Short-range order and origin of the low thermal conductivity in compositionally complex rare-earth niobates and tantalates

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 conductivity. Of these, 29 single (nominally cubic) fluorite phases most them new compositionally complex (medium- or high-entropy) Furthermore, doping 2% light element cations reduce This data set enables discovery negative correlation between conductivity averaged radius ratio 3+/5+ cations. While this is still below threshold for forming long-range ordered weberite-type phases, suggests related short-range order, indeed revealed by diffuse scattering X-ray diffraction neutron total scattering. Specifically, used characterize five selected specimens. A better fit structure found at nanoscale. The characteristic length (domain size) appears larger more insulative materials. As it approaches Ioffe-Regel limit, phonon limit breaks down “diffusons” give rise observed amorphous-like