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143

The clathrate I compounds M8Sn44g2 (M = K, Rband Cs) contain an anionic framework Sn44

8–, inwhich the alkali-metal cations occupy Sn20 andSn24 polyhedra (Fig. 1). According to extensive X-ray diffraction studies the Sn framework containsvacancies arising from a merely 66% occupation ofthe Sn1 site [1]. As structural defects affect thephysical properties, the thermoelectricity ofM8Sn44g2 compounds differs significantly fromthat of related compounds like M8Si46 clathrateswith fully occupied framework positions [2]. Sincetwo defect positions per unit cell imply that 8 three-fold bonded Sn3 atoms adjacent to the defects havea clearly different environment compared to theremaining 36 Sn atoms, we analyzed the influenceof the structural vacancies by NMR experiments.

The compounds were synthesized from the ele-

ments in the stoichiometric ratio using Nb tubes ascontaines. Though some of the samples showed aslight excess of tin, there was generally no evidencefor impurities according to X-ray diffraction meas-urements, chemical analysis and thermal analysis.

Both 117Sn and 119Sn have a nuclear spin of I =1/2 and therefore only the central transition {1/2↔1/2 can be observed. Hence, the interpretation issimplified even though complicated NMR spectracan still be obtained. Furthermore, 117Sn and 119Sn

have a small natural abundance of 7.6 % and 8.6 %,respectively.

The static 119Sn NMR spectra obtained at roomtemperature in a magnetic field of B0 = 7.04 T arepresented in Fig. 2 a-c. Due to the spectral widththe measurements have to be done using the point-to-point method with selective excitation of smallspectral windows to avoid signal distortions. Thewidth of the signals decreases from the K to the Cs

119Sn NMR of Alkali Metal Tin ClathratesFrank Haarmann, Michael Baitinger and Yuri Grin

Fig. 1: Section of the clathrate I network containing apentagondodecahedron around M1 atoms and a tetra-kaidecahedron around M2 atoms ( M= K, Rb, Cs).

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Fig. 2: (a-c) 119Sn NMR spectra of M8Sn44g2 with M =K, Rb, Cs. (d) frequency dependence of the spin-latticerelaxation time T1 in Cs8Sn44g2. The full lines representa guide to the eye. The symbols in the spectra representthe integrated intensity of selective Hahn echo experi-ments.

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compound. All three spectra have a three- or four-peak structure with different peak intensities.Nevertheless, the complex line shape prevents anassignment of the individual signal components.

A special feature of the NMR signals forM8Sn44g2 with M = K, Rb, and Cs is the extreme-ly large broadening of the peaks. Magic angle spin-ning (MAS) experiments were preformed onCs8Sn44g2 to reduce the broadening and to deter-mine the centers of gravity for shift assignment ofthe individual signals. These measurements resultin no significant changes compared to the staticspectrum. For nuclei without quadrupole momentonly inhomogenities of the sample and/or distribu-tions of chemical shifts or Knight shifts can pro-duce such an effect.

To investigate the origin of the shift distributionsspin-lattice and spin-spin relaxation measurementswere done and are still in progress. The frequencydependence of the spin-lattice relaxation time T1 of119Sn in Cs8Sn44g2 measured by selective satura-tion Hahn echo experiments is shown in Fig. 2 (d).The small frequency range of the signal shows sig-nificantly slower relaxation which indicates thenon-equivalence of the atoms contributing to thispart of the signal with respect to the other part ofthe signal.

Spin-spin relaxation measurements show a longT2 compared to the spectral width of the signal. Thenatural width of the signal (T2*) calculated byassuming dipolar coupling dominates the signaland is in the order of the measured T2. This is anevidence for an inhomogeneous NMR signal. Hahnecho experiments with increasing distance (í) ofthe p/2 –í–p pulses (ensuring the signal-to-noiseratio of different spectra is nearly equal) show alsodifferent parts of the signal. This can be seen in thedifferences of both spectra presented in each sub-figure of Fig. 3 (a-c).

