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

• A symmetry-balanced equiatomic In 0 . 5 Ga 0 . 5 Se phase is established as a reference system for disentangling symmetry-driven electronic–optical–phonon coupling in layered III–VI chalcogenides. • First-principles calculations reveal a direct band gap of ∼ 1.9 eV and demonstrate that optical anisotropy originates from symmetry-constrained Se p x ,p y –cation s hybridization. • The dielectric zero-crossing points are identified as interband plasma-like resonances rather than free-carrier (Drude) effects. • Phonon dispersion confirms dynamical stability, while Raman-active modes are assigned through symmetry analysis and atomic displacement patterns. • Calculated phonon frequencies show good agreement with experiment, with deviations below 5%. 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, In 0 . 5 Ga 0 . 5 Se 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 Raman-active 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.