High-Pressure Raman Scattering of CaWO4 Up to 46.3 GPa: Evidenceof a New High-Pressure Phase
Résumé
The high-pressure behavior of CaWO4 wasanalyzed at room temperature by Raman spectroscopy.Pressure was generated using a diamond-anvil cell and Ne aspressure-transmitting medium. The pressure range of previousstudies has been extended from 23.4 to 46.3 GPa. Theexperiments reveal the existence of two reversible phasetransitions. The first one occurs from the tetragonal scheelitestructure to the monoclinic fergusonite structure and isobserved at 10 GPa. The onset of a previously unknownsecond transition is found at 33.4 GPa. The two high-pressurephases coexist up to 39.4 GPa. The Raman spectra measuredfor the low-pressure phase and the first high-pressure phase areconsistent with previous studies in the pressure range wherecomparison is possible. The pressure dependence of all the Raman-active modes is reported for different phases. We also reporttotal-energy and lattice-dynamics calculations, which determine the occurrence of two phase transitions in the pressure rangecovered by the experiments. The first transition is in full agreement with experiments (scheelite-to-fergusonite). According tocalculations, the second-highest pressure phase has an orthorhombic structure (space group Cmca). Details of this structure, itsRaman modes, and its electronic band structure are given. The reliability of the reported results is supported by the consistencybetween the theoretical and experimental values obtained for transition pressures, phonon frequencies, and phonon pressurecoefficients.