Ferroelastic phase transformations in trigonal double molybdates and tungstates

Investigated are ferroelastic phase transformations in crystals of trigonal double molybdates and tungstates (TDM(T)) with MR(XO4)2 (M=K, Rb; R=Sc, Fe, In; X=Mo, W) formula. Presented is phenomenologic description of phase transformations, the conclusions of which coincide with the results of experimental investigations. Full information on microscopic distortions and changes of space symmetry of TDM(T) was obtained by irradiation of ESR spectra of crystals, containing admixtures of Cr3+, Fe3+, Gd3+ and Er3+ ions, isomorphically substituting R3+ cations of the main lattice. Soft modes lower than the temperature of phase transformation T1 were found of light combination scattering method. It is stated, that consequent phase transformations from trigonal into monoclinic phase (T1) and further into triclinic (T2) are explained by TDM(T) structure instability in the symmetric phase of Dsub(3d)sup(3) relatively to one of the lattice oscillations, answering an uncontrolled two-dimensional representation in the K=B3/2 point of the Dsub(3d)sup(3) group. It is concluded, that transformations pass into the Csub(2h)sup(6) (Z=4) phase with inversion preservation in the R3+ cation state