Er3+ doped KYF4 crystals show two structural transitions in the pressure range of 0 to ~70 kbar, as indicated by the abruptchanges of photoluminescence (PL) they exhibit. The crystal with 0.05 at.% Er3+ ions (KY0.9995Er0.0005F4) showed a higher firsttransition pressure (around 40 kbar) and a lower second transition pressure (around 55 kbar) than the crystal with 10 at.%Er3+ ions (KY0.9Er0.1F4), which indicates that rigidity reversal between the two crystals occurs at the first transition. Thisbehavior is explained in terms of the pillaring effect where Y3+ ions are acting as pillars so that the internal stress on the Er3+ions is alleviated while in the first phase. The Y3+ ions and the Er3+ ions are differentially compressed until the first transitionwhere the radii of both species become equal. After the first transition, the Y3+ ions stop functioning as pillars, leading to anenhanced deformation of the Er3+ ions. This effect is evident in the increased pressure-shift rates of the PL in the second phase.

Keywords: Photoluminescence, Fluorescence, Radiative process, Structural transition, High pressure, KYF4