CONTROL ON THE COPPER VALENCE AND PROPERTIES BY OXYGEN-CONTENT ADJUSTMENT IN THE LACUO3-Y SYSTEM LESS-THAN-OR-EQUAL-TO-Y-LESS-THAN-OR-EQUAL-TO-0.5)

The LaCuO3-y perovskite is considered as the first member of the O1(n- 1)n series of ''layered cuprates.'' Highly oxidized, stoichiometric La CuO3 is stabilized under very high oxygen pressures, and was synthesiz ed in a cubic-anvil-type high-pressure apparatus at 5 GPa and 1400 deg rees C using excess amounts of KClO4 as an external oxidizing agent. U pon heating under ambient pressure the rhombohedral high-pressure phas e loses oxygen yielding tetragonal, monoclinic, and orthorhombic forms of LaCuO3-y as intermediate products before the final decomposition i nto La2CuO4 and CuO or Cu2O around 800 degrees C. All three oxygen-def icient LaCuO3-y phases could be isolated and their stability limits an d corresponding oxygen contents conveniently investigated by annealing stoichiometric LaCuO3 in a thermobalance of high sensitivity in order to in situ detect the exact amount of oxygen loss. The nominal copper valence values calculated from the oxygen contents are compared and d iscussed with XPS data as well as with the results evaluated from magn etic susceptibility measurements. (C) 1997 Academic Press.