Evidence for first-order orthorhombic-tetragonal and nearly tricritical tetragonal-cubic antiferrodistortive phase transitions in Ca_(0.43)Sr_(0.57)TiO_3

摘要

Temperature dependence of the symmetry-adopted strains (secondary order parameter) associated with the antiferrodistortive-phase transitions involving change in space groups from Pm3m to I4/mcm and I4/mcm to Pnma in Ca_(0.57)Sr_(0.43)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 β~0.33 ±0.03 in the temperature range 0.70 ≤ T ≤ 0.98T_c for the I4/mcm to Pm3m transition is due to a nearly tricritical transition. The Pnma to I4/mcm transition is unambiguously identified as first order.