Effect of temperature on the structure of manganites

The effect of temperature on the structure of some "normal" manganites has been studied by high temperature X-ray diffraction and by differential thermal analysis. At rcom temperature they are isomorphous with hausmannite, with c/a>1. On heating to elevated temperatures, the c,a, and c/a parameters are found to remain constant until just before a critical transformation temperature whereupon c/a falls abruptly to unity, coinciding with the structure transforming from tetragonal to cubic. This abrupt transformation is reminiscent of a first order process. The structure of the tetragonal forms at various temperatures, and the cubic form just after transition, have been determined by X-ray diffraction. The theoretical model for the variation of distortion with temperature has been discussed, and a correlation between the transition temperature and the cell distortion has been shown. An anomalous behaviour has been observed for magnesium manganite because of its tendency to change its cation distribution and alter, on heating, from a "normal" to a "random" structure. The site preference energy for the cations in MgMn2O4 has been calculated from the change in cation distribution with temperature. The D.T.A. data support the transition temperatures of various manganites as obtained by X-ray and also lead to the determination of the entropy of transformation