Raman scattering study on pressure-induced phase transformation of marokite (CaMn2O4)

An in-situ Raman Spectroscopic study was conducted to explore the pressure-induced phase transformation of CaMn₂O₄ to pressures of 73.7 GPa. Group theory yields 24 Raman active modes for CaMn₂O_4, of which 20 are observed at ambient conditions. With the slight compression below 5 GPa, the pressure-induced contraction compensates the structural distortion induced by a Jahn–Teller (JT) effect, resulting in the occurrence of the zero pressure shifts of the JT-related Raman modes. Upon elevation of pressure to nearby 35 GPa, these Raman modes start to display a significant variation in pressure shift, implying the appearance of a pressure-induced phase transformation. Group factor analyses on all possible structure polymorphs indicate that the high-pressure phase is preferentially assigned to an orthorhombic structure, having the CaTi₂O₄ structure. The cooperative JT distortion is continuously reduced in the CaMn₂O₄ polymorph up to 35 GPa. Beyond 35 GPa, it is found that the JT effect was completely suppressed by pressure in the newly formed high-pressure phase. Upon release of pressure, this high-pressure phase transforms to the original CaMn₂O₄ phase, and continuously remains stable to ambient conditions.