Monazite-type BiPO4, LaPO4, CePO4, and PrPO4 have been studied under high pressure by ab initio simulations and Raman spectroscopy measurements in the pressure range of stability of the monazite structure. A good agreement between experimental and theoretical Raman-active mode frequencies and pressure coefficients has been found which has allowed us to discuss the nature of the Raman-active modes. Besides, calculations have provided us information on how the crystal structure is modified by pressure. This information has allowed us to determine the equation of state and the isothermal compressibility tensor of the four studied compounds. In addition, the information obtained on the polyhedral compressibility has been used to explain the anisotropic axial compressibility and the bulk compressibility of monazite phosphates. Finally, we have carried out a systematic discussion on the high-pressure behavior of the four studied phosphates in comparison to results of previous studies.

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.

The monazite-type solid solution La 1−x Pr x PO 4 was synthesized by solid-state reaction and extensively investigated using electron microprobe and thermogravimetric analyses, differential scanning and high-temperature oxide melt solution calorimetry, powder X-ray diffraction, infrared and Raman spectroscopy. Lattice parameters and Ln-O bond lengths show a decrease with increasing Pr content. A small excess volume is observed for the solid solution. IR 1

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We have investigated the incorporation of the luminescent Eu 3+ cation in different Ln PO 4 ( Ln = Tb, Gd 1−x Lu x , x = 0.3, 0.5, 0.7, 1) host phases. All samples were analyzed with powder X-ray diffraction (PXRD), Raman spectroscopy, and site-selective time-resolved laser-induced luminescence spectroscopy (TRLFS) directly after synthesis and after an aging time of one year at ambient conditions. The PXRD investigations demonstrate the formation of a TbPO 4 phase in an uncommon anhydrite-like crystal structure evoked by a pressure-induced preparation step (grinding). In the Gd 1−x Lu x PO 4 solid solution series, several different crystal structures are observed depending on the composition. The TRLFS emission spectra of LuPO 4 , Gd 0.3 Lu 0.7 PO 4 , and Gd 0.5 Lu 0.5 PO 4 indicate Eu 3+ –incorporation within a xenotime-type crystal structure. TRLFS and PXRD investigations of the Gd 0.7 Lu 0.3 PO 4 composition show the presence of anhydrite, xenotime, and monazite phases, implying that xenotime no longer is the favored crystal structure due to the predominance of the substantially larger Gd 3+ –cation in this solid phase. Eu 3+ –incorporation occurs predominantly in the anhydrite-like structure with smaller contributions of Eu 3+ incorporated in monazite and xenotime. The electronic levels of the Eu 3+ –dopant in Gd 0.3 Lu 0.7 PO 4 and Gd 0.5 Lu 0.5 PO 4 xenotime hosts are strongly coupled to external lattice vibrations, giving rise to high-energy peaks in the obtained excitation spectra. The coupling becomes stronger after aging to such an extent that direct excitation of Eu 3+ in the xenotime structure is strongly suppressed. This phenomenon, however, is only visible for materials where Eu 3+ was predominantly incorporated within the xenotime structure. Single crystals of Eu 3+ –doped LuPO 4 show no changes upon aging despite the presence of vibronically coupled excitation peaks in the excitation spectra measured directly after synthesis. Based on this observation, we propose a lattice relaxation process occurring in the powder samples during aging, resulting in Eu 3+ migration within the crystal structure and Eu 3+ accumulation at grain b...