Acta Crystallographica Section B

Structural Science

Volume 56, Part 4 (August 2000)

research papers

Acta Cryst. (2000). B56, 648-653 [ doi:10.1107/S0108768100003621 ]

Aplanarity of CO₃ groups: a theoretical investigation

B. Winkler, J. Zemann and V. Milman

Abstract: Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO $_{3}$ groups in some carbonates such as dolomite, CaMg(CO), aragonite, CaCO, and norsethite, BaMg(CO), is a ground-state property. This distortion stabilizes dolomite by 500 J mol $^{-1}$ . Up to at least 6 GPa, the aplanarity of CO groups in dolomite is independent of pressure. In aragonite the aplanarity increases slightly on increasing pressure, while a significant tilting of the CO groups occurs. The calculations do not support previous findings of anomalously low values for the pressure derivative of the bulk moduli, $B^\prime$ , of aragonite and dolomite. Instead, the computed pressure dependences of the unit-cell volumes correspond to $B^\prime$ = 5.0 (5) for aragonite and $B^\prime$ = 4 (1) for dolomite, when fitted with a third-order Birch-Murnaghan equation-of-state.

Keywords: Aplanarity; Density functional theory; Carbonates.