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Abstract:
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.