High-Pressure Structural Study on the Effects of Pressure-Transmitting Media on Bi14MoO24 and Bi14CrO24 Compounds

An investigation of isostructural compounds Bi₁₄MO₂₄, where M is either Cr or Mo, has been conducted under high-pressure (HP) conditions, reaching up to 30 and 40 GPa, respectively. In situ synchrotron X-ray diffraction was employed to monitor alterations in the crystal structure induced by pressure. The compounds have been studied in two different experimental conditions, under hydrostatic conditions using He as pressure transmitting medium (PTM) or under plastic deformation without PTM. Remarkably, the use of helium gas as a PTM revealed isomorphic phase transitions unobserved previously when other PTMs were used. In the case of Bi₁₄MoO₂₄, in addition to the previously documented tetragonal (I4/m) to monoclinic (C2/m) transition, two new isomorphic (monoclinic–monoclinic) phase transitions have been identified. Conversely, Bi₁₄CrO₂₄ exhibits two novel isomorphic (tetragonal–tetragonal) transitions preceding the tetragonal-monoclinic transformation. While analogous phase transitions were identified in experiments performed without PTM, an earlier pressure onset of phase transition was noted. Moreover, a new cubic phase coexisting with HP modification was observed in Bi₁₄CrO₂₄, providing insights into the behavior of these compounds under extreme conditions of shear-stress.