Structural phase transitions in KCaF₃

Abstract

A focusing X-ray diffractometer, consisted of a Johansson type monochromator and a position-sensitive proportional counter, has been developed. By using the diffractometer, a study of the structural phase transition in the perovskite compound KCaF₃ has been carried out. The mechanism for the transition is discussed in terms of the condensation of soft modes. It is found that KCaF₃ shows two structural-phase-transitions at 556 and 565 K, which are characterized as first-order transitions; at 565 K the M^z ₃ and R^x ₂₅ soft modes condense simultaneously, while at 556 K the R^y ₂₅ soft mode condenses.

Key Words:

• Structural phase transitions
• KCaF₃