Structural and Electronic Properties
BaMgLaBiO6 has a cubic double perovskite structure (general formula \({\text{A}}^{{\prime }}{\text{A}}^{{\prime }{\prime }}{\text{B}}^{{\prime }}{\text{B}}^{{\prime }{\prime }}{\text{X}}_{6})\) with a space group of F-43m. It is the extension of perovskite structure ABX3. The oxygen atoms (O2) are bounded with Ba+2 to form cuboctahedra BaO12 and they share corners with other equivalent BaO12. Similarly, the oxygen atoms (O2) are bounded with Mg+2, La+3, and Bi+5 to form cuboctahedra MgO12, LaO12, and BiO12 that also share corners with other cubooctahedra. The tilt angle of corner-sharing octahedral is\({0}^{0}\). The bond lengths for all Ba-O and Mg-O, La-O and Bi-O are 2.93 \({\text{A}}^{0}\), 2.05 \({\text{A}}^{0}\), 2.93 \({\text{A}}^{0}\) and 2.10 \({\text{A}}^{0}\) respectively. In a linear distorted geometry, O2 is bounded to two Ba+2, one Mg+2, two La+3 and one Bi+5 atom. Figure 1(a) shows the crystallographic unit cell of double perovskite BaMgLaBiO6.
The unit cell of BaMgLaBiO6 has been optimized for the ground state stable atomic configuration and to achieve optimized lattice constants. The following Murnaghan’s equation of state is used to calculate the ground-state lattice parameters.
\(\text{E}\left(\text{V}\right)={\text{E}}_{0}+\left[\frac{{\left(\frac{{\text{V}}_{0}}{\text{V}}\right)}^{{\text{B}}_{0}}}{{\text{B}}_{0}-1}+1\right]-\frac{\text{B}{\text{V}}_{0}}{{\text{B}}_{0}-1}\) | (1) |
where E0 is the minimum energy and V0 is the volume at T=0k. The bulk modulus is B and the pressure derivative of B is B0 at the equilibrium volume. The optimization curve is presented in Fig. 1b. On the parabolic curve, the obtained minimum energy value shows the ground state energy and the corresponding volume is the ground state volume.
In addition, the formation energy is also calculated to check the possibility of synthesizing this compound. The formation energy is basically the difference between the energies of the stable phases of the elements and the total energy of the compound.
\({\text{E}}_{\text{F}\text{E}}={\text{E}}_{\text{B}\text{a}\text{M}\text{g}\text{L}\text{a}\text{B}\text{i}{\text{O}}_{6}}-({\text{E}}_{\text{B}\text{a}}+{\text{E}}_{\text{M}\text{g}}+{\text{E}}_{\text{L}\text{a}}+{\text{E}}_{\text{B}\text{i}}+6{\text{E}}_{\text{O}})\) | (2) |
It can be seen from the calculated formation energy (Table 1) that the studied compound can be synthesized. The calculated lattice parameters, bulk modulus B(GPa), pressure derivative of B and ground state energy\({\text{E}}_{0}\left(\text{R}\text{y}\right)\) are presented in Table 1.
Table 1
Optimized parameters of perovskite BaMgLaBiO6
Lattice parameters\(\text{a} \left({\text{A}}^{0}\right)\) | Bulk modulus B(GPa) | \({\text{B}}^{{\prime }}\) | \({\text{E}}_{0}\left(\text{R}\text{y}\right)\) | Formation Energy |
BaMgLaBiO6 | 7.91 | 153.2 | 4.84 | -7742.214 | -4.11552 |
The band structure is the key factor in describing the electronic behavior of any compound. Figure 2 shows the band structure of BaMgLaBiO6 perovskite calculated with generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) potentials along with the high symmetry points of 1st Brillouin zone.
The conduction band is observed above the Fermi level (0 eV) and the valence band below it. The energy band-gap lies between the maxima of valance and minima of the conduction band. A direct band-gap is observed and it increased from GGA to mBJ potential which confirms the semiconductor nature of this perovskite. The calculated values of band-gap are 2.7 eV and 3.8 eV with GGA and mBJ potential, respectively. The band-gap nature can be further investigated with the density of states (DOS) plots. The understanding of atomic/orbital contribution towards the band structure can be achieved with DOS. Figure 3 shows the total density of states as a function of energy. The selected energy range is from -15 eV to 15 eV. The conduction band is represented with a positive energy range and the negative energy range corresponds to the valence band. A clear band-gap is observed near the Fermi level in the plot of the total density of states with both potentials. The contribution of different states above Fermi level in the conduction band (CB) and below Fermi level in the valence band (VB) start from the energy band-gap. The Ba-d and La-d states show their main contributions in the conduction and valence band. The s and p states of Mg and Bi have contribution in valence and conduction bands, while the O-p state shows its major contribution valence band. The contribution of other states for all atoms is negligibly small.
Electron density plots reveal the bonding nature between the atoms in the compound. The electron density along the (110) plane for BaMgLaBiO6 is shown in figure 4. A week covalent bonding is observed between the Ba, and La atoms. Oxygen (O) atoms show the ionic bonding nature along with the Bi atoms. A strong covalent bonding can be seen between the Bi and Mg atoms. The overall response is comprised of both ionic and covalent bonding.