Short-Range Order and Origin of the Low Thermal Conductivity in Compositionally Complex Rare-Earth Niobates and Tantalates

TL;DR

This study investigates the origins of low thermal conductivity in rare-earth niobates and tantalates, revealing that short-range weberite order and compositional complexity significantly influence thermal transport properties.

Contribution

It uncovers the relationship between short-range weberite order and low thermal conductivity in complex rare-earth compounds, supported by extensive experimental data and diffraction analysis.

Findings

Doping with light elements further reduces thermal conductivity.

Negative correlation between thermal conductivity and cation radius ratio.

Short-range weberite order correlates with amorphous-like thermal behavior.

Abstract

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Amongst, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. One new finding is that doping 2 % of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite order, which is indeed revealed by diffuse scattering in X-ray and neutron diffraction. Specifically, neutron diffraction is used characterize five selected specimens. A better fit to a weberite structure is found at nanoscale (~1 nm). The characteristic length (domain size) is smaller but with stronger short-range ordering in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and "diffusons" give rise to the observed amorphous-like thermal conductivity. Disordered oxygen sublattices are also confirmed by neutron diffraction.