Compositional and temperature evolution of crystal structure of new thermoelectric compound LaOBiS2-xSex

TL;DR

This study investigates the crystal structure and thermal properties of LaOBiS2-xSex, revealing how Se substitution enhances thermoelectric performance and induces atomic vibrations that reduce thermal conductivity, suggesting new strategies for thermoelectric materials.

Contribution

The paper provides detailed structural analysis of LaOBiS2-xSex, linking Se substitution to enhanced thermoelectric properties and low thermal conductivity through atomic displacement behavior.

Findings

Se substitution increases in-plane chemical pressure.

Bi atoms exhibit large c-axis atomic displacement at high temperatures.

In-plane Bi rattling may lead to low thermal conductivity.

Abstract

We examined the crystal structure of the new thermoelectric material LaOBiS2-xSex, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS2-xSex can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters (Uiso) for Bi and Ch atoms in the BiCh2 conduction layers. The anisotropic analysis of the atomic displacement parameters (U11 and U33) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c-axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c-axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states.