1. Introduction
Calcium carbonate (CaCO3) is inorganic compounds which can be commonly found on earth and is widely used in geological, biological, technological concerns, industry, chemistry, construction, ocean acidification and biomineralization, etc.[1, 2] CaCO3 is an important part of the global carbon cycle. It releases CO2 solidified in rocks into the atmosphere through weathering, on the other hand, calcium carbonate can re-fix CO2 in the atmosphere under the process of sedimentation.[3]Over the past few decades, a lot of researches have been done on three anhydrous crystalline polymorphs of CaCO3, including calcite, aragonite, and vaterite, while two of them were hydrated crystalline phases—monohydrocalcite[4, 5](CaCO3·1H2O) and ikaite[6, 7](CaCO3·6H2O). What’s more, in the recent research, an interesting calcium carbonate hemihydrate with monoclinic structure has been unexpectedly discovered for the first time by Z.Y. Zou. et al[8]. Whose discovery expanded our knowledge of the CaCO3 family and implicated in biomineralization, geology, and industrial processes.
With the development of computing, materials computational science is an effective means of simulation in both computational accuracy and computational efficiency. While the first-principles calculations is one of excellent effective tools to calculate the properties of materials including metal material, inorganic non-metal material, biological materials, functional materials, semiconductor materials and composite materials, etc. The vibrational spectrum of calcite had been researched through an ab initio quantum-mechanical calculation by M. Prencipe. et al[9] who found this method was in agreement with experimental values. Moreover, a large number of articles about the experimental and theoretical researches of calcium carbonate hydrates were appeared. Monohydrocalcite and ikaite were research by many scholars have studied over the last few decades. Monohydrocalcite was a rare mineral in geological settings, which can be found in seawater environment and played a role inremediation material for hazardous oxyanions.[5, 10]. It was also regarded as an adsorbent to remove phosphate from solution and the mechanism of phosphate on monohydrocalcite including ionic strengths, reaction times, and temperatures, etc., which were discussed elaborately by S Yagi et al[11]. In addition, the structures of both CaCO3·6H2O and CaCO3·1H2O had been studied by R Demichelis et al through the PBE0 level of theory[12]. A.M Chaka[13] had researched the thermodynamics of hydrated calcium carbonates and calcium analogues of magnesium carbonates by ab initio. In order to determine the stability of calcium carbonate polymorphs, the incorporation ab initio thermodynamics based on density-functional theory and experimental chemical potentials for H2O-rich and CO2-rich systems were used. Furthermore, she also discussed carbonate crystallization pathways in detail. The hydrogen bonding in ikaite was investigated by I. P Swainson et al[14]. They found the linear thermal expansion coefficients were quite anisotropic due to being smaller in the direction of the C-O bond. In addition, S.N Costa et al[15] hadbeen researched the structure, electronic, optical and vibrational properties of hydrated calcium carbonate crystals CaCO3·(1H2O,6H2O).
To our best knowledge, there are no articles to investigate the electronic, optical and mechanical properties of calcium carbonate hydrates for the novel CaCO3·1/2H2O, which limits their applications for the controlling the formation of crystalline calcium
carbonates, biominerals and global carbon cycle, etc. Therefore, in this work, the structural stability, electronic, optical and mechanical properties of calcium carbonate hydrates (CaCO3·x H2O; x= 1/2, 1 and 6) are investigated by the first-principles calculation, which provide guidance for experiment and its application, such as biomineralization, geology, and industrial processes.