Synthesis and vibrational spectroscopic characterisation of synthetic hydrozincite and smithsonite

Hydrozincite and smithsonite were synthesised by controlling the partial pressure of CO2 . Previous crystallographic studies concluded that the structure of hydrozincite was a simple one. However both Raman and infrared spectroscopy show that this conclusion is questionable. Multiple bands are observed in both the Raman and infrared spectra in the (CO3)2-antisymmetric stretching and bending regions of hydrozincite showing that the symmetry of the carbonate anion is reduced and in all probability the carbonate anions are not equivalent in the hydrozincite structure. Multiple OH stretching vibrations centred in both the Raman and infrared spectra show that the OH units in the hydrozincite structure are non-equivalent. The Raman spectrum of synthetic smithsonite is a simple spectrum characteristic of carbonate with Raman bands observed at 1408, 1092 and 730 cm-1. The symmetry of the carbonate anion in hydrozincite is C2v or Cs. This symmetry reduction results in multiple bands in both the symmetric stretching and bending regions. The intense band of hydrozincite at 1062 cm-1 is assigned to the ν1 (CO3)2- symmetric stretching mode. Three Raman bands assigned to the ν3 (CO3)2- antisymmetric stretching modes are observed for hydrozincite at 1545, 1532 and 1380 cm-1. Multiple infrared or Raman bands are observed in 800 to 850 cm-1 and 720 to 750 cm-1 regions and are attributed to ν2 and ν4 bending modes confirming the reduction of the carbonate anion symmetry in the hydrozincite structure. A Raman band for hydrozincite at 980 cm-1 is attributed to the δ OH deformation mode