3.2 Electronic structure and chemical bonding characteristic
As
we all know that band structure helps us to distinguish the electrical
behavior of the compound: whether the compound is the conductor,
semiconductor or insulator. The band structure and total density of
states (TDOS) and partial density of states (PDOS) of hydrated
crystalline calcium carbonate are shown in Fig. 3. We focus on the band
shape near the Fermi level, therefore, we don’t draw the energy band
away from it. From Fig. 3, most obviously, they exhibit indirect
insulating properties. At the G point, the valence bands of the
CaCO3·x H2O (x= 1/2, 1 and
6) are very flat, while the conduction band has some curvatures,
indicating that the effective masses of holes are much larger than the
electron masses. In order to grasp and describe the structural
stability, the TDOS and PDOS are discussed here and shown in Fig. 3(b),
(d) and (f). The calculated values of TDOS for all calcium carbonate
hydrates are nearly zero at Fermi energy (EF),
indicating that they have covalent-ionic feature. What’s more, it is
clearly observable that the TDOS of calcium carbonate hydrates are
primarily contributed by the H-1s , C-2p and O-2pstate near the EF. From the 5 to 8 eV, H-1s and
C-2p states are the main effects to TDOS. Furthermore, the charge
interaction near EF forms the C-O ionic bond, reflecting
hybridization between C and O orbital. Furthermore, we can see that the
valence band maximum (VBM) is mainly contributed by the O-2porbital, while the conduction band minimum (CBM) is mainly contributed
by the C-2p orbital. The energy gaps of the calcium carbonate
hydrates around the Fermi level are 4.571 eV (B→G transition from
valence band to conduction band), 5.069 eV (A→G transition from valence
band to conduction band) and 5.336 eV (Z→G transition from valence band
to conduction band) for CaCO3·1/2H2O,
CaCO3·6H2O and
CaCO3·H2O, respectively, which are in
agreement with the calculated values (monohydrated: 5.53-5.60 eV;
hexahydrated: 5.14eV) by Costa S.N. et
al.[15]. The error
range is 0.071eV to 0.264eV between them due to different calculated
parameters, indicating the calculation method is accurate.
The calculated charge densities of calcium carbonate hydrates are
presented in (100) planes and shown in Fig 4. The blue region and red
region represent the accumulation of electronic charge and the depletion
of electronic charge, respectively. It further indicates that C-O ionic
bonds are observed in calcium carbonate hydrates, which conforms to
analysis on the TDOS and PDOS in Fig. 3. What’s more, we can clearly see
that calcium carbonate hydrates hascomplex bonding.
CO32+ has the covalent-ionic bonds for
all calcium carbonate hydrates, which can find in Fig. 4. What’s more,
in order to describe the bond strength quantitatively, the Mulliken’s
overlap bond population is employed in this work, and the detail
discussion can be found in the supplementary materials in Fig. S1,
Fig.S2 and Table S1.