From Co ₃ O ₄ to ZnCo ₂ O ₄ : Evolution of Ram

ABSTRACT

Despite extensive Raman studies of Co ₃ O ₄ , uncertainties remain in the vibrational‐mode assignments of Co ₃ O ₄ and ZnCo ₂ O ₄ , particularly concerning infrared‐active phonons, cation disorder, and high‐wavenumber bands. Hybrid density functional theory calculations were therefore employed to investigate their structural, electronic, magnetic, Raman, and infrared properties under different magnetic and cation configurations. The calculations reproduce the antiferromagnetic ground state of Co ₃ O ₄ and the nonmagnetic closed‐shell character of normal ZnCo ₂ O ₄ . Cation inversion in ZnCo ₂ O ₄ substantially reduces the band gap and introduces magnetic tetrahedral Co ²⁺ states. The calculated Raman and infrared spectra agree well with available experimental data and demonstrate the sensitivity of the vibrational response to tetrahedral‐site occupancy and local electronic structure. Zn substitution modifies the high‐wavenumber Raman intensity distribution, whereas inversion generates additional Raman features and shifts the mode from approximately 734 to 706 cm ⁻¹ , indicating distortion and weakening of the octahedral Co–O framework. Pronounced LO–TO splitting is also predicted for the high‐wavenumber polar phonons in both spinels. Eigenvector analysis reveals cooperative angular distortions within the interconnected lattice, demonstrating that wavenumber alone cannot identify the microscopic vibrational character. These results clarify the effects of Zn substitution and cation inversion on cobalt‐spinel lattice dynamics.