Specific heat and phase diagrams for uniaxially stressed KMnF3

The authors have investigated the cubic-to-tetragonal structural phase transition of KMnF3. The phase diagrams as a function of temperature and uniaxial pressure along (100) and (110) have been measured for pressures up to p=0.6 kbar by specific heat measurements. The specific heat curves show Landau temperature dependence but the Landau theory does not explain the observed phase diagrams. The variation of the hysteresis of the phase transition temperature and the Landau parameters as a function of p along (100) indicates the existence of two consecutive tricritical points joined by a second-order line. For p along (110) the measurements suggest that the phase transition is second order above a tricritical Lifshitz point at p=0.45 kbar.