Jered Chaney, Javier Santillan and Quentin Willaims
Univ. of Calif., Santa Cruz
jds@es.ucsc.edu
The behavior of BaCO3-witherite under pressure is an analogue for that of the geologically important calcium carbonate system. BaCO3 crystallizes in the aragonite structure, but with a cation that is substantially larger than calcium. Materials with large cations are known to undergo phase transitions at lower pressure than similarly-structured materials with smaller cations. We examine BaCO3 under pressure to determine the nature of the bonding changes associated with any phase transitions in this material. Previous Raman spectroscopic work has shown that a phase transition occurs near 7 GPa in this material, but the structure of the high pressure phase has remained enigmatic. We utilize infrared spectroscopy coupled with the diamond anvil cell to probe the vibrational spectrum of BaCO3 to pressures of 45 GPa, significantly higher than those of previous measurements. 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 7.2 GPa. At the phase transition, the carbonate symmetric stretch becomes unresolvable, and the in-plane and asymmetric stretching vibrations split into two components. We infer that the unit cell of the high pressure phase has doubled in size, and that the carbonate group remains nearly planar. No further phase transitions are observed to 45 GPa, implying that this phase is stable to volumetric compressions of ~30%. Our results demonstrate that the carbonate unit remains notably stable in this phase, even under dramatic volumetric compressions.