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Zusammenfassung:
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.
