The structure-property relationships of several conducting transition metal oxides, as well as their preparative methods, are presented in this paper. The importance of preparing homogeneous phases with precisely known stoichiometry is emphasized. A comparison is also made of the various techniques used to prepare both polycrystalline and single crystal samples. For transition metal oxides, the metallic properties are discussed either in terms of metal-metal distances which are short enough to result in metallic behavior, or in terms of the formation of a П* conduction band resulting from covalent metal-oxygen interactions. Metallic behavior is observed when the conduction bands are populated with either electrons or holes. The concentration of these carriers can be affected by either cation or anion substitutions. The discussion in this presentation will be limited to the elements Re, Ti, V, Cr, Mo, and Cu.

In the scope of the Goodenough model, a brief review of the electronic properties of some 3d transition elements (M = Fe, Co, Ni and Cu ) is given; the main relevant electronic and structural parameters are emphasized. Then a special attention is focussed on N/(N−1) and N/(N−2) electron mixed valence compounds.

Finally some particular features of copper are discussed and their relevance to high Tc superconductors is pointed out:

-the srong covalency of Cu-O bondings involving a possible delocalization of the hole between trivalent copper and oxygen,

-the existence of low and high spin electronic configurations for CuIII,

-the disproportionation of Cu2+ and its role in the formation of hole pairs,

Cu-O distances very close for the three successive oxidation states of copper favouring the disproportionation process.

The structural evolution versus oxygen adsorption ↔ desorption of the LnBa2Cu3O6+x series, the influence of rare-earth substitution on oxygen network and copper coordination (CN = 2, 4, 4 + 1, 5) in Bi2Sr2Ca1−yLnyCu2O8 series (Ln = La, Yb; 0≤y≤1) are followed. Images of parent mixed valence oxides, i.e. vanadium bronzes, with their single or double layers structures versus intercalated ions with ideal intergrowth structures like the new Na0.70V2O5 oxybronze are presented. The ability to exhibit double non stoichiometry, i.e. LixV3O8−y (0≤y≤1) in the homologous series LixV3(1+n)O8+7n (0≤n≤∞) is explained.

As long ago remarked by WADSLEY, perovskite-like solids have a clear tendency to be non-stoichiometric and both the geometrical requirements of cation coordination and the oxidation state can control the way in which non-stoichiometry is accommodated.

For example, in the case of iron, which shows a particular preference for coexisting octaedral and tetrahedral coordination and in which oxidation states ranging formally from 2 + to 4 + are common in oxides, a Family of Phases AnFenO3n−1, has been described. It includes the members n=2,3,4 and ∞. When the stoichiometries are different from these integral n values, disordered 1−D intergrowthhs are observed. Subsequent work has shown that all the intermediate compositions, whether ordered or disordered in 1−D, can still become non-stoichiometric by oxidation at high temperatures (T>1200°C in air) and this originates the formation of 3D microdomains. Of these, up to nine different sets have been shown to coexist in the same crystal.

Similarly, the copper based HTSC, which have a lot in common with perovskite, have shown a marked tendency for being oxygen deficient. In particular, Ybacuo, Ba2YCu3O7±6 has been shown to exist in a wide oxygen range extending from, at least RO7.2 to RO6 (R<>Ba2YCu3). Within that range, and in spite of an apparent structurally monophasic, and thermodynamically bivariant, behaviour, a succesion of ordered states of the oxygen vacancies in 1,2 and 3 dimensions has been found. This, again, suggests the existence of intermediate ordered phases which, on the other hand, can also be non-stoichiometric. Results of our work along these lines is presented and discussed.

Magnetic moment and resistivity measurements are summarized for the La2−xSrxNiO4 system. They show that the system undergoes a structural phase separation below 70 K. The majority of the sample is antiferromagnetic (AF) below 650 K (for x = 0) or 20 K (for x = 0) or 20 K (for x ≃ 0.2 while the minority phase (≤1% at 4.2 K) is superconducting (SC). An analysis of the results leads to the conclusion that SC and AF coexist in the minority part of the sample; the pairing takes place between 2p holes which propagate through a lattice containing localized 3d moments which, in the ground state, are ordered antiferromagnetically.

