UC Irvine

A two-phase refinement of a crystal for the compound La2CuO4+δ (δ=0.032), based on the neutron diffraction data collected at 15 K with the D9 diffractometer at ILL, using {double barred pipe}=0.48 A ̊, has been carried out. One phase (30%) consists of stoichiometric La2CuO4 domains, while the other (70%) of oxygen-rich La2CuO4.048 domains. The percentages of each phase, which have been refined together with the other parameters (scale factor, positional parameters, thermal factors and occupancy factors of the oxygen atoms), agree very well with the value determined from χAC measurements. The La2CuO4.048 structure is essentially the same as the average structure reported in ref. [1], the only difference being the oxygen content which is found to be δ= δ 0.70. The extra oxygen, O(4), is found to be in between two LaO layers in a similar position as the oxygen atoms located between two Nd layers in the N2CuO4 structure. The insertion of extra oxygen causes the displacement of some of the oxygen O(1) towards the O(3) positions. Different models are proposed for the distortion induced by this insertion according to the experimental value, 3.3 [6], found for the ratio of the amount of O(3) to that of O(4). If this ratio is assumed to be 3 and the O(3) atoms are localized about the insertion, then the formation of a short O(4)-O(3) bond would occur. The models not requiring the formation of the short bond correspond to ratios of 2 or 4. In the latter case there would be four displaced O(1), but due to the rigidity of the oxygen octahedra only two O(3) would be bonded to O(4), the other two being the apically opposite oxygen atoms of the same octahedra. © 1990.