Mat. Res. Bull., Vol. 24, pp. 321-330, 1989. Printed in the USA. 0025-5408/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc. PROPERTIES AND STRUCTURES OF R2_xAxCuO 4 PHASES: R = La, Pr and Nd; A = Sr, Pb and Cd J. Gopalakrishnan ", M. A. Subramanian, C. C. Torardi, J. P. Attfield, and A. W. Sleight Central Research & Development Department # E. I. du Pont de Nemours & Co. Experimental Station, P. O. Box 80262 Wilmington, Delaware 19880-0262 (Received December 15, 1988; Communicatedby W.B. White) ABSTRACT The R2CuO 4 phases for R = Pr and Nd were prepared and found to be semiconducting as previously reported. Crystal structures of both compounds were examined by profile refinements of neutron powder diffraction data. A series of phases of the type R2_xSrxCuO 4 which are isostructural with R2CuO 4 for x up to 0.I were prepared and found to remain semiconducting. Chemical analysis indicates the presence of Cu III, yet these materials are not superconducting. This supports the view that R2CuO 4 phases should be regarded as Mott insulators. A new series of phases possessing the tetragonal K2NiF 4 structure appear in the Pr2_xSrxCuO 4 system for 1.0<x<1.25. The new phases are semiconducting. We also find semiconductivity and not superconductivity for La2_xAxCUO4 where A is Pb or Cd. Reasons are discussed for why Ba, Sr, Ca and Na substitutions into La2CuO 4 lead to superconductivity whereas the Pb and Cd substitutions do not. MATERIALS INDEX: copper, lead, cadmium, lanthanum, strontium, praseodymium, oxides Introduction Understanding the new high temperature superconductors requires an understanding of the family of R2CuO 4 compounds (R = ~Permanent Address: Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India #Contribution No.: 4843 321