Since the discovery of superconductivity above 30 K by Bednorz and M\"uller in the La copper oxide system, the critical temperature has been raised to 90 K in Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and to 110 and 125 K in Bi-based and Tl-based copper oxides, respectively. In the two years since this Nobel-prize-winning discovery, a large number of electronic structure calculations have been carried out as a first step in understanding the electronic properties of these materials. In this paper these calculations (mostly of the density-functional type) are gathered and reviewed, and their results are compared with the relevant experimental data. The picture that emerges is one in which the important electronic states are dominated by the copper $d$ and oxygen $p$ orbitals, with strong hybridization between them. Photon, electron, and positron spectroscopies provide important information about the electronic states, and comparison with electronic structure calculations indicates that, while many features can be interpreted in terms of existing calculations, self-energy corrections ("correlations") are important for a more detailed understanding. The antiferromagnetism that occurs in some regions of the phase diagram poses a particularly challenging problem for any detailed theory. The study of structural stability, lattice dynamics, and electron-phonon coupling in the copper oxides is also discussed. Finally, a brief review is given of the attempts so far to identify interaction constants appropriate for a model Hamiltonian treatment of many-body interactions in these materials.

Electronic structure of the high-temperature oxide superconductors

A review is given of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets. These compounds include R2Fe14B, R2Fe14C and R2Co14B and the large class of ternary rare earth compounds having the tetragonal ThMn12 structure. Special emphasis is given to the changes in magnetic properties of R2Fe17 compounds observed after interstitial solution of C or N atoms. The magnetic properties of all these compounds are discussed in terms of current models based on intersublattice and intrasublattice exchange and the interplay between the rare earth sublattice anisotropy and 3D sublattice anisotropy. A substantial portion of the review is devoted to manufacturing routes of permanent magnets and a description of the coercivity mechanisms operative in the magnets. A comparison is made of the performance and economic costs of various types of magnets and novel applications are briefly discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/54/9/001/pdf

New developments in hard magnetic materials

Calorimetric and IR spectroscopic studies of phase transitions in methylammonium trihalogenoplumbates (II)

Structural changes of the human retinoblastoma gene have been demonstrated previously in retinoblastoma and some clinically related tumors including osteosarcoma. Structural aberrations of the retinoblastoma locus (RB1) were observed in 25% of breast tumor cell lines studied and 7% of the primary tumors. These changes include homozygous internal deletions and total deletion of RB1; a duplication of an exon was observed in one of the cell lines. In all cases, structural changes either resulted in the absence or truncation of the RB1 transcript. No obvious defect in RB1 was detected by DNA blot analysis in primary tumors or cell lines from Wilms' tumor, cervical carcinoma, or hepatoma. These results further support the concept that the human RB1 gene has pleiotropic effects on specific types of cancer.

http://science.sciencemag.org/content/sci/242/4876/263.full.pdf

Structural rearrangement of the retinoblastoma gene in human breast carcinoma.

Probably the most unusual crystal-chemical aspect of the new high-temperature oxide superconductors is their significant variability in oxygen content. For the YBa2Cu3O7–δ family of compounds, the oxygen stoichiometry varies in the range 0 < δ < 1 under normal synthetic conditions, with the optimal 91-K superconductive transitions obtained near δ=0 (see, for example, ref. 1). Work presented so far on the dependence of both superconducting and normal-state properties on oxygen stoichiometry has considered only the value of δ in YBa2Cu3O7–δ. Here we present data that indicate the issue to be considerably more interesting and complex, and propose that the microscopic variables thus far not considered include the distribution of oxygen over the available sets of sites in the structure, with that distribution strongly dependent on synthetic conditions. In addition to the dependence of the bulk transition temperture (Tc), normal-state resistivity and crystallographic symmetry on the microscopic distribution, we find the magnitude of the magnetic susceptibility in the normal state to be strongly affected by the microscopic state of the oxygen subsystem. We also report the existence of a single-phase 60-K bulk superconductor at stoichiometry YBa2Cu3O6.6–6.7, which we postulate to have an ordered array of oxygen vacancies, and two distinct plateau regions in the variation of Tc with x which are either unobserved or only hinted at under other synthetic conditions.

