Electronic structure, lattice dynamics and thermodynamic stability of paramelaconite Cu4O3

• Antiferromagnetic frustration confirmed by calculating exchange coupling constants.
• Mixed valency elucidated by analyzing spin-orbital character of hole states.
• Main features of ab initio vibrational spectrum assigned to ionic species.
• Vibrational entropy shown to be crucial for decomposition reaction of Cu4O3.

An ab initio study of the electronic structure, lattice dynamic and thermodynamic properties of paramelaconite Cu4O3 is reported. The insulating, mixed-valence character of Cu4O3 is elucidated by analyzing the band structure and the spin-orbital symmetry of the Cu-3d hole states. Exchange coupling constants between Cu2+ ions are computed which confirm the frustrated antiferromagnetism of the spin lattice. The lattice dynamics is studied from first principles and main features of the vibrational spectrum are assigned to the different chemical species Cu+, Cu2+ and O. The thermodynamic stability of Cu4O3 is investigated by calculating the free energy of the decomposition reaction into CuO and Cu2 O as a function of temperature. The results agree with the available experimental data and indicate that the vibrational entropy plays an essential role for the decomposition reaction, which occurs at elevated temperatures.