Aspects of macroscopic phase separation and interstitial oxygen ordering in oxygen doped La2CuO(4 + delta)

摘要

NMR and neutron diffraction measurements reveal that macroscopic phase separation and the tetragonal to orthorhombic (TO) structural phase coincide at two distinct points in the temperature-doping phase plot for oxygen doped La2CuO(4 + delta). Thus the TO phase line coincides with the phase separation line. This is evidence that the macroscopic phase separation is inhibited in the tetragonal phase. We propose that the interstitial oxygen has higher mobility in the orthorhombic phase and that insufficient mobility suppresses macroscopic phase separation in the tetragonal phase. Neutron diffraction measurements also reveal superlattice peaks which indicate ordering of the interstitial oxygen. Our NMR measurements, have demonstrated a distribution of tilts of the CuO(sub 6) octahedra. We propose a sawtooth modulation of the octahedral tilt in which the sign of the tilt changes when the tilt reaches a maximum value can explain this distribution. The large openings in the La-O layer resulting from the abrupt switch of the sign of the tilt provide an attractive location for the interstitial oxygen. This mechanism would lead to stripe ordering of the interstitial oxygen.