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 CaMn2O4, 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. (C) 2002 Elsevier Science (USA). All rights reserved.