Oxygen stoichiometry, superconductivity and normal-state properties of YBa2Cu3O7-δ

Materials within the phase diagram Y2O3–BaO–CuO have recently generated great interest because of the observation of superconductivity with a critical temperature (Tc ) of ≳90 K1,2; the current consensus is that YBa2Cu3O7−x (x ≈ 0.2) is the superconducting phase (see, for example ref. 3). Hazen et al.4 concluded, from a single-crystal X-ray diffraction experiment, that its structure is based on the corner-linked octahedral perovskite structure: the tetragonal unit cell (space group P&4bar;m2) arises from a tripling of the c-axis resulting from the ordering of yttrium and barium cations and associated oxygen defects. Here we present high-resolution neutron powder diffraction data for this phase with x = 0.15(7). Our results agree with ref. 4 with respect to cation positions, but the location of one set of oxygen defects is substantially different and necessitates the lowering of lattice symmetry from tetragonal to orthorhombic (space group Pmmm), in agreement with separate findings5–8. In contrast to the 35-K superconductor, La1.85Ba0.15CuO4, the corner-linked CuO4 planar groups are connected not only as sheets in the a–b plane but also as chains parallel to the ft-axis. The average copper valence is 2.23 (5). From bond valence arguments we infer that the Cu2+ and Cu3+ ions preferentially occupy square pyramidal and square planar sites respectively. Crystal chemical considerations suggest that these structural features may be involved in the superconducting mechanism.

Structure and crystal chemistry of the high-Tc superconductor YBa2Cu3O7−x

The compound YBa2Cu3O7–δ has recently received intense interest because of its high-temperature (Tc≈95K) superconducting properties1. Shortly after its discovery, many authors independently confirmed that the structure could be regarded as an ortho-rhombically distorted tripled perovskite2–5. We have investigated La3Ba3Cu6O14+x because it is chemically similar to YBa2Cu3O7–δ and because initial work6 indicated a poor structure determination based on a cell not dissimilar from YBa2Cu3O7–δ. Here we report time-of-flight neutron powder diffraction results which confirm that the structure of La3Ba3Cu6O14+x is isomorphous with the tetragonal variant of YBa2Cu3O7–δ; in particular, the copper coordination and calculated valencies of both compounds agree closely. The apparent contradiction of stoichiometries between these two phases is resolved by ordering of the large cations consistent with a formulation La(La0.25Ba0.75)2Cu3O7+1/2x. Stoichiometries for both quenched and oxygen-annealed materials are in excellent agreement with those obtained by Er-Rakho et al.6, yielding more than seven oxygens per formula unit. The excess oxygen is located in the planes containing the chains of CuO4 planar groups in YBa2Cu3O7–δ. The present results extend the upper limit of oxygen stoichiometry from RA2Cu3O7 to RA2Cu3O7.2, but the materials studied here remain semiconducting down to 7 K (Fig. 1). Thus, although there is a remarkable structural stability from RA2Cu3O6 (ref. 7) to RA2Cu3O7.2, superconductivity appears to be confined to the range RA2Cu3O6.5 to RA2Cu3O7.

The structure of the non-superconducting phase La 3 Ba 3 Cu 6 O 14+x and its relation to the high- T c superconductor YBa 2 Cu 3 O 7–δ

High resolution neutron powder diffraction studies of manganese-and chromium-substituted Y 2 Fe 14 B compounds reveal the presence of a substantial preferential site occupation of manganese and chromium on the six non-equivalent transition metal sites. In particular, the 8j 2 site is found to be well populated by manganese and chromium atoms. The observed distribution of magnetization in manganese-substituted compounds suggests marked local environmental effects and a substantial contribution of manganese to the site magnetization. A ferromagnetic coupling at the 8j 2 site and a more complex situation at the other sites is suggested. An even more enhanced local environmental effect on the distribution of the magnetization is observed in chromium-substituted compounds.

Magnetic structure and preferential site occupation in manganese- and chromium-substituted Y2Fe14B compounds

High Resolution Powder Diffraction at ISIS

High resolution powder diffraction experiments were carried out on the clathrate compound [C6D4(OD)2]3(D2S)0.96 at several temperatures. Above the 7.6 K phase transition the molecules are preferentially orientated lying flat in the ab plane of the R 3 structure and are disordered between six equivalent sites. Below the transition the molecular orientation becomes more localised, probably between 2 sites.

W. T. A. Harrison

Papers

Structure and crystal chemistry of the high-Tc superconductor YBa2Cu3O7−x

The structure of the non-superconducting phase La 3 Ba 3 Cu 6 O 14+x and its relation to the high- T c superconductor YBa 2 Cu 3 O 7–δ

Magnetic structure and preferential site occupation in manganese- and chromium-substituted Y2Fe14B compounds

High Resolution Powder Diffraction at ISIS

Neutron scattering studies on the hydroquinone clathrate compound of deuterium sulphide