The identification of high-pressure phase transition sequence in selected tungstates and molybdates

Tungstates and molybdates possessing the scheelite- and wolframite-type (if present) structures hold a significant functional value. Their high-pressure phase diagrams are very complicated and controversial, and even some parts have not been characterized yet. In this study, we investigate the sequence of pressure driven structural phase transitions up to 100 GPa in these tungstate and molybdate families via first-principles structure predictions. Based on our structural predictions, it is possible for isostructural tungstates and molybdates to exhibit a phase transition sequence that is either similar or identical. Examples of these compounds are CaWO4, CaMoO4, and CdMoO4, in addition to EuWO4 and EuMoO4. However, the phase transition sequences of some tungstates and molybdates, especially those with different divalent cations, display noteworthy variations, revealing the intricate influence of ionic radii and electronic properties on crystal configurations. To obtain a deeper understanding of the high-pressure phase transition behavior of tungstates and molybdates, we analyze the high-pressure phase diagrams of MgWO4, SrWO4, and CaMoO4, representative examples of wolframite-type tungstate, scheelite-type tungstate, and scheelite-type molybdate, respectively, using x-ray powder diffraction. Our x-ray diffraction experiments and structure predictions consistently verify that the orthorhombic Cmca phase is a high-pressure phase of SrWO4. Structural configurations and mechanical properties of these predicted structures are discussed, and electronic properties are given. This study could have important implications for the fields of seismology and geophysics, as well as the utilization of these materials in various capacities, such as photocatalysts, photoanodes, and phosphors.