Thermophysical properties of rare-earth stannates: Effect of pyrochlore structure

Compounds with an A2B2O7-type pyrochlore structure have been identified as a class of materials with low thermal conductivity. To ascertain the effect of the pyrochlore structure, the thermophysical properties of rare-earth stannates which constitute the longest isostructural series with this unique crystal structure were measured and the thermal conduction behavior was investigated by analyzing the variation in the inverse phonon mean free path with temperature in terms of the phonon-scattering theory. The results show that the contraction of rare-earth ions and the accompanying deviation from the ideal pyrochlore structure result in a simultaneous increase in the Young's modulus and the thermal expansion coefficient. Scattering due to the strain field fluctuation caused by the systemic displacement of 48f oxygen ions rather than scattering from the so-called 8a site “oxygen vacancies” and thermal defects predominates in reducing the phonon mean free path and consequently the thermal conductivity at relative low temperature. At a certain critical temperature, the thermal conduction behavior of the stannates undergoes a transition to a minimum phonon mean free path mechanism, which is responsible for the remarkable variation in temperature dependence of thermal conductivity across the compositions.