Pressure-induced structural phase transition of KY0.9Er0.1F4

By investigating the photoluminescence (PL) of KY 0.9 Er 0.1 F 4 crystals under high pressure, we find that a structural phase transition occurs at a pressure around 57 kbar. In the new phase, the upconversion efficiency of the PL increases. After reaching the new phase, the crystal does not return to its original phase, not even after the pressure is completely released. From the measurement of the time-resolved PL, we find that the admixture of 4f and 5d wavefunctions is suddenly reduced as the transition occurs; accordingly, the new phase has a lattice with higher symmetry and smaller spacing than the original one.