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

Abstract

The pressure-induced phase transition of monazite-type ${\mathrm{LaPO}}_{4}$ at $\ensuremath{\approx}26$ 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 ${\mathrm{LaPO}}_{4}$ 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 ${\mathrm{NdPO}}_{4}$ and ${\mathrm{GdPO}}_{4}$ to be 35 and 45 GPa, respectively.