Effects of pressure on the structure and lattice dynamics of TmPO4: Experiments and calculations

Abstract

Thulium phosphate (TmPO${}_{4}$) with the tetragonal zircon-type structure has been studied using angle-dispersive powder x-ray diffraction and Raman spectroscopy in a diamond anvil cell up to 55 GPa (at room temperature). The results from both experimental methods reveal a pressure-induced structural phase transition from zircon to a scheelite-type structure at 20 GPa. At the same pressure, a second phase with the monazite structure is also formed at a low concentration, which decreases and eventually vanishes with increasing pressure. Our ab initio total-energy and lattice-dynamics calculations are in good agreement with experimental findings. The calculations indicate that the main transition to the scheelite structure is related with the softening of a silent ${B}_{1u}$ mode. Upon decompression, TmPO${}_{4}$ reverses back to the original zircon structure showing significant hysteresis. The results are discussed in relation to the observed general structural systematics and phase transition sequences in zircon-type $A$PO${}_{4}$ orthophosphates under pressure.