Pressure impact on the structure, elasticity, and electron density distribution of CaSi2O5

Ab initio molecular dynamics simulations were used to reveal the mechanism of the fivefold to sixfold transition in Si coordination numbers of CaSi${}_{2}$O${}_{5}$. The longest first-neighbor Si-O distance drops from 2.8 to 1.8 \AA{} upon the triclinic to monoclinic transition. We find significant bulk modulus softening during the structure crossover, which is due to appearance of intermediate Si-O connections in the triclinic phase under slightly nonhydrostatic stress. Nonetheless, no soft phonon modes were found in either structure, indicating that both structures are dynamically stable. Across the transition, ${c}_{33}$ doubles and ${c}_{35}$ increases sixfold in magnitude due to the formation of new Si-O bonds. Chemical bonding analysis reveals distinctions in the electron localization function and bond ellipticity between the regular (1.8 \AA{}) and the dangling Si-O bonds (2.8 \AA{}), both of which suggest an impending disassociation of the dangling Si-O bond.