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.