Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y2Ti2O7, have been determined over the temperature range 2K⩽T⩽300K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ D, of ca. 970K) and an assignment of the remaining 63 optic modes, as well as a correction for Cp −Cv . From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15K)−H 0=34.69kJmol−1 and S(T=298.15K)−S 0=211.2JK−1 mol−1. The thermal conductivity shows a peak at ca. θ D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y2Ti2O7 is 2.8Wm−1 K−1, close to the calculated theoretical thermal conductivity, κ min, for fully coupled phonons at high temperatures.