Phase diagram of KMn1−xCaxF3 (x<0.05) determined by high-resolution x-ray scattering

Abstract

Precise high-resolution x-ray-diffraction measurements of the lattice parameters as a function of temperature are reported for the mixed system ${\mathrm{KMn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{F}}_3$ in the concentration range x<0.05. From the splitting of the cubic (0,0,4) Bragg reflection it is possible to determine the transition temperatures of the structural phase transitions which occur in this system and to establish the phase diagram in the concentration range x<0.05. Three structural phase transitions are observed in this system and the transition temperatures at which they occur are enhanced when ${\mathrm{Mn}}^{2+}$ ions are substituted by ${\mathrm{Ca}}^{2+}$ ions. The enhancement rates are, respectively, 5.8, 18, and 14 K/at. % ${\mathrm{Ca}}^{2+}$ ions. This shows that a crossover between the ${\mathit{T}}_{\mathit{c}1}$ and the ${\mathit{T}}_{\mathit{c}2}$ transitions should occur at the extrapolated critical concentration x=0.075.