We have studied mixed-phase samples of bulk LazCuO4 using neutron powder diffraction, thermogravimetric analysis and resistivity measurements. Samples fired at 950°C and slowly cooled form in the K2NiF4 structure with CuO impurities for z<2 and La2O3 impurities for z>2. Neither La-vacancies nor stacking defects were found. The bulk resistivity of the samples show semiconducting behavior which is shorted at 38K by small amounts of superconducting material. The superconductivity is due to the presence of an oxygen-rich phase with stoichiometry La2CuO4.08. The occurrence of superconductivity in pure La2CuO4.08 with the same Tc as observed in the doped La1.85 Sr0.15CuO4 indicates that the excess oxygen provides the same level of doping as metal-ion substitution.

We describe the ordered oxygen arrangements in Y1Ba2Cu3O7−x samples prepared in precisely controlled oxygen environments using a solid-state ionic tehnique. The so-called “Ortho II”phase, characterized by a 〈1/200〉 superlattice wave vector, exists at oxygen contents from 6.28 to 6.65, with electrical properties that range from insulating to superconducting at up to 60 K. Additional ordered structures occur at oxygen contents from 6.65 to 6.90. All of the ordered structures are consistent with the removal of entire chains of oxygen along the b axis, which minimizes the number of three-fold coordinated copper and creates Cu+1 ions even in oxygen-rich samples. We found no evidence for phase separation between 6.0 and 7.0 or between 6.5 and 7.0. Phase separation, if it occurs at all, occurs on the 60 K and 90 K plateaus.

The nonstoichiometric and thermodynamic behavior of the YBa2Cu3Ox system as a function of oxygen partial pressure and temperature is being investigated by means of a coulometric titration technique. The oxygen content of the superconductor can be varied coulometrically by well-defined small amounts and the equilibrium partial pressures determined from the EMF. The oxygen stoichiometry in YBa2Cu3Ox as a function of oxygen partial pressure shows a change of curvature around x = 6.55–6.75 and an inflection around x ≅ 6.65 at temperatures between 400–500°C. These new low temperature data are consistent with the presence of a miscibility gap at lower temperatures, which is similar to that postulated in several theoretical papers.

Thermogravimetry (TG) is a sensitive and powerful tool for investigation of oxygen nonstoichiometry and related properties in mixed-valence oxide systems and has been extensively applied to a series of high-Tc oxide superconductors. Nonstoichiometry, phase stability ranges, and oxygen diffusion as investigated mainly through the TG technique in several systems of high-Tc oxides are reviewed.

One of the intriguing questions involving the copper containing high temperature superconductors is the relationship between the hole concentration and the transition temperature, Tc. We address this question in terms of a proposed “master curve” on a Tc, vs hole concentration plot. This curve shows a maximum at 90K at a hole concentration of approximately 0.2 – 0.22 holes per sheet copper for the 1–2–3 system. In this work we studied the Nd1+xBa2−xCu3Oy and YBa2Cu3−xZnxOy systems to check the validity of this “master curve”. Both Nd and Zn substitutions lower Tc, Nd by reducing the number of holes, consistent with the model given by the curve. The effect of Zn substitution is ambiguous, since both the holes are increased and the Cu-O planes are perturbed. Compensation doping, using both Nd and Zn, shows that the major effect of Zn is to disrupt the Cu-O planes.

Substitution of Ca(II) for Y(III) into tetragonal YBa2Cu3O6 has been achieved and a superconducting transition has been observed for a material of nominal composition (Y0.5Ca0.5) Ba2Cu3O6 (Tc (onset) ˜50K) [1]. Observations using high resolution electron microscopy show samples with x < 0.3 consist of a complex mixture, including (Y, Ca) Ba2Cu3O6+δ, YBa2Cu3O6+δ, and BaCuO2. Further, structural refinement using neutron diffraction data provide evidence of a solid solution limit at x ˜ 0.3. A direct analogy can be drawn between superconducting (Y1−xCax)Ba2Cu3O6 (0.1 < x < 0.3) and superconductors of the type (Y1−xCax)Pb2Sr2Cu3O8 and (La2−xM(II)x)CuO4. Substitution of the Ca(II) for Y(III) effectively increases the formal copper oxidation state. The refined structural model is fully consistent with partially oxidized CuO2 sheets separated by linear O-Cu(I)-O units.

Two new compounds in the Ba-Y/Ca-Cu-O systems have been grown as single crystals from a Ba(OH)2H2O flux. The compounds Ba3(Y/Ca, Cu)2O7−δ and Ba(Ca, Cu)O3−δ have the Sr3Ti2O7 and the perovskite structure, respectively.

