Steric and electronic effects relating to the Cu2+ Jahn-Teller distortion in Zn1-xCuxAl2O4 spinels

Zn1-xCuxAl2O4 (0 <= x <= 1) compositions have been synthesized by solid-state route, and a color scale going from white (x = 0) to brownish-red (x = 1) with intermediate colors as pale green for x = 0.10 and pale brown for x = 0.30 can be observed. XRD-data refinements on the whole solid solution have allowed defining two critical areas where structural features such as cell parameter and inversion rate as well as cation-oxygen bond distances in 8a and 16d sites of the spinel network exhibit clear unique variations. By observing the direct environment of tetrahedral and octahedral sites potentially occupied by Cu2+ Jahn-Teller cations, the two critical compositions have been estimated to x(1) =1/6 and x(2) = 4/7. Actually from the x(1) and x(2) copper contents, the probability of getting Cu2+-Cu2+ pairs involving tetrahedral-octahedral or two octahedral sites respectively is high. Then, the stabilization of Cu2+ JT ions in distorted octahedral site, identified by ESR, is in competition with the occurrence of Cu2+ in tetrahedral sites, and both electronic and steric effects of Cu2+ JT cations lead to the explanation of the evolution of the inversion rate in this series. The study of the optical absorption properties clearly shows that the position and the intensity of the various absorption bands are influenced by the distribution of Cu2+ ions in tetrahedral and octahedral sites, the creation of Cu2+-Cu2+ pairs around the x(1) and x(2) critical compositions, and the increasing of Cu-O bond covalency. Such an evolution of structural features correlated with electronic properties where various critical compositions have been identified can be generalized to other spinel oxides, considering local distortion around Jahn-Teller ions.