Raman Scattering And Dielectric Studies Of Phase Transitions In Alkali Chromates (dipole-glass, Ferroelastic, A(2) Bx(4)-compounds, Superionic, Pre-transitions).

The results of Raman scattering and dielectric measurements are reported for the alkali chromate A(,2)CrO(,4) (A = K, Rb, CS). Evidence for a dipole-glass transition is observed in potassium chromate at T(,f) (TURNEQ) 983 K. The dielecric constant along the pseudohexagonal axis shows a weak anomaly at T(,c) (TURNEQ) 960 K (the (alpha)-(beta) transition), and pronounced frequency dependent cusps at slightly higher temperatures. This is interpreted in terms of a glassy state arising from randomness introduced by the presence of vacancies. The (alpha)-(beta) transition was also observed in rubidium and cesium chromate ((TURN)1025 K and 1040 K respectively), however the freezing peak in the dielectric constant could not be detected within the temperature range of our experimental apparatus. Evidence of various phase transformations at lower temperatures is presented, and a scenario of successive phase transitions in which the anion tetrahedra become increasingly disordered is discussed. These are viewed as pre-transitions into the high temperature paraelastic phase since the average structure remains orthorhombic (isomorphous with (beta)- K(,2)SO(,4)) below T(,c). The lower temperature transformations show the following signatures. At 100 K in K(,2)CrO(,4) (('(TURN))75 K in Rb(,2)CrO(,4) and ('(TURN))100 K in Cs(,2)CrO(,4)) a lock-in of the internal mode positions occur with anomalous damping of (suspected) librational modes. As temperature increases, suspected librational modes become degenerate at 328 K in K(,2)CrO(,4) (145 K in Rb(,2)CrO(,4)) which correlates with dielectric anomalies perpendicular to the pseudohexagonal axis. Finally, near 650 K, the alkali chromates exhibit an exponential increase in conductivity (activation energy ('(TURN))0.9 eV) which correlates with a partial decoupling of the anion tetrahedra from the crystal field demonstrated by internal mode degeneracies. This, and the fact that the external mode Raman spectra become nearly featureless at these temperatures suggest superionic behavior. These materials also exhibit anomalous Raman selection rules which have yet to be explained. They may be attributed to superlattice reflections observed in electron and x-ray diffraction experiments.Ph.D.Condensed matter physicsPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/127890/2/8612616.pd