Electron Scattering in Perovskite-Oxide Ferroelectric Semiconductors

Abstract

This paper reports new data on the hydrostatic-pressure dependence of the conductivity and static dielectric constant, as well as new Hall-effect data taken over a wide temperature range, in the perovskite-oxide ferroelectrics KTa${\mathrm{O}}_{3}$, SrTi${\mathrm{O}}_{3}$, BaTi${\mathrm{O}}_{3}$, and $\mathrm{K}{\mathrm{Ta}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ ($x=0.002, 0.1, \mathrm{and} 0.35$). From the linear dependence of the electron mobility $\ensuremath{\mu}$ and the reciprocal dielectric constant $\frac{1}{\ensuremath{\epsilon}}$ on pressure in all materials studied, we conclude that $\ensuremath{\mu}\ensuremath{\propto}\frac{1}{\ensuremath{\epsilon}}+\mathrm{const}$. Based on the observed temperature dependence of the Hall mobility and the Curie-Weiss behavior of $\ensuremath{\epsilon}$, we conclude further that in the perovskite oxides the temperature dependence of the mobility is given by the empirical expression $\ensuremath{\mu}\ensuremath{\propto}{T}^{\ensuremath{-}3.5}(T\ensuremath{-}{T}_{0}+{T}^{*})$, $T>{T}_{0}$, where ${T}^{*}$ is an experimental constant and ${T}_{0}$ is the Curie-Weiss temperature (${T}_{0}=4\ifmmode^\circ\else\textdegree\fi{}$K and ${T}^{*}\ensuremath{\approx}140\ifmmode^\circ\else\textdegree\fi{}$K for KTa${\mathrm{O}}_{3}$, ${T}_{0}=32\ifmmode^\circ\else\textdegree\fi{}$K and ${T}^{*}\ensuremath{\approx}77\ifmmode^\circ\else\textdegree\fi{}$K for SrTi${\mathrm{O}}_{3}$, ${T}_{0}=287\ifmmode^\circ\else\textdegree\fi{}$K and ${T}^{*}\ensuremath{\approx}70\ifmmode^\circ\else\textdegree\fi{}$K for K${\mathrm{Ta}}_{0.65}$${\mathrm{Nb}}_{0.35}$${\mathrm{O}}_{3}$, and ${T}_{0}=395\ifmmode^\circ\else\textdegree\fi{}$K and ${T}^{*}\ensuremath{\approx}85\ifmmode^\circ\else\textdegree\fi{}$K for BaTi${\mathrm{O}}_{3}$). Our experimental results are shown to be inconsistent with previously reported suggestions that the electron mobility in the perovskite oxides is dominated by LO-mode scattering. We suggest, instead, that the electron mobility is determined by a new and strong electron-phonon interaction involving critical-point lattice-polarization fluctuations associated with the soft TO mode.