Structural study in the (La,Nd) 2− xSr xCuO 4 system

Abstract

The presence of layers of corner-sharing square CuO 4 units is a common structural feature of the superconducting copper oxide ceramics. There are three closely related structure types with stoichiometry (RE) 2CuO 4, each containing isolated sheets of fourfold (T ′ structure), fivefold (T ∗ structure) or sixfold (T structure) Cu-O coordination. For the longest rare earth cation (La), (RE) 2CuO 4 crystallizes in a slightly distorted K 2NiF 4 T structure. Change of the lanthanide ion from La 3+ to Pr 3+ or Nd 3+ leads to the closely related structure called T ′ structure (8). This is due to the intermediate size of the rare earth ion in the range (Pr–Gd). The relative simplicity of these rare earth systems allows us to examine how structure is related to ionic size effects and cation ordering. The study of the solid solubility of La 2CuO 4 (T) and Nd 2CuO 4 (T ′) provides a measure of the relative stabilities of these structural types. This work reports the crystal structure of the La 2− x− y Nd y Sr x CuO 4 ( x=0.1, y=0.1, 0.3), determined by neutron powder diffraction ( T=10 K, RT), with data obtained at Studsvik Neutron Research Laboratory (Sweden). These specimens crystallize in the Cmca space group, showing the T structure and thus the solubility of Nd in this network.