Scheelite to fergusonite phase transition in YLiF4 at high pressures

Yttrium lithium orthofluoride YLiF4 with a tetragonal scheelite structure (I4(1)/a, Z=4) has been studied with angle-dispersive x-ray powder diffraction in a diamond anvil cell at room temperature. Upon compression to about 10.0 GPa, the c/a axial ratio increases, demonstrating that the tetragonal distortion of the fluorite superstructure is enhanced. At 10.6 GPa, there occurs a transformation to a fergusonite structure (I2/a, Z 54) that involves small distortions of the cation matrix and significant displacements of anions in the simple cubic packing. There is no detectable discontinuity in the evolution of the unit cell volumes during the I4(1) /a --> I2/a transformation. Changes of the lattice parameters and axial ratios are similar to those found for temperature-induced ferroelastic scheelite-fergusonite transitions in rare earth orthoniobates and orthotantalates. A second sluggish phase transition to an as yet unidentified polymorph of YLiF4 occurs above 17.0 GPa.