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Low‐thermal conductivity ceramics in monazite‐type REPO4 (RE=La, Ce, Nd, Sm, Eu, Gd) ceramics are expected to have potential in structural (refractories, thermal insulator) and nuclear applications. To this end, the present study determines their thermal conductivities and examines how differences of the rare earth ions change their thermal conductivity at different temperatures. The results show that their conductivities are remarkably low from 25° to 1000°C. In addition, different conductivity variation mechanisms exist that change gradually upon altering from LaPO4 to GdPO4 at low and high temperatures. At relatively lower temperatures (≤400°C), the thermal conductivities of all the REPO4 ceramics decrease nearly at first, reach a minimum value, and then rise with gradual altering from LaPO4 to GdPO4. It may be due to the combined effects of the increase of both the anharmonicities in lattice vibrations and the bond strength. As the temperature increases, the conductivity trends become obscure, and the conductivities of the monazite‐type REPO4 approach their minimum thermal conductivities when the temperature is above 800°C.

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Low‐thermal‐conductivity rare earth zirconates (Re2Zr2O7) have recently been identified as promising thermal barrier coating materials. We observed an order–disorder transition in the (Sm1−xYbx)2Zr2O7 series with the changing x value and investigated the thermal conductivity variation. Structural analysis by X‐ray diffraction and Raman spectroscopy shows that the (Sm1−xYbx)2Zr2O7 series undergo a discontinuous phase transition from an ordered pyrochlore phase to a disordered fluorite one between the x=1/6 and x=1/3 compositions. Meanwhile, both of the sound velocity and Young's modulus reveal a dramatic reduction, indicating the lattice softening accompanying the order–disorder transition. The thermal conductivities of the (Sm1−xYbx)2Zr2O7 series are different from the conventional behavior of a simple alloying system and show a minimum thermal conductivity value at the transition composition (Sm2/3Yb1/3)2Zr2O7, which possibly arises from the enhanced phonon scattering due to the lattice softening.

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