At present two competing models describing thespectra are under consideration:– The signal is due to three or four groups of chem-ical or Knight shifts corresponding to the differentpositions Sn1, Sn2, and Sn3 (Sn31 and Sn32). – The signal consists of a wide distribution of che-mical or Knight shifts having the origin in slightlydifferent positions of Sn1, Sn2, and Sn3 (Sn31,

Sn32) and of a broad asymmetric spectrum whichis due to the anisotropy of the chemical shieldingtensor caused by the lone pair of electrons locatedat Sn3 (Sn32) atoms neighboring a vacancy in theSn1 position.

References

[1] M. Baitinger, Dissertation, Technische UniversitätDarmstadt (2000).

[2] G. S. Nolas, B.C. Chakoumakos, B. Mahieu, G. J.Long, T. J. R. Weakley, Chem. Mater. 12, 1947(2000).

Fig. 3: Spin-spin relaxation of the 119Sn signal ofM8Sn44g2 with M = K, Rb, Cs. The red lines representthe spectra with short spin-spin relaxation, the blue linesrepresent the spectra with advanced spin-spin relax-ation. The black lines indicate the difference of bothspectra.

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145

Nuclear magnetic resonance (NMR) of nuclei witha quadrupole moment (I > 1/2) is sensitive to theelectric field gradient at the examined positions.Lattice distortions, which are due to atomic disor-der or other defects of a crystal, have a great influ-ence on the electric field gradient and lead to abroadening of the satellite transitions whereas thecentral transition is not affected according to firstorder perturbation theory [1].

X-ray diffraction experiments and measurementsof the mass density of aluminum diboride Al0.9B2

lead to a defect structure model with vacancies atthe Al site. The composition determined by thereexperiments is Al0.9B2, even for single crystalsgrown from a melt of composition Al:B = 49:1.

The 11B isotope has a small quadrupole momentand the experiments were done using a magneticfield of B0 = 7.04 T. Therefore, first order pertur-bation theory is appropriate to analyze the data. Inaddition, 11B has a high sensitivity for NMR exper-iments. The electric field gradient at the B site isdetermined by the covalent B-B bond and the coor-dination of the Al atoms (Fig. 1). A vacancy at aposition of an Al atom produces a significantchange of the electric field gradient at the B sitecompared to full occupation of all Al sites.Moreover, a relaxation of the lattice due to theinfluence of the vacancies can be expected. Thesewill also influence the electric field gradients ofneighboring positions.

An 11B NMR spectrum centered at a frequency ofn0 ¹ 96.26 MHz is presented in Fig. 2a. The cen-tral transition was detected without distortions dueto the measurement technique within a singleFourier transform spectrum. To measure the wholesignal of such a broad spectrum without artifact,selective excitation of a narrow spectral windowmust be done using a Hahn echo sequence withlow-power pulses of long duration. The wholespectrum is achieved by sweeping the frequencypoint by point.

The quadrupole frequency nQ = 540 kHz 620 kHz determined by least squares fitting of thespectrum is in good agreement to earlier results[3,4]. A slight asymmetry of the central transitionis due to an anisotropy of the Knight shift whichcan be estimated to DK ¹ 130 ppm. Such ananisotropy has been predicted by ab initio calcula-tions. The absolute value of the Knight shift of K ={10.5 ppm was determined in magic angle spinning(MAS) experiments [4]. The asymmetry parame-

11B NMR of Al0.9B2

Ulrich Burkhardt, Vladimir Gurin, Frank Haarmann and Yuri Grin

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Fig. 2: 11B NMR spectrum of Al0.9B2: ♦ selective excita-tion Hahn echo experiments; full line: calculated spec-trum for I=3/2 with nQ=540 kHz, hQ=0, DK=130 ppm,hK=0. (b) Magnification of subfigure (a).

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Fig. 1: Coordination polyhedra of B: (a) ideal and (b)one of the real configurations (g = vacancy).

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ters of quadrupole coupling and Knight shift can beneglected (hQ ¹ hK ¹ 0). This result is alsoobtained by a least-squares fit of the spectrum.

A broadening of the satellites with respect to thecentral transition is obvious (Fig. 2b). For a bettercomparison, a calculated profile is added to the fig-ure. The broadening of the satellites is due to a dis-tribution of quadrupole frequencies which mostlikely result from the influence of the vacancies atthe Al sites. Thus, the results of our NMR experi-ments are in agreement with the model derivedfrom X-ray diffraction data and mass density meas-urements [2].

