Articles written in Bulletin of Materials Science

    • Physical properties of the semiconducting delafossite AgNiO$_2$


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      The delafossite AgNiO$_2$ was prepared by hydrothermal route. The X-ray diffraction shows a single phase indexed in a rhombohedral unit cell (R$\bar{3}m$) with a particle size of 12 nm. The Raman spectroscopy confirmed the singlephase. 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 ${\rightarrow}$ 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 ${\sim}$5 kOe, suggesting a paramagnetism of AgNiO$_2$ nanocrystallites with a low spin (LS) Ni$^{3+}$ 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 (${\sigma}_{300K}$ = 1.8 ${\Omega}^{–1}$ cm$^{–1}$) is in conformity with the non-cooperative effect of the Jahn–Teller Ni$^{3+}$ 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.

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