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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.

No MeSH data available.


The integral breadth at 150 K versus that at 10 K for reflections in the range 12.9 < 2θ < 46.8° [λ = 0.50036 (7) Å] with no peak overlap, and that are supposed to split in a transition .
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fig11: The integral breadth at 150 K versus that at 10 K for reflections in the range 12.9 < 2θ < 46.8° [λ = 0.50036 (7) Å] with no peak overlap, and that are supposed to split in a transition .

Mentions: Finally, as shown by the MEM and NXMEM maps, the electron density on the Sn atom sharpens the lower the temperature and no features appear along the 〈111〉 direction. Again, this tends to support the hypothesis that the structure is cubic at 20 K. Pseudomerohedral twinning of the rhombo­hedral lattice on a cubic lattice leads to perfect Laue symmetry . Therefore, to check further for the presence of a phase transition we have measured powder X-ray diffraction on the same powder specimen of sample B from 200 K to 10 K and back to 200 K, employing a closed-cycle cryostat. No peak splitting occurs down to 10 K (see supporting information) and the integral breadths at 150 and 10 K appear to be almost unchanged (Fig. 11 ▸). The integral breadths β obtained by fitting single peaks at different temperatures, not corrected for instrumental broadening, show a slight increase below 100 K (see supporting information). However, we notice that such broadening appears mainly at low angles and it occurs not only for those reflections that split if the cubic cell has distorted, but also for the (00l) reflections which should remain unaltered if the transition has occurred, e.g. (002) and (004). This means that the broadening is more generally ascribable to strain, which could be caused by changes in the defect distribution or by a thermal gradient between the side of the capillary in contact with the copper sample holder and the one that is not.


Carrier concentration dependence of structural disorder in thermoelectric Sn 1 − x Te
The integral breadth at 150 K versus that at 10 K for reflections in the range 12.9 < 2θ < 46.8° [λ = 0.50036 (7) Å] with no peak overlap, and that are supposed to split in a transition .
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig11: The integral breadth at 150 K versus that at 10 K for reflections in the range 12.9 < 2θ < 46.8° [λ = 0.50036 (7) Å] with no peak overlap, and that are supposed to split in a transition .
Mentions: Finally, as shown by the MEM and NXMEM maps, the electron density on the Sn atom sharpens the lower the temperature and no features appear along the 〈111〉 direction. Again, this tends to support the hypothesis that the structure is cubic at 20 K. Pseudomerohedral twinning of the rhombo­hedral lattice on a cubic lattice leads to perfect Laue symmetry . Therefore, to check further for the presence of a phase transition we have measured powder X-ray diffraction on the same powder specimen of sample B from 200 K to 10 K and back to 200 K, employing a closed-cycle cryostat. No peak splitting occurs down to 10 K (see supporting information) and the integral breadths at 150 and 10 K appear to be almost unchanged (Fig. 11 ▸). The integral breadths β obtained by fitting single peaks at different temperatures, not corrected for instrumental broadening, show a slight increase below 100 K (see supporting information). However, we notice that such broadening appears mainly at low angles and it occurs not only for those reflections that split if the cubic cell has distorted, but also for the (00l) reflections which should remain unaltered if the transition has occurred, e.g. (002) and (004). This means that the broadening is more generally ascribable to strain, which could be caused by changes in the defect distribution or by a thermal gradient between the side of the capillary in contact with the copper sample holder and the one that is not.

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&#8197;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&ndash;2%) and ordering of Sn vacancies possibly occurs on warming, as corroborated by the appearance of multiple phases and strain above 400&#8197;K. The possible presence of disorder and anharmonicity is investigated in view of the low thermal conductivity of SnTe. Refinement of anharmonic Gram&ndash;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 &lang;100&rang; 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.