Monazite-type SrCrO4 under compression

We report a high-pressure study of monoclinic monazite-type SrCrO₄ up to 26 GPa. Therein we combined x-ray diffraction, Raman, and optical-absorption measurements with ab initio calculations, to find a pressure-induced structural phase transition of SrCrO₄ near 8-9 GPa. Evidence of a second phase transition was observed at 10-13 GPa. The crystal structures of the high-pressure phases were assigned to the tetragonal scheelite-type and monoclinic AgMnO₄-type structures. Both transitions produce drastic changes in the electronic band gap and phonon spectrum of SrCrO₄. We determined the pressure evolution of the band gap for the low- and high-pressure phases as well as the frequencies and pressure dependencies of the Raman-active modes. In all three phases most Raman modes harden under compression, however the presence of low-frequency modes which gradually soften is also detected. In monazite-type SrCrO₄, the band gap blueshifts under compression, but the transition to the scheelite phase causes an abrupt decrease of the band gap in SrCrO₄. Calculations showed good agreement with experiments and were used to better understand the experimental results. From x-ray-diffraction studies and calculations we determined the pressure dependence of the unit-cell parameters of the different phases and their ambient-temperature equations of state. The results are compared with the high-pressure behavior of other monazites, in particular PbCrO₄. A comparison of the high-pressure behavior of the electronic properties of SrCrO₄ (SrWO₄) and PbCrO₄ (PbWO₄) will also be made. Lastly, the possible occurrence of a third structural phase transition is discussed.