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.