Costa Stefane N, Freire Valder N, Caetano Ewerton W S, Maia Francisco F, Barboza Carlos A, Fulco Umberto L, Albuquerque Eudenilson L
Departamento de Física, Centro de Ciências, Universidade Federal do Ceará , Caixa Postal 6030, Campus do Pici, 60455-760, Fortaleza, Ceará, Brazil.
Instituto Federal de Educação, Ciência e Tecnologia do Ceará , Avenida 13 de Maio 2081, Benfica, 60040-531 Fortaleza, Ceará, Brazil.
J Phys Chem A. 2016 Jul 21;120(28):5752-65. doi: 10.1021/acs.jpca.6b05436. Epub 2016 Jul 13.
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.
借助已发表的实验和理论数据，在广义梯度近似和色散校正方案下应用密度泛函理论，评估了水合作用对一水合方解石（CaCO₃·H₂O，六方晶系，P31，Z = 9）和六水合方解石（CaCO₃·6H₂O，单斜晶系，C2/c，Z = 4）晶体的结构、电子、光学和振动性质的影响。我们发现，相对于无水结构，水的存在使一水合方解石的主带隙增加了0.4 eV，而六水合方解石的带隙实际上没有变化。然而，当从一水合晶体转变为六水合晶体时，带隙类型从间接变为直接。模拟的振动红外光谱和拉曼光谱与实验数据取得了良好的一致性，在837 cm⁻¹处观察到了六水合方解石相对于一水合方解石的红外特征峰。还确定了晶格、水分子和CO₃²⁻的其他重要振动特征峰。对声子色散曲线的分析表明，随着方解石水合程度的增加，纵向光学-横向光学声子分裂变小。六水合方解石的热力学性质随温度的变化与方解石非常相似，而一水合方解石则表现出非常不同的行为。
