Thermal conductivity of rare-earth titanate pyrochlores

Abstract

Pyrochlore oxides with the chemical formula ${R}_{2}{B}_{2}{\mathrm{O}}_{7}$ ($R=$ rare earth and $B=$ tetravalent metal) exhibit very low thermal conductivities (\ensuremath{\kappa}) and are considered as promising materials for thermal barrier coatings. To achieve better understanding of their intrinsic thermal properties, this study reports \ensuremath{\kappa} for single crystalline ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Dy}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, and for a series of polycrystalline ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, in the temperature range from 300 to 773 K. For the single crystals, \ensuremath{\kappa} values are evaluated in comparison to spinel ${\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}$ and garnet ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ using the Slack equation. It is more difficult to obtain satisfactory analysis for the pyrochlores, and this is attributed to the strong involvement of low-lying optic phonons and very low energies of zone-boundary acoustic phonons. For polycrystalline ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, \ensuremath{\kappa} values similar to single crystals are obtained. Comparison with published data shows a systematic trend of $\ensuremath{\kappa}({R}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7})>\ensuremath{\kappa}({R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7})>\ensuremath{\kappa}({R}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7})$ at 300 K, which reflects the increasing ionicity from ${\mathrm{Sn}}^{4+}$ to ${\mathrm{Ti}}^{4+}$ to ${\mathrm{Zr}}^{4+}$. For the three ${R}_{2}{B}_{2}{\mathrm{O}}_{7}$ systems, \ensuremath{\kappa} appears to decrease with either decreasing ionic radius or increasing cation mass of $R$.