Pressure-stable optical anisotropy and band gap tunability in NaBi(S,Se,Te)2 and mixed-chalcogen NaBiXY compounds

Abstract

We report a first-principles investigation of the structural, electronic, and optical properties of NaBiXY (X, Y=S, Se, Te) ternary chalcogenides, with a focus on optical anisotropy and pressure response. Total energy and formation energy calculations reveal that all compounds, except NaBiS2 and NaBiSe2, favor a trigonal crystal structure over the cubic NaCl-type phase. All systems exhibit indirect band gaps, with values tunable through chalcogen substitution. NaBiTe2 presents the smallest gap, while NaBiSeS has the largest. Band gap variation under hydrostatic pressure is nearly linear across the series. Optical property analysis shows pronounced dielectric anisotropy, particularly in the low-energy region below 6 eV, with the in-plane component (εxx) exceeding the out-of-plane component (εzz). Substituting Te with Se or S shifts the primary peaks of the dielectric function to higher photon energies with reduced intensity. The optical spectra remain stable under moderate pressure, highlighting environmental robustness. These results identify NaBiXY compounds as promising pressure-resilient and compositionally tunable semiconductors with directional optical behavior, suitable for future photonic and optoelectronic applications © 2025 The Authors