References

[1] M. Mehring and O. Kanert, Z. Naturforsch. 24a,332 (1969).

[2] U. Burkhardt, V. Gurin, F. Haarmann, H. Borrmann,W. Schnelle, A. Yaresko and Yuri Grin, J. Solid StateChem. 2003 in press and Development of theInstitute and Scientific Report “Aluminium dibori-de” MPI CPfS (2000).

[3] J. P. Kopp and R. G. Barnes, J. Chem. Phys. 54,1840 (1971).

[4] S. H. Baek, B. J. Suh, E. Pavarini, F. Borsa, R. G.Barnes, S. L. Bud’ko and P. C. Canfield, Phys. Rev.B 66, 104510 (2002).

147

The two „twin isotopes“ of gallium (71,69Ga withnuclear spin of 3/2 and abundance of 60.4 % and39.6 %, respectively) were used as local probe todetect structural information in a series of binarygallides TMGa (TM = Ni, Pd, Pt). The NMR lineshape of gallium containing materials and the posi-tion of the center of gravity are strongly dependenton the electric field produced by the environment(Electric Field Gradient, EFG). It is known thatstrong quadrupolar interaction causes very broadspectra. Due to the fairly high quadrupolar momentof both isotopes, the quadrupolar interaction isvery often large, so that the {3/2 ↔ {1/2 and 1/2 ↔3/2 transitions do not occur in the same spectralwindow as the main transition which is itself broad-ened by second order quadrupolar interaction. Themain transition may reach several MHz inlinewidth. Because of the lower quadrupolarmoment, the higher resonance frequency and sensi-tivity, 71Ga is the favorite nucleus. Furthermore, itis a very advantageous situation to have two NMRactive isotopes to ensure the experimental resultsand the simulations as well. Therefore, we meas-ured the resonance signals for both isotopes.

NiGa

NiGa crystallizes in the cubic CsCl structure. Dueto the cubic site symmetry Oh of Ga atoms, noquadrupolar contributions are expected. The 71Gastatic powder pattern at room temperature isobserved at a Knight shift K = 0.206(5) %. In theMAS spectra with a maximal rotation frequency of12 kHz the linewidth remains constant. Because ofthe weak Pauli-paramagnetic character of NiGa wecan exclude an influence from magnetic momentsof Ni on the linewidth. Therefore, it can beassumed that the linewidth is caused by a distribu-tion of shifts, due to small disorder effects. Thephase NiGa has a large homogeneity range [1] andthe occurrence of substitutional disorder is knowneven for the equimolar composition [2]. By thiseffect the environment of some Ga atoms ischanged. This causes small alterations of the

Knight shift and gives rise to tiny quadruolar inter-actions, leading to the broadening of the NMR sig-nal both in the static powder pattern and in theMAS spectrum. The temperature dependent 71Gaand 69Ga NMR spectra show a small shift to high-er frequencies with higher temperatures. The signalwidths decrease with increasing temperature, asexpected.

PdGa and PtGa

PdGa and PtGa crystallize in the FeSi structuretype with the space group P213 [3, 4]. The FeSistructure can be described as a strongly distortedNaCl variant with an increase of the coordinationnumber from 6 to 7 for both atom types (Fig. 1) [5].In the crystal structures of PdGa and PtGa, theshortest distances between transition metal and gal-lium occur along the three-fold axes: d(Pd–Ga) =254 pm and d(Pt–Ga) = 257 pm (from our X-raysingle crystal structure refinements).

Due to the C3 symmetry of the Ga site and theinfluence of the short distance mentioned above,the NMR spectra are dominated by second orderquadrupolar interactions. The 71Ga point-by-pointNMR spectrum at 4.2 K of PdGa including the

71,69Ga NMR Spectroscopy of Transition-Metal MonogallidesAnnegrit Rabis, Rainer Giedigkeit, Frank R. Wagner, Andrei A. Gippius*, Michael Baenitzand Yuri Grin

Fig. 1: Crystal structure of PdGa and PtGa, respective-ly. The shortest distances along the three-fold axes aremarked red.

