A new model for the phase transitions due to octahedra rotations in the perovskite structure

Abstract

We give an order-disorder picture of the phase transitions due to MX 6 octahedra rotations in the perovskite structure. The lattice distortions and the occurrence of a central peak are discussed in terms of the intrinsic defects resulting from the disorder associated with the rotation of MX 6 rigid octahedra

The temperature dependence of the structures of RbCaF3, and the determination of the low temperature phases

“…This correction for thermal librations is similar to the one proposed by Glazer [20]. The corrected CaF bond lengths la and T, are reported in figure 3. All these data will be discussed in section 4.…”

“…-While, in the perovskite compounds family, the ferroelectric phase transitions are now well described by the orderdisorder model proposed by Comes for BaTi03 [27], the antiferrodistorsive ones, related to octahedra tilts, remain an unsolved problem. The order-disorder description proposed by Rousseau [3] figure 4 together with the experimental and the classical displacive ones [26]. One can see the improvement brought by the orderdisorder description.…”

“…In fact, such a paradox can be easily explained if it is assumed an order-disorder character for this phase transition [3].…”

“…In the other technique, model have to be used to relate the measurements to the octahedron rotation angle, and more over, some parameters whose value is not known accurately have to be introduced. To explain the mechanism of these transitions, a model is put forward [3] which, to be confirmed, needs measurements of the absolute values of the tilt angles. Only a diffraction method is able to give these angles by the direct determination of ionic coordinates.…”

“…For this computation the very large thermal motion of fluorine must be taken into account [19]. When [26] (dashed line) and the order-disorder model [3] (solid line). This correction for thermal librations is similar to the one proposed by Glazer [20].…”