A precursor effect in the lattice constant at the 186 K-structural phase transition in KMnF3

A small amount of critical contraction of the lattice constant of KMnF3 has been found just above the cubic-to-tetragonal transition temperature (TR ). The critical contraction shows a cusp-shaped anomaly; the magnitude reaches 1.05 × 10−4, Å at TR . This phenomenon can be regarded as an effect due to critical dynamics. The contraction is analyzed on the basis of single-and double-well potentials. In the first case, the critical contraction can be explained by taking account of the enhancement of the mean-squared displacement of fluorine atoms; the critical exponent of the displacement is determined to be 0.79 ± 0.03. In the second case, on the other hand, the critical contraction can be related to precursor local-order; clusters with cubic symmetry, rather than tetragonal, are appropriate. The precursor local order is determined to be 0.29° at TR . It is concluded that the single-wall potential model is more appropriate for the interpretation of the critical contraction.