Interstitial Zn Atoms Do the Trick in Thermoelectric Zinc Antimonide, Zn4Sb3: A Combined Maximum Entropy Method X‐ray Electron Density and Ab Initio Electronic Structure Study

Chemistry - A European Journal - Tập 10 Số 16 - Trang 3861-3870 - 2004
Fausto Cargnoni1, Eiji Nishibori2, Philippe Rabiller3, Luca Bertini1, G. Jeffrey Snyder4, Mogens Christensen5, Carlo Gatti1, Bo B. Iversen5
1CNR-ISTM, Instituto di Scienze e Tecnologie Molecolari via C. Golgi 19, 20133 Milano, Italy,
2Department of Applied Physics, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
3Université de Rennes 1, UMR CNRS 6626, 35042 Rennes, France
4Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
5Department of Chemistry, University of Aarhus, 8000 Aarhus C, Denmark

Tóm tắt

AbstractThe experimental electron density of the high‐performance thermoelectric material Zn4Sb3has been determined by maximum entropy (MEM) analysis of short‐wavelength synchrotron powder diffraction data. These data are found to be more accurate than conventional single‐crystal data due to the reduction of common systematic errors, such as absorption, extinction and anomalous scattering. Analysis of the MEM electron density directly reveals interstitial Zn atoms and a partially occupied main Zn site. Two types of Sb atoms are observed: a free spherical ion (Sb3−) and Sb24−dimers. Analysis of the MEM electron density also reveals possible Sb disorder along thecaxis. The disorder, defects and vacancies are all features that contribute to the drastic reduction of the thermal conductivity of the material. Topological analysis of the thermally smeared MEM density has been carried out. Starting with the X‐ray structure ab initio computational methods have been used to deconvolute structural information from the space‐time data averaging inherent to the XRD experiment. The analysis reveals how interstitial Zn atoms and vacancies affect the electronic structure and transport properties of β‐Zn4Sb3. The structure consists of an ideal A12Sb10framework in which point defects are distributed. We propose that the material is a 0.184:0.420:0.396 mixture of A12Sb10, A11BCSb10and A10BCDSb10cells, in which A, B, C and D are the four Zn sites in the X‐ray structure. Given the similar density of states (DOS) of the A12Sb10, A11BCSb10and A10BCDSb10cells, one may electronically model the defective stoichiometry of the real system either by n‐doping the 12‐Zn atom cell or by p‐doping the two 13‐Zn atom cells. This leads to similar calculated Seebeck coefficients for the A12Sb10, A11BCSb10and A10BCDSb10cells (115.0, 123.0 and 110.3 μV K−1atT=670 K). The model system is therefore a p‐doped semiconductor as found experimentally. The effect is dramatic if these cells are doped differently with respect to the experimental electron count. Thus, 0.33 extra electrons supplied to either kind of cell would increase the Seebeck coefficient to about 260 μV K−1. Additional electrons would also lowerσ, so the resulting effect on the thermoelectric figure of merit of Zn4Sb3challenges further experimental work.

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Tài liệu tham khảo

10.1016/S0022-3697(96)00228-4

10.1016/S0364-5916(02)00008-1

10.1016/S0925-8388(03)00074-4

For papers on thermoelectrics see for example:

10.1126/science.272.5266.1325

10.1126/science.285.5428.703

10.1002/1521-3757(20001016)112:20<3759::AID-ANGE3759>3.0.CO;2-C

10.1002/1521-3773(20001016)39:20<3613::AID-ANIE3613>3.0.CO;2-D

10.1038/35098012

G. J. Snyder M. Christensen E. Nishibori T. Caillat B. B. Iversen Nature Mater.2004 in press.

10.1016/0022-5088(78)90109-1

10.1103/PhysRevB.57.6199

10.1524/zkri.216.2.71.20335

 

10.1038/377046a0

10.1002/1521-3757(20010119)113:2<411::AID-ANGE411>3.0.CO;2-6

10.1002/1521-3773(20010119)40:2<397::AID-ANIE397>3.0.CO;2-3

10.1126/science.1078481

10.1107/S0021889802018691

Bader R. F. W., 1990, Atoms in Molecules: A Quantum Theory, 10.1093/oso/9780198551683.001.0001

10.1107/S0108768194010360

Merli M., 2003, Eur. J. Mineral., 15

Nishibori E., 2001, Nucl. Instrum. Methods Phys. Res. Sect. A, 467

10.1107/S002188980200050X

Cotton F. A., 1999, Advanced Inorganic Chemistry

 

Iversen B. B., 1995, Philos. Mag. A, 72

10.1107/S0108767397000792

V. R. Saunders R. Dovesi C. Roetti M. Causà N. M. Harrison R. Orlando C. M. Zicovich‐Wilson CRYSTAL98 User's Manual University of Torino Torino (Italy) 1998.

 

10.1063/1.464913

10.1103/PhysRevB.40.3399

 

10.1063/1.448800

10.1063/1.448975

C. Gatti TOPOND98 User's Manual CNR‐ISTM Milano (Italy) 1999.

L. Bertini C. Gatti unpublished results 2003.

10.1017/CBO9781139644075

10.1063/1.1397324

10.21236/ADA196638

Crystal structures with ZnZn internuclear distances shorter than 2.0 Å were all discarded in our analysis. This distance threshold when applied to the average Zn–Zn distance in bulk Zn corresponds to a repulsive energy whose magnitude is 4.8 times the binding energy of the crystal. The equilibrium distance in the bulk for the two different sets of six closest neighbours equals 2.886 and 3.120 Å respectively.

The reaction which involves mixing of Zn bulk metal with the optimal 12‐Zn cell structure to yield the best 13‐Zn cell structure Zn (bulk)+A12Sb10→A11BCSb10 has ΔE=−8 kcal mol−1. This shows that the 13‐Zn cell structure is only slightly unstable with regard to disproportionation into metallic Zn and a 12‐Zn structure. Furthermore A12Sb10is slightly unstable with regard to the formation of ZnSb and Zn metal. These reactions when combined corroborate the dominance of 13‐Zn atom cells over Zn‐12 and Zn‐14 cells in the amorphous Zn12.816Sb10phase and they also explain the occurrence of ZnSb and Zn metal islands in experimental structure determinations.

The (BD)12Sb10structure was not considered since it implies that the ZnZn distances are too short (1.87 Å).

10.1016/0165-1633(85)90051-6

There are 12 nonequivalent arrangements of A10BCDSb10that are favoured over the coexistence of IDand VAin A11DSb10when these two point defects are inserted in a pair of A12Sb10and A11BCSb10cells (final products in the last row against those in the first row of Table 5).

For instance ifp1 is set to 0.05 and equal top2 the Seebeck coefficients of the majority 13‐Zn atom cells would rise to about 240 μV K−1 while that of the minority 12‐Zn atom cells would decrease to 43 μV K−1.

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Chemistry - A European Journal

Tập 10 Số 16

3861-3870

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