High-Pressure Structural\nStudy on the Effects of Pressure-Transmitting\nMedia on Bi<sub>14</sub>MoO<sub>24</sub> and Bi<sub>14</sub>CrO<sub>24</sub> Compounds

An investigation of isostructural compounds Bi<sub>14</sub>MO<sub>24</sub>, where M is either Cr or Mo, has been conducted under\nhigh-pressure\n(HP) conditions, reaching up to 30 and 40 GPa, respectively. In situ\nsynchrotron X-ray diffraction was employed to monitor alterations\nin the crystal structure induced by pressure. The compounds have been\nstudied in two different experimental conditions, under hydrostatic\nconditions using He as pressure transmitting medium (PTM) or under\nplastic deformation without PTM. Remarkably, the use of helium gas\nas a PTM revealed isomorphic phase transitions unobserved previously\nwhen other PTMs were used. In the case of Bi<sub>14</sub>MoO<sub>24</sub>, in addition to the previously documented tetragonal (<i>I</i>4/<i>m</i>) to monoclinic (<i>C</i>2/<i>m</i>) transition, two new isomorphic (monoclinic–monoclinic)\nphase transitions have been identified. Conversely, Bi<sub>14</sub>CrO<sub>24</sub> exhibits two novel isomorphic (tetragonal–tetragonal)\ntransitions preceding the tetragonal-monoclinic transformation. While\nanalogous phase transitions were identified in experiments performed\nwithout PTM, an earlier pressure onset of phase transition was noted.\nMoreover, a new cubic phase coexisting with HP modification was observed\nin Bi<sub>14</sub>CrO<sub>24</sub>, providing insights into the behavior\nof these compounds under extreme conditions of shear-stress.