that when antiferromagnetic behavior isobserved in the copper oxides, a separation may occur between the empty o'band states of the Cu(lI'I) couple and the occupied o' band states of theCu(1lI/lI) couple. This is shown in Fig. 9 where the Fermi energy is placed nearthe top of the filled n' band. The antiferromagnetic behavior of Cuo is nottypical, since the data reported by Roden (201 do not show a maximum of "\ atthe aritiferromagnetic ordering temperature. The unusual ordering can be related -:

to the magnitude, or even existence, of the o' bandsplitting. In the case whereCu(lIl) is formed, the sign of the carrier results unambiguously from the

formation of holes in the top of the n' band. \Vhere Cu(l) is formed, in additionto any electrons entering the o' (Cull/I) band, holes in the ir band are alsocreated by thermal excitation. Since the o" bands are narrnw, the greatermobility of the holes in the n' band will determine the sign of Ilhe domin'antcarrier. Koffeyberg [241 has intdicated that C'uO can only be prepared as p-type.

ACKNOWI.EDGEMENTS

'Phis research was partially supported by G'TE Labs, Inc., Walt ham,Massachusetts, by the Offi re of Naval l .,ch and by lht' Natiml Sciel ceFouillat ion, G(rant. No. )M -8901:!70.

Q copper ( )OxygenFig. 8. Structure of CuO

Ff g. 9. Schematic Band Structuirefor CuO

REFERENCES

1. W. biltz, A. A. Lehrer and K. leisel, Nachr. Ges. Wiss. Gottingen,Math. Physik Klasse, 191 (1931).

2. W. Biltz, A. A. Lehrer and K. Meisel, Z. Anorg. Allgem. Chem. 20 7113 (1932); '214, 775 (1933).

3. N. Nechankin, A. D. Kurtz, C. F. Hiskey, J. Am. Chem. Soc., 73,2828 (1951).

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6. D. G. Wickham and E. R. Whipple, Talanta 10, 314 (1963).

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13. B. 0. Marir der, F. Dorm and M. Se]eborg, ibid., 16, 293 (1962).

14. J. B. Goodenough, "Magnetism and the Chemical Bond,"lnterscience Monographs on Chemistry, Inorganic ChemistrySection, Vol. 1, F. A. Cotton, Ed., Intc science )ivision, JohnWiley & Sons, Inc., New York, NY and London 196:3.

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J. Inorg. Chem., 8(4), 841 (1969).

1 6. 6. lliigg, Z. Phys. Chem., -2.9, 192 (19"35). 0

17. S. Asbrink and L. J. Norrb , Acla ('ryst., 1126, 8 (1970).

18. N. Datta and J. W. ,leffery, Act a Cryst., [F14, 22 (1 978).

19. M. O'Keefe and 1. S. Slone, ,J. Plhys. Chom. Sol., '-"1, 2';1 (1962).

20. [1. Roden, E. Iraumi and A. l'riiiimih, Solid Sltale Comlin.. 64, No.7, 1OF-] (1987).

21. H. I . Yart,,, .1I.M. 'l'raltIiui Imd and G. Shi rane , lh . Rev. IB, 3', \o. 117.1 (1 19P0.

22. .1. t3. (iof)'V 1'io ;'.l., ,., ,I;(1. l' uic:tioii, 9(6), 6 9 (1 37).

23. J. B. Goodenough, A. Manthiram, Y, Dai and A. Campion,Superconductor Sci. and Technology (in press).

24. F. P. Koffeyberg and F. A. Benko, J. Appl. Phys, 53(2), 1173(1982).

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