Sr1−xCaxTiO3: AnXYQuantum Ferroelectric with Transition to Randomness

Abstract

A series of twenty homogeneous ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$ mixed crystals has been measured dielectrically between 4.2 and 300 K. In the tetragonal phase, the dielectric constant perpendicular to the $c$ axis becomes peaked above ${x}_{c}=0.0018$, the quantum mechanical onset for displacive ferroelectricity. The polarization $\ensuremath{\perp}c$ can be switched between the two equivalent $a$ axes, i.e., the system is an $\mathrm{XY}$, $n=2$, quantum ferroelectric. Above ${x}_{r}=0.016\ifmmode\pm\else\textpm\fi{}0.002$, the $\ensuremath{\epsilon}(T)$ peaks round in a distinct manner which we attribute to the onset of a random-field-induced domain state.