Phase transitions in RbCaF3

Abstract

Single crystals of RbCa${\mathrm{F}}_3$ have been successfully prepared by a Bridgeman method. The crystals are found to have the cubic perovskite structure at ambient temperature, but the structure is of lower symmetry at temperatures below 198 K. At lower temperatures the polarizing microscope reveals optical birefringence and well-defined domain structure. Detailed measurements of the birefringence and the paramagnetic resonance of impurity ${\mathrm{Fe}}^{3+}$ show a transformation to a tetragonal structure at 198 °K. This phase transition is at least near second order and appears similar to the antiferroelectric transitions known in SrTi${\mathrm{O}}_3$ and KMn${\mathrm{F}}_3$ (the latter has a small first-order contribution) except very near $T_c$ where it differs from both. The measurements imply a temperature variation of the transition order parameter that is described by a critical index $\beta$ which is close to 0.25 very near $T_c$, roughly 0.33 at somewhat lower temperature, and 0.50 or Landau-like at still lower temperatures. Preliminary measurements of the dielectric function and specific-heat anomalies have also been made. Only small anomalies are found at 198 K. However, these measurements along with the paramagnetic-resonance measurements suggest that other phase transitions occur at about 43 and 10 K.