Erwan André

The Raman spectrum of CaCO3 polymorphs calcite and aragonite: A combined experimental and computational study

Powder and single crystal Raman spectra of the two most common phases of calcium carbonate are calculated with ab initio techniques (using a hybrid functional and a Gaussian-type basis set) and measured both at 80 K and room temperature. Frequencies of the Raman modes are in very good agreement between calculations and experiments: the mean absolute deviation at 80 K is 4 and 8 cm(-1) for calcite and aragonite, respectively. As regards intensities, the agreement is in general good, although the computed values overestimate the measured ones in many cases. The combined analysis permits to identify almost all the fundamental experimental Raman peaks of the two compounds, with the exception of either modes with zero computed intensity or modes overlapping with more intense peaks. Additional peaks have been identified in both calcite and aragonite, which have been assigned to (18)O satellite modes or overtones. The agreement between the computed and measured spectra is quite satisfactory; in particular, simulation permits to clearly distinguish between calcite and aragonite in the case of powder spectra, and among different polarization directions of each compound in the case of single crystal spectra.

Modelling the Structure and Vibrational Properties of Layered Double Hydroxides

A model structure is proposed for the layered double hydroxide [Mg2Al(OH)6] + · Cl (Mg/Al ratio R = 2). The structural parameters of this model have been optimized by DFT calculations and both PXRD and vibrational spectra have been simulated. The comparison with experimental data confirms the validity of the model. Thanks to the synergy between experiment and theory it is now possible to analyze in detail the vibrational signals and understand the interactions between the different components of this material.