Exploring the high-pressure behavior of the three known polymorphs of BiPO4: Discovery of a new polymorph

We have studied the structural behavior of bismuth phosphate under compression. We performed x-ray powder diffraction measurements up to 31.5 GPa and ab initio calculations. Experiments were carried out on different polymorphs; trigonal (phase I) and monoclinic (phases II and III). Phases I and III, at low pressure (0.2-0.8 GPa), transform into phase II, which has a monazite-type structure. At room temperature, this polymorph is stable up to 31.5 GPa. Calculations support these findings and predict the occurrence of an additional transition from the monoclinic monazite-type to a tetragonal scheelite-type structure (phase IV). This transition was experimentally found after the simultaneous application of pressure (28 GPa) and temperature (1500 K), suggesting that at room temperature the transition might by hindered by kinetic barriers. Calculations also predict an additional phase transition at 52 GPa, which exceeds the maximum pressure achieved in the experiments. This transition is from phase IV to an orthorhombic barite-type structure (phase V). We also studied the axial and bulk compressibility of BiPO4. Room-temperature pressure-volume equations of state are reported. BiPO4 was found to be more compressible than isomorphic rare-earth phosphates. The discovered phase IV was determined to be the less compressible polymorph of BiPO4. On the other hand, the theoretically predicted phase V has a bulk modulus comparable with that of monazite-type BiPO4. Finally, the isothermal compressibility tensor for the monazite-type structure is reported at 2.4 GPa showing that the direction of maximum compressibility is in the (010) plane at approximately 15 (21) degrees to the a axis for the case of our experimental (theoretical) study.

💡 Research Summary

This paper presents a comprehensive investigation of the high‑pressure behavior of bismuth phosphate (BiPO₄) by combining synchrotron X‑ray powder diffraction experiments up to 31.5 GPa with first‑principles density‑functional calculations. Three known polymorphs were examined: a trigonal phase I and two monoclinic phases II and III. At modest pressures (0.2–0.8 GPa) both phase I and phase III undergo a reconstructive transition to the monazite‑type monoclinic structure (phase II). Once formed, phase II remains stable at room temperature up to the maximum experimental pressure of 31.5 GPa, indicating that the monazite structure is the most energetically favorable configuration under compression.

Ab initio calculations corroborate the experimental observations and predict two additional high‑pressure phases. The first, a tetragonal scheelite‑type structure (phase IV), is calculated to become stable near 28 GPa. This transition was experimentally realized only when pressure (28 GPa) was combined with high temperature (1500 K), suggesting that kinetic barriers prevent the I → IV transformation at ambient temperature. The second predicted transition occurs at about 52 GPa, where phase IV would convert to an orthorhombic barite‑type structure (phase V). Because the experimental setup did not exceed 31.5 GPa, phase V remains a theoretical prediction awaiting verification.

The authors determined pressure–volume equations of state for each phase using a third‑order Birch–Murnaghan fit. BiPO₄ is found to be more compressible than isomorphic rare‑earth phosphates, with bulk moduli that increase from the monazite phase to the scheelite phase. Notably, phase IV is the least compressible polymorph, while the calculated phase V would possess a bulk modulus comparable to that of the monazite structure.

In addition to bulk compressibility, the work provides the full isothermal compressibility tensor for the monazite‑type phase at 2.4 GPa. Both experimental and theoretical tensors reveal that the direction of maximum compressibility lies in the (010) crystallographic plane, inclined by approximately 15° (experiment) and 21° (theory) relative to the a‑axis. This quantitative description of anisotropic compression is valuable for interpreting strain‑induced property changes in BiPO₄‑based materials.

Overall, the study delivers a detailed high‑pressure phase diagram for BiPO₄, identifies a new scheelite‑type polymorph, predicts a further barite‑type transition, and supplies robust thermodynamic and elastic data. These findings not only deepen the fundamental understanding of BiPO₄ under extreme conditions but also provide essential parameters for the design of functional devices—such as phosphors, catalysts, or radiation‑shielding components—where the material may experience high pressures or temperatures.