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 APO 4 orthophosphates under pressure.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 23 Jan 2012|