In situ Raman spectroscopy of pressure‐induced phase transformations in polycrystalline TbPO₄, DyPO₄, and Gd_xDy_(1−x)PO₄

Abstract

Xenotime DyPO4 and GdxDy(1−x)PO4 (x = 0.4, 0.5, 0.6) (tetragonal I41amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman‐active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7‐8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO4 occurred at a pressure of 15.3 GPa. DyPO4 underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (GdxDy(1−x)PO4) were in the range 10‐12 GPa. The GdxDy(1−x)PO4 solid solutions yielded partially reversible phase transformations, retaining some of the high‐pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO4 decreased the transformation pressure relative to pure DyPO4. The ability to modify the phase transformation pressures of xenotime rare‐earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.

High pressure crystal structures of orthovanadates and their properties

“…Compounds with mixed compositions can have a different behavior that their end members when studied under high-pressure [87,88]. For instance, it has been observed that Tb0.5Gd0.5PO4 does not undergo a transition from zircon to scheelite or monazite as expected for TbPO4 and GdPO4 [17].…”

Crystal chemistry and thermodynamic properties of zircon structure-type materials

“…For the rare earth orthovanadates, the ionic radii define a hard cut off between LaVO4 and CeVO4. This is slightly different from that of the rare earth orthophosphates, as GdPO4, TbPO4, and DyPO4 all exist as polymorphs of either monazite or xenotime (Ni et al 1995;Ushakov et al 2001;Clavier et al 2018;Musselman et al 2018), but this should be viewed as a high temperature phase transition from monazite to xenotime, which is discussed in the following section.…”

“…The α ® β transition occurs above 1473 K (Achary et al 2007;Errandonea et al 2009a). The monazite to zircon phase transition has been reported for three rare earth orthophosphates, GdPO4, TbPO4, and DyPO4 (Ni et al 1995;Ushakov et al 2001;Clavier et al 2018;Musselman et al 2018). These three orthophosphates can exist as either the monazite or xenotime structures, the latter of which is more stable for Gd, Tb, and Dy at high temperatures (Ushakov et al 2001; Thorite (ThSiO4) is one oddity amongst the zircon structure materials.…”

“…DOI: https://doi.org/10.2138/am-2022-8632. http://www.minsocam.org/Clavier et al 2018;Musselman et al 2018). Once the zircon structure is formed, most materials do not exhibit further phase transitions at higher temperatures.…”

Influence of citrate on phase transformation and photoluminescence properties in LaPO4 and LaPO4:Eu

“…Several pressure-induced phase transformations of ABO 4 (A ¼ Ba, Ca, Sr, Tb, Dy; B ¼ Cr, W, P) compounds have been discovered. [15][16][17][18] In particular, the organic ligand-induced phase transformation of REPO 4 compounds at low temperature has been studied. Yan et al 19 reported an ethylenediaminetetraacetic acid (EDTA)-mediated hydrothermal route to synthesize different phase cerium orthovanadate (CeVO 4 ) microcrystals.…”