Pressure-Driven Phase Transition in InPO4: The Elastic Response of CrVO4-Type, Scheelite, and Zircon Polymorphs

In this work, we present a theoretical study of InPO4 under pressure. Total-energy calculations based on density functional theory were performed to explore the crystal structure of InPO4 in light of the recent X-ray diffraction characterization of this compound under pressure. A phase coexistence was observed above 10 GPa, involving the ambient-pressure CrVO4-type structure and the high-pressure scheelite and zircon phases. Therefore, the previously performed analysis of InPO4 behavior under pressure is extended by simulating X-ray spectra and interplanar distances for the three polymorphs. In addition, the elastic behavior of the three phases is analyzed to assess the elastic stability of InPO4 under pressure and to compute the mechanical properties and elastic anisotropy. Our findings significantly extend previous experimental results on the compressibility of InPO4, which were limited to the ambient-pressure phase. Moreover, our results unambiguously reveal a marked difference in the elastic properties of the scheelite and zircon phases under pressure, showing that the zircon phase is elastically unstable at high pressures. This suggests that the reported coexistence of phases may result from kinetic barriers or from non-hydrostatic conditions within the diamond anvil cell caused by the pressure-transmitting medium.