Evidence for first-order orthorhombic-tetragonal and nearly tricritical tetragonal-cubic antiferrodistortive phase transitions inCa0.43Sr0.57TiO3

Abstract

Temperature dependence of the symmetry-adopted strains (secondary order parameter) associated with the antiferrodistortive-phase transitions involving change in space groups from $Pm\overline{3}m$ to $I4/mcm$ and $I4/mcm$ to $Pnma$ in ${\text{Ca}}_{0.57}{\text{Sr}}_{0.43}{\text{TiO}}_{3}$ have been investigated using powder x-ray diffraction data in the temperature range of 300--950 K. It is argued that the experimentally determined critical exponent $\ensuremath{\beta}\ensuremath{\sim}0.33\ifmmode\pm\else\textpm\fi{}0.03$ in the temperature range $0.70\ensuremath{\le}T\ensuremath{\le}0.98{T}_{c}$ for the $I4/mcm$ to $Pm\overline{3}m$ transition is due to a nearly tricritical transition. The $Pnma$ to $I4/mcm$ transition is unambiguously identified as first order.