Cubic-tetragonal transition in KMnF3: IR hard-mode spectroscopy and the temperature evolution of the (precursor) order parameter

Precursor effects were observed in KMnF3 using infrared hard- mode spectroscopy. The intensity of the infrared-active mode near 265 cm(-1) follows the thermodynamic order parameter, Q, in the tetragonal phase (I4/mcm). An additional weak signal is found in the cubic phase (Pm (3) over barm). The order parameter step at T-o = 185.95 K (L = 0.129 J g(-1)) is smeared with excess intensity due to the tetragonal short range order extending to T > 215 K. The intensity follows the predictions of Landau theory with a defect field G(defect) = -hQ, h = 6 J mol(-1). The observed excess intensities are compared with the appearance of precursor elastic softening. It is concluded that the precursor softening and the local tetragonal short range order are likely to be related to an extended defect structure, such as a tweed pattern, which may be stabilized by extrinsic defects.