Measurement of the Lattice Constant of KMnF₃around the 186 K Phase-Transition: Determination of the Critical Exponent

The lattice constant of KMnF 3 has been measured by means of the high-angle double-crystal X-ray diffractometry (HADOX). The relative accuracy of the lattice constant to one part in 10 6 is attained. The value of the critical exponent β for the static rotation angle of MnF 6 octahedra below the structural phase-transition temperature (186.65 K) is determined to be 0.2857 ±6 from the lattice constant a ( T ); the value is significantly smaller than 0.38 predicted for a 3D-Heisenberg system and even than 0.315 for a 3D-Ising system, with an isotropic short range interaction. This suggests that the transition occurs in the vicinity of a tricritical point. The lattice constants themselves also exhibit a critical behaviour; the value of the critical exponents are 2 β for a ( T ) and β for c ( T ).