Pressure-induced tricritical phase transition from the scheelite structure to the fergusonite structure in LiLuF4

LiLuF4 scheelite (I41/a, Z = 4) has been investigated at high pressures using synchrotron angle-dispersive x-ray powder diffraction in a diamond anvil cell at room temperature. At 10.7 GPa, it reversibly undergoes a tricritical phase transition to the fergusonite structure (C 12/c 1, Z = 4), a distorted modification of the scheelite type. No other phase transition occurs in this material up to 19.5 GPa, the highest pressure in this study. Such a high-pressure behaviour is compared with the pressure-induced transformations in LiYF4 and LiGdF4, adding on to our knowledge of the structural systematics in LiMF4 compounds.