High-pressure phase ofLaPO4studied by x-ray diffraction and second harmonic generation

Abstract

The pressure-induced phase transition of monazite-type LaPO 4 at ≈26 GPa is studied by single-crystal x-ray diffraction and second harmonic generation (SHG) up to 31 GPa. The structure of the postmonazite phase of LaPO 4 has been obtained and it is shown that it corresponds to a post-barite-type structure with an acentric space group P 2 1 2 1 2 1 . A strong increase of the SHG signal at the transition confirms that the high-pressure polymorph is noncentrosymmetric. The phase transition involves a significant discontinuous decrease of the unit-cell volume by 6%, which is mainly due to a strong contraction of the a lattice parameter. Enthalpy differences between polymorphs with monazite-, barite-, and post-barite-type structures have been obtained from density-functional-theory-based calculations. They also yield a transition pressure of 21 GPa for the monazite to postbarite phase transition, but indicate that the barite-type phase is significantly more stable, implying that the transition into this phase may be kinetically hindered. As the transition pressure for the monazite-type to post-barite-type structure is reproduced with reasonable accuracy, we predict the corresponding phase-transition pressures of NdPO 4 and GdPO 4 to be 35 and 45 GPa, respectively.

High‐pressure phase transitions and properties of MTO4 compounds with the monazite‐type structure

“…From powder XRD an orthorhombic barite‐type structure (space group Pnma ) has been proposed for the HP phase . More recently, precise single‐crystal XRD measurements assigned an orthorhombic post‐barite‐type structure (space group P 2 1 2 1 2 1 ) for the post‐monazite phase of LaPO 4 . Interestingly, a similar structure, described by the same space group, has been found to be the post‐monazite structure in CeVO 4 .…”

“…A number of new polymorphs have been discovered and their crystal structure has been determined by X‐ray diffraction (XRD). In the case of alkaline‐earth phosphates, it is known that the monazite structure remains stable at least up to 30 GPa . From powder XRD an orthorhombic barite‐type structure (space group Pnma ) has been proposed for the HP phase .…”

“…An interesting fact to remark on the transitions induced by pressure in monazites is that, with the exception of the isostructural transition of PbCrO 4 , the rest of the transitions induced by pressure in the monazite always involve a large volume collapse ; that is, a densification of the material. Such reduction of the volume implies a first‐order character for the phase transition.…”

“…Among all these compounds, monazites have been the last group to be studied, in part due to the complexity of the monazite structure. Most efforts have been dedicated to the study of monazite‐type phosphates under high‐pressure . However, research has been also carried out more recently in vanadates and chromates .…”

Neutron diffraction reveals the existence of confined water in triangular and hexagonal channels of modified YPO4 at elevated temperatures

“…At ambient condition, depending on the ionic radii of RE ion and synthesis route, the compounds can crystalize in tetragonal, hexagonal and monoclinic phases 13,[15][16][17][18] .A smaller ionic radius compared to the ionic radius of Gd generally adopts the tetragonal structure, whereas other orthophosphates have the lower-symmetry monoclinic structure. Depending on the synthesis conditions, GdPO4, TbPO4, DyPO4, and HoPO4 can adopt either zircon or monazite structure 13,16,[19][20][21] . LaPO4 crystallizes in two polymorphs, namely, the tetragonal phase (xenotime structure) and the monoclinic phase (monazite structure).…”