Phase r, which is stable at room temperature, is of the lowest density, comparable only with that of phase a. Therefore, the transition B - r leads to a decomposition of the structure with the formation of small crystals. As it is shown in Fig.12, the sequence of transformation during cooling always gives phase b. It is not possible to obtain the polymorph r by cooling the phase L'. Both transformations b-r and r-L' and a are very similar. Small displacements of structural elements are sufficient to cause a transformation without the breaking of primary coordination bonds. Also the enthalpy of these transformations is low