A high pressure infrared spectroscopic study of PbCO3-cerussite: constraints on the structure of the post-aragonite phase

Abstract

We have measured the infrared spectrum of aragonite-structured PbCO3-cerussite to 41 GPa at 300 K in the diamond anvil cell. We observed a phase transition from an orthorhombic to a trigonal structure beginning at ~15 GPa, manifested by a splitting of the ν2-out-of-plane bending vibration and a broadening and dramatic decrease in amplitude of the ν1-symmetric stretching vibration of the carbonate group. While the locations of the ν1-symmetric stretching and ν4-in-plane bending bands are similar between the low- and high-pressure phases, their mode shifts and peak shapes change markedly near the transition. In particular, the ν1 symmetric stretch has an essentially zero pressure shift in the high pressure phase, and its dramatically enhanced peak width indicates that it may be symmetry forbidden. The decreased mode shifts of the carbonate vibrations after the phase transition suggest that the carbonate group is less compressible in the new structure. The spectral changes observed are consistent with a small, trigonal unit cell, with space group $${P\bar{3}{1c}}$$ and two formula units, instead of a previously proposed orthorhombic cell with sixteen formula units. This structure is identical to that of the high-pressure phase of BaCO3, and likely CaCO3 as well. Our results thus indicate that the post-aragonite, high-pressure phase of divalent-cation carbonates may be a comparatively high-symmetry trigonal structure.