Landau theory and phase diagram of KMn1-xCaxF3 ferroelastic crystal near the tricritical point: calorimetric and order parameter study
The cubic to tetragonal phase transition in the solid solution K(Mn,Ca)F3 has been investigated by conduction calorimetry and x-ray diffraction. The behaviour of the excess specific heat, latent heat and spontaneous strain has been explained in terms of a 2-4-6 Landau potential. The coefficients A and C, prefactors of Q2 and Q6 in the free energy expansion, are practically constant with composition, but B (the prefactor of Q4) and TC are functions of composition. The tricritical point occurs when the sign of B changes from negative (in pure KMnF3) to positive (in Ca rich samples). However, the variation of the parameters B and TC with dopant concentration x is non-linear. The dependence of TC with composition is explained in terms of an internal stress due to the doping ion of Ca and is compared with the effect of external uniaxial stress on pure KMnF3.