Glass-like thermal conductivities in (La1-x1Yx1)2(Zr1-x2Yx2)2O7-x2(La1-x1Yx1)2(Zr1-x2Yx2)2O7-x2 (x = x1 + x2, 0 ⩽ x ⩽ 1.0) solid solutions

The evolution of structure and thermal conductivity (k) has been studied for a range of Y–La2Zr2O7 solid solutions. Within the pyrochlore range (x < 0.40) Y3+ solely substitutes for La3+ below a critical composition factor (x = 0.15), above which it substitutes for both La3+ and the Zr4+. A glass-like k, approaching the amorphous limit, is observed within a certain composition range (0.20 ⩽ x < 0.40). The glass-like k behaviour is attributed to a phonon localization effect that arises from small and weakly bound Y3+ cations (rattlers) oscillating locally and independently in oversized anionic cages [(La/Y)O8]. The ultralow and glassy k makes Y3+-doped La2Zr2O7 pyrochlores promising candidate materials for high temperature thermal barrier coating topcoats.