Decomposition of LiGdF4 scheelite at high pressures

The high-pressure behaviour of LiGdF4 scheelite (I41/a, Z = 4) was studied by measuring its angle-dispersive x-ray powder diffraction patterns as a function of pressure and temperature in a diamond anvil cell and a large-volume Paris–Edinburgh cell using a synchrotron radiation source. Upon compression to about 11 GPa at room temperature, the stable structure is of the scheelite type. At higher pressures and T = 298 K, new reflections occur that cannot be explained with the fergusonite structural model previously observed for LiY F4. Associated with this is the growth of an amorphous component. All the transformations are largely irreversible upon decompression. Annealing of the sample at 13.1 GPa led to a nucleation of a solid solution series LiyGd1−yF3−2y (P63/mmc, Z = 2) and traces of LiF. The new material LiyGd1−yF3−2y (P63/mmc, Z = 2) was recovered to ambient conditions but back-transformed to a Y F3-type phase (Pnma, Z = 4) after regrinding at room temperature for several hours. These observations are discussed in relation to the high-pressure high-temperature systematics of the AMX4-type compounds.