Thermoelectric and structural correlations in (Sr1−x−yCaxNdy)TiO3 perovskites

Abstract

Structural and thermoelectric properties are reported for a specially designed class of $A$-site substituted perovskite titanates, $(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x\ensuremath{-}y}\mathrm{C}{\mathrm{a}}_{x}\mathrm{N}{\mathrm{d}}_{y})\mathrm{Ti}{\mathrm{O}}_{3}$. Two series synthesized with various $A$-site Sr-rich or Ca-rich (Sr-poor) concentrations were investigated using high-resolution neutron powder diffraction as a function of temperature and Nd doping. Each series was designed to have a nominally constant tolerance factor at room temperature. We determine the room temperature structures as tetragonal $I4/mcm$ and orthorhombic $Pbnm$ for the Sr-rich and Ca-rich series, respectively. Three low-temperature orthorhombic structures, $Pbnm, Ibmm$, and $Pbcm$ were also observed for the Sr-rich series, whereas the symmetry of the Ca-rich series remains unchanged throughout the full measured temperature range. Thermoelectric properties of $(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x\ensuremath{-}y}\mathrm{C}{\mathrm{a}}_{x}\mathrm{N}{\mathrm{d}}_{y})\mathrm{Ti}{\mathrm{O}}_{3}$ were investigated and correlated with the structural variables. We succeeded in achieving a relatively high figure of merit $ZT=0.07$ at $\ensuremath{\sim}400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the Sr-rich $\mathrm{S}{\mathrm{r}}_{0.76}\mathrm{C}{\mathrm{a}}_{0.16}\mathrm{N}{\mathrm{d}}_{0.08}\mathrm{Ti}{\mathrm{O}}_{3}$ composition which is comparable to that of the best $n$-type TE $\mathrm{SrT}{\mathrm{i}}_{0.80}\mathrm{N}{\mathrm{b}}_{0.20}{\mathrm{O}}_{3}$ oxide material reported to date. For a fixed tolerance factor, the Nd doping enhances the carrier density and effective mass at the expense of the Seebeck coefficient. Thermal conductivity greatly reduces upon Nd doping in the Ca-rich series. With an enhanced Seebeck coefficient at elevated temperatures and reduced thermal conductivity, we predict that $\mathrm{S}{\mathrm{r}}_{0.76}\mathrm{C}{\mathrm{a}}_{0.16}\mathrm{N}{\mathrm{d}}_{0.08}\mathrm{Ti}{\mathrm{O}}_{3}$ and similar compositions have the potential to become some of the best materials in their class of thermoelectric oxides.