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

Abstract

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.

Introduction

Over the past dozen years, rare-earth zirconates have been widely investigated as promising candidates for thermal barrier coating (TBC) applications [1], [2], [3], [4], [5], [6], [7], [8]. Compared with the conventional TBC material, Y₂O₃-stabilized ZrO₂, they are seen to possess better high-temperature stability, higher thermal expansion coefficient and significantly lower thermal conductivity. The extremely low thermal conductivity of these compounds is usually considered to be chiefly attributed to their unique structure in which a high concentration of crystallographically intrinsic oxygen vacancies exists. These vacancies are regarded as strong scattering centers of phonons by virtue of both missing atoms and missing interatomic linkages [9], which significantly reduce the phonon mean free path and, consequently, the thermal conductivity.

Rare-earth stannates are a series of compounds with similar structures to the rare-earth zirconates. Interestingly, these compounds, Ln₂Sn₂O₇ (Ln = La–Lu, Y), constitute a complete series of isostructural compounds with a pyrochlore structure, whereas the rare-earth zirconates undergo a pyrochlore–fluorite transition, i.e. pyrochlores can only form for the lanthanide series from La to Gd, with the remainder existing as defective fluorites. This implies that the stannates possess a wider composition range with a stable pyrochlore structure than the zirconates [10]. As representative compositions of oxide pyrochlores, rare-earth stannates have attracted intense attention, and their structural details have been thoroughly investigated [11], [12], [13]. The vibrational spectra revealed that the structure of rare-earth stannates is very close to the ideal pyrochlore structure and hence is especially stable [12]. Moreover, Kennedy et al. [13] performed a systematic study of the bond trends in the rare-earth stannates, and the bond valence sum calculations indicated that covalent bonding interactions are important for the stannates, as suggested from the investigation of the force fields of the vibrational spectra by Vandenborre et al. [12].

The isostructuralism of rare-earth stannates offers an opportunity to systemically investigate the thermal behavior of materials with a pyrochlore structure. In addition, the distinctiveness of chemical bonds may lead to some exceptional variation in the thermophysical properties. However, until now, few investigations have been conducted concerning the thermophysical properties of this series of compounds. Schelling et al. [14] carried out a theoretical calculation by molecular dynamics simulation to determine the thermal conductivities of a wide range of compounds with the composition A₂B₂O₇ (A = La, Pr, Nd, Sm, Eu, Gd, Y, Er, Lu; B = Ti, Ru, Mo, Sn, Zr, Pb) and came to the conclusion that the thermal conductivity of these compounds does not depend greatly upon the size of the A³⁺ ion, but tends to decrease with increasing size of the B⁴⁺ ion. In particular, Schelling et al. declared that some of the stannates may have a lower thermal conductivity than the molybdates and the zirconates, and an apparent composition region in the vicinity of Sm₂Sn₂O₇ was observed to achieve a local minimum thermal conductivity in the calculated contour map. This observation is of particular interest for identifying materials with low thermal conductivity. Unfortunately, there has been no further experimental evidence to confirm the calculations.

In the present work, we investigated the thermophysical properties of a series of rare-earth stannates Ln₂Sn₂O₇ (Ln = La, Nd, Sm, Gd, Er, Yb) to determine the effect of the pyrochlore structure on the thermal conduction process. The variations in the elastic modulus and the thermal expansion coefficient as well as X-ray diffraction (XRD) and the Raman spectrum were analyzed to validate the systemic change of crystal structure. The temperature and composition dependence of thermal conductivity were modeled 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. This work aims to provide a general insight into the thermal conduction process of materials with an A₂B₂O₇-type pyrochlore structure.