Physical properties of the semiconducting delafossite AgNiO₂

Abstract

The delafossite AgNiO₂ was prepared by hydrothermal route. The X-ray diffraction shows a single phase indexed in a rhombohedral unit cell (${\text{R}}\overline{3}m$) with a particle size of 12 nm. The Raman spectroscopy confirmed the single phase. The thermal analysis shows a stability up to 290°C. The forbidden band (0.87 eV), determined from the diffuse reflectance, is assigned to the transition: Ag⁺: 4d → hybridized (d_z2–s) – O^2–: 2p orbital. The magnetization M(H), measured at different temperatures, exhibits a low hysteresis at 200°C with a weak remanence of 495 Oe. It increases with the applied field to saturate at ~5 kOe, suggesting a paramagnetism of AgNiO₂ nanocrystallites with a low spin (LS) Ni³⁺ configuration. The thermal variation of the electrical conductivity indicates a semiconducting behaviour with an activation energy (E_a) of 0.013 eV. The high conductivity (σ_300K= 1.8 Ω^–1 cm^–1) is in conformity with the non-cooperative effect of the Jahn–Teller Ni³⁺ ion. The thermo-power shows p-type behaviour coming from oxygen intercalation in the layered lattice. The conduction occurs by polaron hopping between mixed valences Ag^2+/+ and increases with raising temperature, in agreement with a degenerate semiconductor. The valence band, determined from the capacitance measurements in KOH (0.1 M) electrolyte, is made up of Ag⁺: 4d orbital, located at – 4.52 eV below vacuum.