| Autor: |
Costa SN; Departamento de Física, Centro de Ciências, Universidade Federal do Ceará , Caixa Postal 6030, Campus do Pici, 60455-760, Fortaleza, Ceará, Brazil., Freire VN; Departamento de Física, Centro de Ciências, Universidade Federal do Ceará , Caixa Postal 6030, Campus do Pici, 60455-760, Fortaleza, Ceará, Brazil., Caetano EW; Instituto Federal de Educação, Ciência e Tecnologia do Ceará , Avenida 13 de Maio 2081, Benfica, 60040-531 Fortaleza, Ceará, Brazil., Maia FF Jr; Departamento de Ciências Exatas e Naturais, Universidade Federal Rural do Semi-Árido , Campus Mossoró, 59900-000 Mossoró, Rio Grande do Norte, Brazil., Barboza CA; Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte , 59072-970 Natal, Rio Grande do Norte, Brazil., Fulco UL; Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte , 59072-970 Natal, Rio Grande do Norte, Brazil., Albuquerque EL; Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte , 59072-970 Natal, Rio Grande do Norte, Brazil. |
| Abstrakt: |
The role of hydration on the structural, electronic, optical, and vibrational properties of monohydrated (CaCO3·H2O, hexagonal, P31, Z = 9) and hexahydrated (CaCO3·6H2O, monoclinic, C2/c, Z = 4) calcite crystals is assessed with the help of published experimental and theoretical data applying density functional theory within the generalized gradient approximation and a dispersion correction scheme. We show that the presence of water increases the main band gap of monohydrocalcite by 0.4 eV relative to the anhydrous structure, although practically not changing the hexahydrocalcite band gap. The gap type, however, is modified from indirect to direct as one switches from the monohydrated to the hexahydrated crystal. A good agreement was obtained between the simulated vibrational infrared and Raman spectra and the experimental data, with an infrared signature of hexahydrocalcite relative to monohydrocalcite being observed at 837 cm(-1). Other important vibrational signatures of the lattice, water molecules, and CO3(2-) were identified as well. Analysis of the phonon dispersion curves shows that, as the hydration level of calcite increases, the longitudinal optical-transverse optical phonon splitting becomes smaller. The thermodynamics properties of hexahydrocalcite as a function of temperature resemble closely those of calcite, while monohydrocalcite exhibits a very distinct behavior. |
