A high-pressure infrared and Raman spectroscopic study of BaCO3: the aragonite, trigonal and Pmmn structures

BaCO3 is examined under pressure, and following laser heating at pressure, to characterize bonding changes associated with different phases of this material. Infrared spectroscopy is utilized to probe the vibrational spectrum of witherite to 8 GPa and the metastable trigonal P-31c phase of BaCO3 that forms above similar to 7 GPa at 300 K to pressures of 45 GPa. Similarly, Raman spectroscopy is used to examine the vibrations of the orthorhombic Pmmn symmetry phase to 22 GPa and on metastable decompression to 1 GPa: this phase forms at high temperatures at pressures above similar to 8 GPa. For witherite, all of the vibrations of the carbonate unit, except for the out-of-plane bending vibration (which is affected by increases in Ba-O bond strength), shift to higher frequencies to 7 GPa. At the trigonal-phase transition, the infrared carbonate symmetric stretch becomes unresolvable, and the infrared in-plane and asymmetric stretching vibrations split into two components. No further phase transitions are observed at 300 K to 45 GPa, implying that the trigonal phase remains metastable at 300 K to at least this pressure. For the orthorhombic Pmmn phase which is generated following laser heating, our Raman results document that the lattice modes of this phase lie at significantly lower frequency than those of the aragonite-structured witherite: this is consistent with the increase in Ba-O coordination to 12-fold coordinate in this phase from pseudo-ninefold in witherite. The in-plane bending vibration is split within the Pmmn phase due to the lowered symmetry of the carbonate site: this splitting is enhanced by pressure, which indicates that the distortion of the planar carbonate unit is likely increased by compression: this distortion may play a key role in the response of the carbonate group to compression in this phase, and its notable stability under compression.