Band gap closing in La2-xSrxNiO4+δ

Structural and electrical transport measurements are reported for the layered La2-xSrxNiO4+δ oxide for a range of Sr and oxygen contents. The electrical properties change from semiconductor-like for x < 0.8 to metallic for larger Sr content. The room temperature Seebeck coefficient changes from semiconducting and positive for small x toward metallic and negative for large doping levels. From analysis of the temperature dependence of the Seebeck coefficient the authors conclude that the transition from the semiconductor to the metallic side of the phase diagram is achieved by the closing of the band gap. 27 refs., 7 figs., 1 tab