Defects in divided zinc-copper aluminate spinels: Structural features and optical absorption properties

Zn1-xCuxAl2O4 (0 <= x < 0.30) compounds have been synthesized by polyesterification using metallic salts and annealing at low temperatures as well as by conventional solid state. XRD-powder data refinements (Rietveld method) have demonstrated that both compound series crystallize in the spinel structure (Fd (3) over barm) and exhibit similar inversion rates. This low-temperature route lead to metastable phases with crystallite sizes around 40 nm whereas particle sizes are larger than 1 mu m in the case of solid-state route. This preparative method largely described in the literature allows stabilizing reduced copper states thanks to the presence of reductive organic species, which are decomposed below T = 700 degrees C. The absorption spectra of the x = 0.15 composition exhibit strong differences depending on the synthesis route. These differences can be explained by the occurrence of Cu2+/Cu+ mixed valencies in compounds prepared by the low-temperature route; 33% of monovalent copper has been identified in the x = 0.15 composition prepared by low-temperature process, whereas the solid-state compound contains only divalent copper. Reductive properties of polyesterification reaction implying citric acid and low annealing temperature (T = 700 degrees C) are mainly responsible of the occurrence of the Cu2+/Cu+ mixed valencies. Actually, the annealing under air at T = 1000 degrees C of divided zinc-copper aluminates prepared at low temperatures (T = 700 degrees C) leads to the oxidation reaction Cu+ -> Cu2+ + e(-) confirmed by the evolution of magnetic measurements, ESR spectra, and optical absorption properties. Defects such as oxygen vacancies in the anionic network leading to reduction in the cations coordination number could also explain the strong evolution of optical absorption spectra especially around lambda = 700 nm where intervalencies transfer (Cu+/Cu2+) as well as intra-atomic d-d transitions (Cu2+ in a 5-fold coordination) can occur. Finally the occurrence of monovalent and divalent copper at the surface of such divided oxides, probably in tetrahedral sites, has been demonstrated by FTIR spectroscopy using the co-adsorption of CO and NO as probe molecules.