The Crystal Structure of Superconducting La2Cu04.032 by Neutron Diffraction

The structure of superconducting La,CuO,,o,, has been determined by single- crystal neutron diffraction data. The excess oxygen, 04, is located between two adjacent La01 layers. The presence of 04 distorts the apical-oxygen 01 sublattice in such that each 0 4 forms a short bond (1.64A) with one 01, which indicates the formation of a strong covalent bond between these two atoms. The Cu and 0 2 sublattices as well as that of La are not affected by 04, however, 1.6% of the La cations increase their coordination from 9 to 10.

It is well established today that La2Cu04 becomes superconducting with Tc's between 28 K and 40 K either by cation doping on the La-sites or by raising the oxygen content to more than 4. The superconducting properties of "undoped" La2Cu04, synthesized under high oxygen pressure, were first reported by Beille et al. [l]. This publication was followed by several others on undoped La2Cu04 samples synthesized under slightly different oxygen-rich conditions [2, 31. Samples of La2Cu04+6 with somewhat appreciable Meissner effects were obtained reproducibly by Schirber et al. [4] and by Demazeau et al. [5], who carried out the synthesis under high oxygen pressure (a few kbar) at high temperature (500-8OOOC).
Because of the uncertainties about the position and the bonding of the excess oxygen in superconducting La2Cu04+, samples, we have carried out the crystal structure determination of a superconducting La2Cu04,032 single crystal by neutron diffraction data taken at room temperature and 15K.
The crystal was grown in a CuO flux [4]. It was then treated at 500°C under oxygen pressure (pOz = 3 kbar) for 62 h and cooled slowly to room temperature.
The superconducting transition temperature for the single crystal was obtained by a.c. susceptibility measurements. The volume of the superconducting phase was estimated to be about 70% with an onset at 37.5K (Fig. 1).
The neutron diffraction intensities were collected at room temperature and 15 K using the D-9 four-circle diffractometer at the ILL reactor (A = 0.484 17 A). The intensities of ten of the strongest reflections were measured every 5K on both side of the superconducting transition temperature. No measurable change, indicating a structural transition, was detected. The crystal was twinned but the systematic abscences of the space group Cmca allowed the determination of the percentage of each individual in the crystal (a value of 0.46 was obtained in the final refinement).
The refinement of the cation sites occupancy indicates that the La/Cu ratio is exactly 2 in this crystal. Since the cation lattices do not compromise any vacancies, the structure must contain an excess oxygen. We chose to localize this oxygen excess from crystal chemical considerations (analogy to the structure of La2Ni04+, [6]) and then try to refine its position (1/4 y 1/4). We used the low temperature data because in this case the thermal effects are minimized.
We found an excess oxygen 0 4 at x = 1/4, y = 0.243, z = 1/4 with a population p 0 4 = 0.016(2). We found also that the site 0 3 at x = 0.031(5), y = 0.182(2), z = 0.101(5) was occupied with a population p03 = 0.023(2) x 2. Since this position is just 0.75 8, from that of 01 (apical oxygen of the octahedron) it can only be occupied when 01 is empty. Therefore 0 3 should be interpreted as the result of displacements of some of the 0 1 anions. Very likely, the excess oxygen 0 4 is responsible for these displacements. From the ratio of the occupancy factors of 0 3 and 0 4 , it can be deduced that for each 0 4 three 0 1 are displaced to 0 3 .
The results based on the 15 K data after the final refinement are reported in Table I. The results based on the room temperature data were in agreement with the low temperature ones.

Physica Scripta T29
The present structural study shows that each 0 4 atom forms an Oz grouping with one 03. Since the formal valence of such grouping is taken at -2, the oxygen doping does not bring about any change in the formal cation valence. For every 0 4 that enters the structure as -1, one 0 3 decreases its valence from -2 to -1. anions as in the case of La2Cu04,032. However, qualitatively it can be stated that the increase of the coordination number of some of the La cations and the readjustment of the corresponding La-0 distances, should correspond to a readjustment of the electrostatic charges around the La cations. La valence calculations using the Zachariasen formula indicate an average total increase of 0.3 V.U. Since the Cu sublattice is only slightly affected by the introduction of 04, all the cation valence increase occurs on the La sublattice. When the structure of YBa,Cu,O, was determined in detail it was reasonable to attribute the extra charge to the Cu sublattice, but, as indicated later by NMR, EXAFS, and XPS measurements, it actually corresponds to holes in the oxygen 2p band. In the present case as well the cation charge excess could be the result of holes in the same band.
The above discussion is valid only for a model where the distortion induced by a given 0 4 does not overlap with that induced by one of the adjacent 04. If these atoms are distributed isotropically in the structure, then one every eighth cell contains an 0 4 and the corresponding distortion. Consequently, there are not large 04-free areas.
Jorgensen et al. [9] have shown that at 320K a phase separation occurs. From their data they argue that one phase is superconducting while the other is not. The powder pattern can only be indexed on two sets of lattice parameters: the a and b values are identical for the two sets while the two c values differ of about 1/5 x (a-c). The single-crystal and the powder diffraction data could be reconciled if one assumes that the 0 4 atoms are in some way more clustered together than described above. The superconducting samples would be comprosed of two types of domains, one with the 0 4 excess and the other without and these domains would be larger than the diffraction correlation length.
Because of our experimental conditions (short wavelength) we could not possibly detect the phase separation, even if present in our sample. Neutron diffraction work on crystals exhibiting better Meissner effect than that of the present crystal, and using a longer wavelength is in progress.