We discuss the effects of doping on the Cu chain sites in YBa2Cu3−xMxO6+y. The relationship between bond lengths obtained from neutron scattering and charge transfer is evaluated in terms of bond valence. In particular, it is concluded that removing an oxygen from the chains transfers one electron to the planes.

An over view of the crystal chemistry of the layered bismuth, thallium and lead cuprates is presented here. The classification of these oxides is examined, and the problems of non stoichiometry, especially of extented defects and incommensurability of the structures, are discussed in connection with the superconducting properties of these materials. A particular attention is paid for the role of the lone pair cations in the structural properties of these oxides.

Neutron powder diffraction data have been collected from samples of La1.9Ba0.1CuO4 and La1.85Ba0.15CuO4 in the temperature range 11–293 K. The first material undergoes a phase transition from the tetragonal K2NiF4-type structure (space group 14/mmm) to the orthorhombic La2CuO4-type structure (space group Bmab) at about 280 K, and then a second transition to another type of tetragonal (or near-tetragonal) structure with probable space group P42/ncm. The latter transition occurs predominantly between 50–40 K, but about 10% of the orthorhombic phase persists down to 15 K. Rietveld refinement was carried out at selected temperatures in the different structural regions. The differences among the low temperature structures can be cb acterized by tilting of the CuO6 octahedra about different rotation axes on the basis of the simple X-Y model previously proposed by Axe and coworkers. The tilt angle, which is a measure of the long-range order parameter, is about 3.5° at 15 K.

Data sets were also collected from La1.85Ba0.15CuO4 at 293, 90 and 11K. At the two latter temperatures there was some peak broadening consistent with the distortions observed in previous high resolution neutron [Paul and co-workers] and x-ray [Axe and co-workers] powder studies. Rietveld analysis of the data indicated the presence of local octahedral displacements, but it was not possible to determine the nature of the correlations.

Transport measurements were made on single crystals of (La1−xSrx)2CuO4 (x = 0.06 and 0.07) which have sizes large enough so that resistivity could be measured reliably along the c-axis by the usual 4 probe method. It was found that conduction along the c-axis was semiconductive in the low temperature orthorhombic structural region, while it was metallic in the high temperature tetragonal region. The resistivity in the basal plane, however, was not affected by the structural phase transition but could be fitted by the T1.3 dependence over the whole temperature range below 300K. The resistive superconducting transition was found to spread significantly under a magnetic field along the c-axis. But the degree of the spreading was essentially independent of the direction of the measurement current. The results suggest that LSCO is a typical 2D metal but with an unusual conduction mechanism and that its superconducting transition is overwhelmed by critical fluctuation under a magnetic field.

Photoemission spectra from HTSC materials ( primarily 123 -type ), cleaved and measured at 20K, reveal a rich DOS structure which compares favorably with a calculated band structure, except for a residual 0.5 eV shift which may reflect some correlation effects. Band dispersion is observed throughout the valence bands, with clear evidence for a 0.2 eV wide band dispersing through EF. The orbital character at EF is a mix of Cu-3d and O-2p. There is unambiguous evidence for a large BCS-like gap (2Δ≥ 4kTc).

The specific features of the optical spectra in the high-Tc Cu-oxides are discussed. The low-frequency spectral behavior can be explained by the highly correlated electronic model, whereas in the high-frequency spectrum the behavior of the Fermi liquid metal shows up.

The growth of single crystals of Bi2+xSr2−yCuO6±δ from alkali chloride fluxes, typically sodium chloride is reported. Electronic properties of the resultant crystals are controlled by the melt composition and the partial pressure of oxygen during growth. Superconducting crystals with Tc's near 10K are obtained under low oxygen partial pressures from melts which are strontium-rich. Measurements of transport, tunneling and magnetoconductivity in nonsuperconducting crystals show two-dimensional localization within the CuO2 planes. Superconductivity occurs only when transport perpendicular to the planes gives rise to extended states in three dimensions.

High quality large single crystals of La2−xSrxCuO4 were grown by the traveling solvent floating zone method(TSFZ method). The crystals up to about 6 mm diameter and 40 mm length were obtained. The composition of the grown crystals was uniform and was determined to be La1.86Sr0.14Cu0.97O3.89. The single crystals were superconductors with Tc=37.5K and δ Tc=1.1K, and had a significant anisotropy of the electrical resistivities at the non-superconducting state.