Mixed alkali and alkaline-earth borate Li2Sr4B12O23 single crystal

• Calculations show that LSBO crystal is a direct band gap semiconductor.
• Calculated band gap is 4.64 eV (LDALDA), 4.92 eV (GGAGGA) and 5.51 eV (mBJmBJ).
• Calculations bring the birefringence in excellent agreement with experiment.
• mBJmBJ calculations show excellent good agreement with the experimental data.

A comprehensive theoretical investigation of the electronic band structure, density of states, electron charge density distribution and the optical properties for mixed alkali and alkaline-earth borate Li2Sr4B12O23 (LSBO) single crystals were performed. The experimental geometrical structure was optimized by minimizing the forces acting on each atom. Calculations were performed using the full potential linear augmented plane wave plus local orbitals (FPLAPW+loFPLAPW+lo) method within the local density approximation (LDALDA), generalized gradient approximation (GGAGGA) and the recently modified Becke–Johnson potential (mBJmBJ). Our calculations show that LSBO crystal is a direct band gap semiconductor. The calculated band gap is 4.64 eV (LDALDA), 4.92 eV (GGAGGA) and 5.51 eV (mBJmBJ). An earlier calculation using the CASTEP code within LDALDA obtained a band gap of about 4.66 eV. To overcome the well-known LDALDA underestimation of the energy gap we have used GGAGGA and mBJmBJ. We find that mBJmBJ succeed by large amount in bringing the calculated bond lengths in good agreement with the experimental data. Also we found that using mBJmBJ to calculate the optical properties gives a birefringence of about 0.068 (at λ = 586.5 nm) in excellent agreement with the experimental data (0.068 at λ = 586.5 nm). Therefore, we believe that the mBJmBJ calculations reported here show excellent agreement with the experimental data.