Copper valency

Phinyocheep, P. Tang, 1. M. Determination of the Hole Concentration (Copper Valency) in the High Superconductors, /. Chem. Educ. 1994, 71, A115-A118. 

Table 2.2 contains mean values of the Cu—Oz bond lengths ((<7Cu—())), copper valence index [11] (FCu) and partial charge (<2Cu), one-electron energies of the HOMO and LUMO ( HOmo> lumo) levels along with the bonding energy of Cu1 to the hosting cluster. 

The nuclear quadrupole tensor of the Cu(II) ion is discussed in terms of the formalism outlined in Sect. 5.2.1. Kita et al.158) treated the Stemheimer antishielding factor (1 - as an adjustable parameter. The rhombicity of the measured quadrupole tensor was found to originate from the charge distribution on the ligands, the field gradient due to the copper valence electrons being nearly axially symmetric. 

Figure 12 Lattice parameters (a), cell volumes and Tc s (b), copper valences and oxygen contents (c) are shown for compositions in the Bi4Sr4Ca2 xTmxCu4016+y series.

Similar studies on bulk ceramic electrodes make it possible to analyze the samples qualitatively. As a rule, such methods are directed to the determination of the copper valence. From the appearance of an additional peak in the cathodic run of voltammograms [37,44,46,476,534,538,539] or the shift of the reduction peak [534,540], it is assumed that Cu is present in solid cuprates along with Cu ". ... 

Analysis of the copper valence around the two dislocation cores in Fig. 11.7 is shown in Fig. 11.10 (for all of the other elements, valences are within 10%... 

Fig. 11.13. Experimental observations of (T = 4.2 K) as a function of misorientation angle from the results of several groups [11.1-11.3] show an exponential dependence. Where the results were reported at E = 77 K, the values at 4.2 K were extrapolated from the temperature dependence of Jq [11.45]. The grain boundary tunneling current calculated from eq. 11.2 using the grain boundary widths from Fig. 11.12 shows excellent quantitative agreement for a width defined by a copper(I) valence between 1.5 and 1.9. This copper valence corresponds to the copper(I) valence in bulk YBCO when it becomes non-superconducting. The predicted drop in due to the symmetry of the superconducting order parameter is insufficient by two orders of magnitude to account for the observed behavior.

The high-temperature superconductors have a crystal structure of so-called perovskite, whose primary structure is ABO3, where A is a di- or trivalent metal atom with a larger ion size and B is a tetra- or trivalent metal atom with a smaller ion size. In the preceding high-temperature superconductors, the A site is occupied by La3+, Y3+, Ba +, Sr2+, and Ca2+ and the B-site is occupied by Cu2+, Bi3+, and TP+. If the copper valency is 2, the cation valency is insufficient to form the nominal ABO3. This insufficiency is compensated for by... 

The copper valence in most compounds lies between the formal values of Cu and Cu. The appearance of superconductivity and the superconducting transition temperature, T, at which the superconducting state appears on cooling, are closely connected with the composition of these solids, which all exhibit considerable degrees oxygen composition variation (Section 8.4). 

Thermogravimetric reduction. One should be very cautious with thermogravimetric reduction in reducing atmosphere, usually hydrogen (Gallagher et al. 1987, Swaminathan et al. 1988). A certain advantage is the universaUty of the method, as it can be used for all cuprates, independently of the copper valence in the sample, as the final state is metallic copper. Also, it contributes to its popularity that no wet chemistry is involved. 

Dilution of the copper oxide by the barium oxide and melting the fluxes with a higher Ba Cu ratio (0.3, 0.4, 0.6, 0.8) results (fig. 17b) in a pronounced shift of the copper absorption edge to the higher energy direction (1050"C). This probably means that the alkaline-earth element stabilizes the higher copper valence. 

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