Evidence of Structural Phase Transition in K₂CdF₄ Derived from EPR Measurements for Cr³⁺ Centres

Abstract

An EPR investigation of Cr³⁺ in K₂CdF₄ has been made in a temperature range 295–397 K. The temperature dependence of the spectrum shows that a structural phase transition occurs near 376 K. The field-direction dependence of the uncompensated centre at 301 K indicates that there are four equivalent Cr³⁺ sites whose principal z axes in the spin Hamiltonian deviate from the [001] axis by an angle close to 9° in the (110) and (1\bar10) planes. The structure in the low temperature phase is interpreted by the condensation of soft phonon modes at X point of the tetragonal Brillouin zone associated with rotation of CdF₆ octahedra arccund [110] axes.