关键词

• charge distribution
• electron density
• electron distribution
• bond strength & electron density

1. An Ab Initio Molecular Dynamics Simulation of Liquid FeO–SiO2 Silicate System with Sulfur Dissolving

• https://doi.org/10.1007/s11663-021-02263-x

B. Charge Distribution of FeO–SiO2 System

The relaxed structure of liquid FeO-SiO2 that eliminates the influence of atomic vibration is shown in Figure 3(a) with valence charge density depicted on a typical cross-section parallel to the ab plane. This cross-section is selected to include both the Fe-O and Si-O bonds which are more clearly presented in Figure 3(b). From this figure, it is observed that most of the valence charges are concentrated around the oxygen atoms with only a few left surrounding the iron atoms. Near the silicon atoms, valence charges can only be found in the direction pointing to their nearest neighbor oxygen atoms, which means bonds with strong polarity form between silicon and oxygen atoms. From the above analysis, it is reasonable to infer the Si-O bonds to be strong covalent bonds. No prominent polarity can be observed for the Fe-O bonds from the valence charge density distribution, which shows the typical features of ionic bonding. Therefore, it is obtained that the iron and oxygen atoms are primarily connected by ionic bonds.

Bader charge analysis was conducted to quantify the chemical state of each atom in the liquid FeO-SiO2 as shown in Figure 3(c). From the Bader charge distribution (g) of oxygen atoms (Figure 3(d)), it is interesting to find that the Bader charges of oxygen in the FeO-SiO2 silicate are widely distributed within the range from -1.7 to 0.7 e. The Bader charge of Fe atoms is distributed between 0.5 and 1.4e, while Si is mainly concentrated around 3e. It can be found that the charge distribution of Fe and O is relatively wide, so the amount of Fe in the local environment will affect the valence state of O, while Si has little effect on it. Meantime, it is interestingly found that the coordination of Fe shows that there is a part of Fe-Fe coordination in the silicate in Table I. Therefore, the uneven distribution of charges and the cluster effect of cations are a good confirmation of our previous explanations.[18].

2. Comparison of desulfurization mechanism in liquid CaO-SiO2 and MnO-SiO2: An ab initio molecular dynamics simulation

• https://doi.org/10.1016/j.jallcom.2021.163008

3.3. Bader charge of oxygen in molten CaO-SiO2 and MnO-SiO2 silicates

To explore the effect of the two system structures on desulfurization, the bonding situation and the Bader charge of oxygen atoms in the structure before and after sulfur doping was further analyzed.

Firstly, the charge information of the two systems without sulfur doping is calculated. Fig. 4 shows the relaxed structure of liquid CaO-SiO2 and MnO-SiO2 with the charge distribution diagram. The results have eliminated the influence of atomic vibration. From Fig. 4(a), in the CaO-SiO2 system, it can be observed that there are very few charges around calcium atoms, most of the charges are concentrated around oxygen. The valence charge only points in the direction of the oxygen atom closest to the Si atom, which means that a strong polar bond is formed between the silicon and the oxygen atom. In Fig. 4(b), a very strong polar bond between Si and O also was found in MnO-SiO2, however, more electrons around Mn atom compared with Ca, indicating that bonding of Mn-O has a weaker ionicity than that of Ca-O. Due to the existence of the electrons in d orbitals, Mn can devote more electrons than Ca, which is only have electrons in s and p orbitals. Therefore, use the Bader method to further analyze the charge distribution.

The charge distribution of doped and undoped sulfur and the Bader charge of sulfur are shown in Fig. 8. Fig. 8(a)-(d) shows that before doping with sulfur (a and c), the charge density distribution of Ca and O in the CaO-SiO2 system have spherical shape, characteristic of ionic compounds. In the MnO-SiO2 system, the charge density distribution of Mn and O have non-spherical shape, and Mn and O share partial charge. After sulfur doping (b and d), in the CaO-SiO2 system, the charge distribution of Ca still has spherical shape, while the charge around sulfur is less but evenly distributed; however, in the MnO-SiO2 system, the charge around Mn decreases. Comparing (b) and (d) in Fig. 8, the density of states around sulfur in MnO-SiO2 has a triangular shape, indicating that it will be strongly influenced by the outer electrons of Mn. While in CaO-SiO2, the density of states around sulfur has spherical shape. Combining the Bader charge information in Fig. 8(e), the valence of sulfur in the CaO-SiO2 system is around − 1.4 eV, which is lower than that in the MnO-SiO2 system(−0.8~−0.4 eV). Since Ca does not have d electrons, the number of Ca atoms around S does not greatly affect the valence of S. However, the outer d electrons of Mn atoms attract the outer electrons of S atoms to form ionic bonds, increasing the valence of S atoms. Therefore, this phenomenon reveals the nature of the two desulfurization processes. The desulfurization mechanism of the two systems will be discussed in detail below in combination with structural information.

3. Prediction of crystal structure, lattice dynamical, and mechanical properties of CaB2H2

• https://doi.org/10.1016/j.ijhydene.2011.05.038

6. Chemical bonding

The α- and β-CaB2H2 exhibit similar features and in view of that we have only documented the charge density and ELF plots for β-CaB2H2. Fig. 7 and b show the charge density distribution at the Ca, B, and H sites, from which it is evident that the highest charge density resides in the immediate vicinity of the nuclei. Further, the spherical charge distribution shows that the bonding interactions between Ca–H and Ca–B have predominantly ionic character. On the other hand, in the interactions between B–H and B–B are predominantly directional characters. The substantial difference in the electronegativity between Ca and B/H suggests the presence of strong ionic character (i.e., the Ca valence electrons transferred to the H/B sites) and the small difference in the electronegativity between B and H suggests the presence of strong covalent character.

4. Power law relationships between bond length, bond strength and electron density distributions

• https://doi.org/10.1007/s002690050151

The strength of a bond, defined as p=s/r, where s is the Pauling bond strength and r is the row number of an M cation bonded to an oxide anion, is related to a build-up of electron density along the MO bonds in a relatively large number of oxide and hydroxyacid molecules, three oxide minerals and three molecular crystals. As p increases, the value of the electron density is observed to increase at the bond critical points with the lengths of the bonds shortening and the electronegativities of the M cations bonded to the oxide anion increasing. The assertion that the covalency of a bond is intrinsically connected to its bond strength is supported by the electron density distribution and its bond critical point properties. A connection also exists between the properties of the electron density distributions and the connectivity of the bond strength network formed by the bonded atoms of a structure.