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Carrier concentration dependence of structural disorder in thermoelectric Sn 1 − x Te

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ABSTRACT

SnTe is a promising thermoelectric and topological insulator material. Here, the presumably simple rock salt crystal structure of SnTe is studied comprehensively by means of high-resolution synchrotron single-crystal and powder X-ray diffraction from 20 to 800 K. Two samples with different carrier concentrations (sample A = high, sample B = low) have remarkably different atomic displacement parameters, especially at low temperatures. Both samples contain significant numbers of cation vacancies (1–2%) and ordering of Sn vacancies possibly occurs on warming, as corroborated by the appearance of multiple phases and strain above 400 K. The possible presence of disorder and anharmonicity is investigated in view of the low thermal conductivity of SnTe. Refinement of anharmonic Gram–Charlier parameters reveals marginal anharmonicity for sample A, whereas sample B exhibits anharmonic effects even at low temperature. For both samples, no indications are found of a low-temperature rhombohedral phase. Maximum entropy method (MEM) calculations are carried out, including nuclear-weighted X-ray MEM calculations (NXMEM). The atomic electron densities are spherical for sample A, whereas for sample B the Te electron density is elongated along the ⟨100⟩ direction, with the maximum being displaced from the lattice position at higher temperatures. Overall, the crystal structure of SnTe is found to be defective and sample-dependent, and therefore theoretical calculations of perfect rock salt structures are not expected to predict the properties of real materials.

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NXMEM electron-density maps for Sn (left) and Te (right) in the (001) plane from 20 to 300 K for sample B. Contour lines have been set at 64, 128, 256, 512, 1024, 2048, 4096 and 8192 e Å−3. An additional contour line has been added as guide to locate the maximum corresponding to the nuclear position.
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fig8: NXMEM electron-density maps for Sn (left) and Te (right) in the (001) plane from 20 to 300 K for sample B. Contour lines have been set at 64, 128, 256, 512, 1024, 2048, 4096 and 8192 e Å−3. An additional contour line has been added as guide to locate the maximum corresponding to the nuclear position.

Mentions: Significant correlations are present when GC coefficients are refined for both Sn and Te. Conversely, the MEM offers a non-parametrized description of the electron or nuclear density (Sakata & Sato, 1990 ▸; Collins, 1982 ▸), and MEM density maps are provided in the supporting information. Recently, the NXMEM procedure has been shown to enhance the nuclear resolution substantially, and thus to enhance the ability to quantify subtle disorder features (Christensen et al., 2015 ▸). Therefore, our study focuses on the NXMEM results (Figs. 7 ▸ and 8 ▸). In both samples A and B the electron density on the Sn site is a maximum at 20 K and decreases with increasing temperature. Compared with Te, the electron density of the Sn atom is lower and more diffuse. This is in close agreement with the higher ADPs refined for Sn in the least-squares modelling.


Carrier concentration dependence of structural disorder in thermoelectric Sn 1 − x Te
NXMEM electron-density maps for Sn (left) and Te (right) in the (001) plane from 20 to 300 K for sample B. Contour lines have been set at 64, 128, 256, 512, 1024, 2048, 4096 and 8192 e Å−3. An additional contour line has been added as guide to locate the maximum corresponding to the nuclear position.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5391859&req=5

fig8: NXMEM electron-density maps for Sn (left) and Te (right) in the (001) plane from 20 to 300 K for sample B. Contour lines have been set at 64, 128, 256, 512, 1024, 2048, 4096 and 8192 e Å−3. An additional contour line has been added as guide to locate the maximum corresponding to the nuclear position.
Mentions: Significant correlations are present when GC coefficients are refined for both Sn and Te. Conversely, the MEM offers a non-parametrized description of the electron or nuclear density (Sakata & Sato, 1990 ▸; Collins, 1982 ▸), and MEM density maps are provided in the supporting information. Recently, the NXMEM procedure has been shown to enhance the nuclear resolution substantially, and thus to enhance the ability to quantify subtle disorder features (Christensen et al., 2015 ▸). Therefore, our study focuses on the NXMEM results (Figs. 7 ▸ and 8 ▸). In both samples A and B the electron density on the Sn site is a maximum at 20 K and decreases with increasing temperature. Compared with Te, the electron density of the Sn atom is lower and more diffuse. This is in close agreement with the higher ADPs refined for Sn in the least-squares modelling.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

SnTe is a promising thermoelectric and topological insulator material. Here, the presumably simple rock salt crystal structure of SnTe is studied comprehensively by means of high-resolution synchrotron single-crystal and powder X-ray diffraction from 20 to 800 K. Two samples with different carrier concentrations (sample A = high, sample B = low) have remarkably different atomic displacement parameters, especially at low temperatures. Both samples contain significant numbers of cation vacancies (1–2%) and ordering of Sn vacancies possibly occurs on warming, as corroborated by the appearance of multiple phases and strain above 400 K. The possible presence of disorder and anharmonicity is investigated in view of the low thermal conductivity of SnTe. Refinement of anharmonic Gram–Charlier parameters reveals marginal anharmonicity for sample A, whereas sample B exhibits anharmonic effects even at low temperature. For both samples, no indications are found of a low-temperature rhombohedral phase. Maximum entropy method (MEM) calculations are carried out, including nuclear-weighted X-ray MEM calculations (NXMEM). The atomic electron densities are spherical for sample A, whereas for sample B the Te electron density is elongated along the ⟨100⟩ direction, with the maximum being displaced from the lattice position at higher temperatures. Overall, the crystal structure of SnTe is found to be defective and sample-dependent, and therefore theoretical calculations of perfect rock salt structures are not expected to predict the properties of real materials.

No MeSH data available.


Related in: MedlinePlus