Structural aspects of the phase separation in La2CuO4.032

Abstract The average structure of superconducting La 2 CuO 4.032 has been determined by single-crystal neutron diffraction data. The excess oxygen is located between two adjacent LaO layers. Its presence distorts the apical-oxygen sublattice in such a way that a short O O bond is formed (1.64A). By scanning several hkl reflections, we have confirmed that a phase separation occurs below room temperature. The peaks of one phase are in agreement with the space group Cmca and the unit cell proposed by Jorgensen et al. and Cox et al.. However, a monoclinic unit cell is needed to index the second phase reflections.


INTRODUCTION
It is well established today that La2CuO4 becomes superconducting with Tc's between 28K and 40K either by cation doping on the La-sites or by raising the oxygen content to more than 4.

NEUTRON DIFFRACTION
We determined the structure of a superconducting (To --40K) La2CuO4.032single crystal from neutron diffraction data taken at room temperature and 15Kl.(Figure 1).Cmca space group was used in the refinement.We found that the La/Cu ratio is exactly 2 and that an excess oxygen 04 is present at x--l/4, y=0.243(3), z--l/4 with an occupation factor p(O4)--0.016 (2).We found also that the site 03 at x=0.031(5), y=0.182(2), z=0.100(5) was occupied with an occupation factor p(O3)=0.024(2) x 2. Since this position is just 0.75A from that of O1, it can only be occupied when O1 is empty.We found indeed that p(O1)+p(O3)=l.P(O3) is also equal to 3 p(O4).Consequently, it seems that each 04 is The structure of La2CuO4.032.Only half cell is shown.
responsible for the displacement of three O1 to 03.If 04 is surrounded by three 03 and one O1, one distance 03---04 is =1.64/~ which is indicative of a strong covalent bond and of the formation of an (02) 2-peroxyde grouping.

PHASE SEPARATION
Jorgensen et al.2 have shown from powder neutron diffraction studies that a phase separation occurs near 320K~ They indexed the neutron powder pattern of the two phases using Cmca space group for one (non superconducting, phase 1), and Fmmm space group for the other one (superconducting, phase 2).Their results were corroborated by Cox et al.3 who used x-ray synchrotron powder diffraction data taken at Brookhaven NSLS.
In order to verify whether or not our La2CuO4.032crystal presented also the phase separation we have scanned several hkl reflections as a function of temperature (300K-120K) using xray (~CuKtx) and neutron (~,= 1.265,~) diffraction.We confirmed Jorgensen and Cox results about the phase separation, which means that the structure determined from neutron data 1 is only an average structure.However, our results are not compatible with the Fmmm space group.We have tried to refine the structure of La2CuO4.032 using Fmmm space group and the neutron single crystal data.The results are not as satisfactory as those obtained with Cmca.An improvement is obtained when the oxygen atoms are allowed to vary over the general xyz position, one of them coresponding to Cmca.We also noticed that the reflections forbidden by F centering were not systematically calculated stronger than observed when Cmca space group was used, which would be the case if these reflections belonged to one phase only.Furthermore, 0kl reflections with k even and 1 odd, which are forbidden by F centering, exhibit also phase separation (Figure 2).This was more clearly observed on neutron data as these reflections are mainly due to the oxygen atoms.The peaks were fitted with Gaussian functions.Those corresponding to the nonsuperconducting phase (Cmca) can be perfectly indexed with Cox et al. and Jorgensen et al. cell parameters.On the contrary, a monoclinic unit cell (a unique axis) is needed to index the second-phase reflections.This phase transition implies that an additional twinning FIGURE 1 FIGURE 2 (o-20 scan of 0 14 5 reflection at 300K and 120K (neutron diffraction).Crosses represent experimental points.The arrows indicate the peaks' positions as determined from fitting with Gaussian functions.