Structural transitions in (La,Ln)₂CuO₄ and La₂(Cu,Ni)O₄ systems

Abstract

Solid solutions of the formula La_2−xLn_xCuO₄ (Ln=Pr, Nd) possess the orthorhombic structure of La₂CuO₄ for small values of x and transform to the tetragonal Nd₂CuO₄ structure at a critical value of x. At the critical composition, there is an abrupt change in specific volume as well as the c̲/a̲ ratio. The material exhibits temperature-independent electrical resistivity below the critical value x and semiconducting behaviour above it. The specific volume and c̲/a̲ ratio smoothly decrease with increase in x in the La₂Cu_1−xNi_xO₄ system, although the solid solution possess the tetragonal K₂NiF₄ structure when x>0.1. Compositions with x>0.1 exhibit a gradual semiconductor metal transition similar to that of La₂NiO₄, the transition temperature decreasing with increasing x.