Cubic-tetragonal transitions in perovskites:: Experimental data and a landau potential for KMnF3

X-ray rocking and calorimetry are used to obtain experimental data on the cubic-tetragonal phase transition in pure KMnF3. These results are compared with a 2:4:6 Landau potential with A = 3.76 J K-1 mol(-1) B = -106.4 J mol(-1), C = 806.3 J mol(-1), T-C = 185.2 K, and good agreement is found. The qualitative effect of substitutional solid solution is discussed, and the Landau potential is found to be consistent with the observed crossover between first- and second-order behaviour resulting from the replacement of Mn by Ca.