First-principles study of ABSe₃ (A = Li, Na) Perselenoborates: A Promising Family of Nonlinear Optical Materials

The properties of perselenoborates ABSe₃ (A = Li, Na) are investigated for the first time using density functional theory (DFT). These compounds crystallize in the orthorhombic system, adopting the Pca2₁ and Pna2₁ space groups for LiBSe₃ and NaBSe₃, respectively. A plane-wave pseudopotential (PW-PP) approach combined with the GGA-PBE functional employed to examine their structural, elastic, electronic, and optical properties. In parallel, the linear combination of atomic orbitals (LCAO) method used to explore the electronic band structure with GGA-PBE, HSE06, and B3LYP functionals, as well as the second harmonic generation (SHG) response using GGA-PBE.

The results reveal that both compounds are indirect semiconductors, with band gap values that vary depending on the functional used. The density of states (DOS) analysis confirms the covalent nature of the BSe bonds. Optical calculations show significant birefringence in both materials, and the (BSe₄)⁵⁻ anionic units found to dominate the SHG response and birefringence, while the alkali metal cations have minimal influence. Mechanical analysis confirms the structural stability and elastic anisotropy of both phases. Thermal calculations indicate low thermal conductivity and relatively low melting points for LiBSe₃ and NaBSe₃, which may limit their use in high-temperature environments.

Overall, these findings suggest that LiBSe₃ and NaBSe₃ are promising candidates for nonlinear optical applications, particularly in frequency conversion and birefringent devices. Their combination of anisotropic optical response, semiconducting behavior, and low thermal conductivity supports their potential for integration into thermally sensitive photonic systems.