NEW PHASES INDUCED BY HYDROGEN REDUCTION AND BY SUBSEQUENT OXIDATION OF LA2CUO4, PR2CUO4, ND2CUO4, SM2CUO4, EU2CUO4, GD2CUO4

We report the synthesis of new compounds L2CuO3.5 (L=Pr,Nd,Sm,Eu,Gd) and La2CuO3.67, obtained by heating conventionally prepared L2CuO4 powders in a 5 mol % H2/He atmosphere at low temperatures (300°C). The structures of the La, Pr, and Nd compounds appear related to that of Sr2CuO3, whereas those of the Sm, Eu, and Gd compounds are different. A new Nd2CuO4-type (T) La2CuO4+ system was synthesized by oxidation of La2CuO3.67 at 300 500°C; the oxygen contents are exceptionally high, ranging from 4.10 to 4.42. Annealing La2CuO3.67 above 620°C in oxygen converts it to the original K2NiF4-type (T-phase) structure, but with excess oxygen (up to 4.06) and with smaller orthorhombicity compared to the La2CuO4 starting material. Oxidation of the other L2CuO3.5 phases (L=Pr,Nd,Sm,Eu,Gd) above 300°C leads to recovery of the original T structures, with oxygen contents ranging from 3.98 (Eu,Gd) to 4.03 (Nd). Magnetization measurements in a field of 50 G did not reveal superconductivity in any of the samples above 5 K. © 1990 The American Physical Society.