Symmetry-balanced electronic structure, optical anisotropy and lattice dynamics of equiatomic In–Ga–Se: A first-principles study

Equiatomic cation ordering in layered III–VI chalcogenides provides a unique platform for disentangling the intrinsic coupling between electronic states, optical anisotropy, and lattice vibrations. In this work, In0.5Ga0.5Se is employed as a symmetry-balanced reference system to clarify how equal In/Ga substitution modifies the microscopic origin of optical and vibrational responses in layered semiconductors. First-principles calculations show that the equiatomic composition preserves a direct band gap of approximately 1.9 eV. The band-edge electronic structure is governed by hybridized Se p and In/Ga s states, which determine the symmetry-allowed optical transition channels and enhance the polarization-dependent dielectric response. The calculated dielectric spectra reveal a pronounced contrast between in-plane and out-of-plane optical excitations. This anisotropy originates from symmetry-constrained interband transition channels and the preferential in-plane orientation of Se p orbitals, rather than from compositional asymmetry. Phonon calculations combined with Raman spectroscopy further demonstrate that equiatomic cation balance leads to a characteristic redistribution of vibrational spectral weight, enabling reliable identification of Ramanactive modes and their displacement patterns. The absence of imaginary phonon frequencies confirms the dynamical stability of the equiatomic lattice. The phonon-derived heat capacity follows Debye-like behavior at low temperatures and approaches the Dulong–Petit limit at elevated temperatures. By isolating symmetry effects from compositional imbalance, the present results provide a physically transparent reference for understanding electronic–optical–phonon coupling in In–Ga–Se solid solutions and related layered chalcogenide semiconductors.