  Rare-earth pyrochlores, commonly exhibiting anomalously low lattice thermal
conductivities, are considered as promising topcoat materials for thermal
barrier coatings. However the structural origin underlying their low thermal
conductivities remain unclear. In the present study, we investigated the phonon
properties of two groups of RE pyrochlores, Ln2Zr2O7 (Ln = La, Nd, Sm, Gd) and
Gd2T2O7 (T = Zr, Hf, Sn, Pb) employing density functional theory and quasi
harmonic approximation. Through the relaxation time approximation (RTA) with
Debye model, the thermal conductivities of those RE pyrochlores were predicted,
showing good agreement with experimental measurements. The low thermal
conductivities of RE pyrochlores were shown to largely come from the
interference between the low-lying optical branches and acoustic branches. The
structural origin underlying the low-lying optical branches was then clarified
and the competition between scattering processes in transverse and longitude
acoustic branches was discussed.