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satellite transitions is presented in Fig. 2 togetherwith the signal observed for the main transition.The Knight shift K = 0.304(10) % is positive andclose to the one observed for a Pd(Ga) alloy with5 % gallium (K = 0.320(10) % [6]). Also of interestis also the fairly large quadrupolar interactionresulting in a quadrupolar coupling constant nNQR =16.6(2) MHz. It points to a high EFG at the galliumsite and supports the finding of a strong lattice dis-tortion detected by X-ray diffraction. Using thegeneral equation |Vzz| = nNQRh / eQ an EFG|Vzz

Ga (PdGa)| = 7.14 × 1021 V/m2 was obtained. ForPtGa the observed Knight shift K = 0.197(10) %and the quadrupolar coupling constant nNQR =14.1(2) MHz are smaller than for PdGa. From thisquadrupolar coupling constant an electric field gra-dient |Vzz

Ga (PtGa)| = 6.04 × 1021 V/m2 is derived.First principles electronic structure calculations forthe experimentally determined crystal structure onthe scalar-relativistic DFT level using the fullpotential APW+lo code WIEN2k [7] resulted in elec-tric field gradients Vzz

Ga(PdGa) = {5.8 × 1021 V/m2

and VzzGa(PtGa) = {5.0 × 1021 V/m2. A calculation of

PtGa using the structural parameters of PdGaresulted in the same value Vzz

Ga, indicating that thedifferent EFGs are primarily caused by structuraldifferences. Structural relaxation has a significantinfluence on these values and test calculations indi-cated an up to 20 % decrease by just optimizing theinternal parameters at the experimental lattice con-

stants determined at T = 293 K. Taking this intoaccount, the agreement with the NMR experiment(at T = 4.2 K) is satisfactory.

Summary

Investigations of binary transition metal monogal-lides TMGa (TM = Ni, Pd, Pt) have shown that gal-lium NMR is a sensitive local probe to extract thestructure information as well as the electronicalenvironment and provides information which can-not be found with other methods. The Knight shiftsfound here are in a good agreement with literaturedata observed for palladium-rich alloys. It could bedemonstrated that the value of the quadrupolar cou-pling constant is sensitive to small structuralchanges. A significantly smaller value has beenfound for PtGa in comparison to PdGa. With theknowledge of the quadrupolar coupling constantsnNQR the absolute values of the EFG can be derived.These values are in a good agreement with thosefound by theoretical calculations which additional-ly provide the algebraic sign. On the other hand,knowledge of experimental EFGs for a systemunder study represents a valuable information forthe evaluation of the quality of certain approxima-tions used in a quantum mechanical calculation.

84 86 88 90 92 94 96 98 100

91,6 91,8 92,0 92,2

a)

Frequency [MHz]

PdGaPtGaNiGa

Frequency [MHz]

Fig. 2: 71Ga-NMR spectrum for PdGa at 4.2 K, inset:central transition for NiGa, PdGa, PtGa.

References

[1] E. Hellner, Z. Metallkd. 41, 480 (1950).[2] K.-J. Best, Z. Metallkd. 62, 419 (1971).[3] E. Hellner, F. Laves, Z. Naturforsch. 2a, 177 (1947).[4] M. K. Bhargava, A. A. Gadalla, K. Schubert, J. Less-

Common Met. 42, 69 (1975).[5] L. Pauling, A. M. Soldate, Acta Crystallogr. 1, 212

(1948).[6] G. A. Matzkanin, D. O. van Ostenburg, J. J. Spokas,

H. G. Hoeve, C. H. Sowers, Bull. Am. Phys. Soc. 13,363 (1968).

[7] P. Blaha, K. Schwarz, G. K. H. Madsen, D. Kvas-nicka, J. Luitz, Computer code WIEN2k, An Aug-mented Plane wave + Local Orbitals Program forCalculating Crystal Properties, Wien2k (KarlheinzSchwarz, Technische Universität Wien, Austria,ISBN 3-9501031-1-2.

________* Moscow State University, Faculty of Physics,

119899 Moscow, Russia