Low-temperature heat capacities of Y2GaSbO7 and R2FeSbO7 (R = Y, Dy) pyrochlore compounds: Quasi-harmonic phonons calculation

Heat capacities at constant pressure, C°p,m, of three polycrystalline pyrochlore compounds, Y2GaSbO7, Y2FeSbO7 and Dy2FeSbO7, have been measured in the temperature range up to 300 K. Temperature variation of the Debye temperatures, θD(T), for Y2GaSbO7 is determined using a quasi-harmonic expression of Debye lattice heat capacity, which has been employed to estimate the phonon heat capacity, Cph, for the other two compounds, e.g., Y2FeSbO7 and Dy2FeSbO7. Theoretical analysis using phonon model shows that the acoustic and optic phonons contribute significantly to the thermodynamic properties. The effect of anharmonicity in the lattice vibrations at higher temperature (above 220 K) was also considered for Y2GaSbO7. The Cph of Y2FeSbO7, calculated using the experimental infra-red and Raman phonon spectra, are in good agreement with the measured heat capacity data and thus provides a good taste of understanding to the molecular dynamics in the pyrochlore structure. The Schottky heat capacity has been deduced from the crystal-field level scheme of Dy3+ ions and added to the Cph of Dy2FeSbO7 to estimate the total heat capacity, which is in reasonable agreement with the experimental heat capacity, C°p,m, of Dy2FeSbO7. From the integrated heat capacity data, the entropy and enthalpy relative to absolute zero have been generated for Y2GaSbO7 and Y2FeSbO7. The room-temperature thermal conductivity was estimated to be ∼2 W⋅m−1⋅K−1 for Y2GaSbO7 implying that Y2GaSbO7 could be a potential candidate for thermal barrier coating material.