Oxidation state superconductor

Technetium is a silvery-gray metal that tarnishes slowly in moist air. The common oxidation states of technetium are +7, +5, and +4. Under oxidizing conditions technetium (Vll) will exist as the pertechnetate ion, TcOr-. The chemistry of technetium is said to be similar to that of rhenium. Technetium dissolves in nitric acid, aqua regia, and cone, sulfuric acid, but is not soluble in hydrochloric acid of any strength. The element is a remarkable corrosion inhibitor for steel. The metal is an excellent superconductor at IIK and below. 

The superconductor YBa2Cu30g.j, contains copper in both the +2 and +3 oxidation states. Procedures are described for synthesizing the superconductor, demonstrating the superconducting effect, and for determining the amount of Cu + and Cu + in the prepared material. 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

Various Ru-oxides, YBa2Cu307, c (I), Ba Ru2/3Gdi/303 (II) as well as Ru-doped a-Fe203 (III), to probe the local chemical structure around the Ru atoms. Compound (I) has interesting properties with x < 0.2 it is a superconductor and with x 1 a semiconductor. Ru oxidation state and coordination are discussed on the basis of measured isomer shifts and quadrupole splittings Ru(IV) ions exclusively occupy Cu-1 sites which form one-dimensional chains... 

Neither the calcium nor the strontium compound is a superconductor. A possible reason may be that the oxidation state of copper is equal, or very close, to 2+. 

Another closely related family of superconductors is represented by the formula TlBa2Can 1Cun02n+3 (n = 1, 2, 3, 4, 5). They contain single layers of T1 and O atoms that separate the perovskite-like Ba-Cu-Ca-O slabs (24)(25)(26)(27) (Figure 6). Distortions in the Tl-O sheets are also found in these compounds (26)(27). Note that if these phases were stoichiometric, copper would always have a formal oxidation state of greater than two. Therefore, the chemical composition of this homologous series allows the existence of holes in the copper-oxygen sheet. 

The most heavily studied high temperature superconductor is YBa2Cus07 x (x = 0 to 1), whose Cu oxidation state is determined by the oxygen content. The parent structure, YBa2Cus07, contains layers... 

Immediately after the discovery of YBa2Cus07 x, numerous groups employed iodometric titration procedures to measure the effective oxidation state of the material, and therefore the value of x. The procedure described below involves two different titrations (10X17X18) and is more accurate than a procedure in which the first titration is omitted (19). Experiment A measures the total copper content of the superconductor and Experiment B measures the total charge of the copper. The two experiments, together, give the average oxidation state of copper. 

Suppose that mass mA is analyzed in Experiment A and volume VA of standard thiosulfate is required for titration. Let the corresponding quantities in Experiment B be mB and VB. Let the average oxidation state of Cu in the superconductor be 2+p. Then p is given by (18)... 

Another popular titrimetric analysis of superconductor oxidation state is based on the oxidation of ferrous ion (21)-(24) ... 

A method that can distinguish between the oxidation states Cus+ and Bi5+ in bismuth-based superconductors has been described (33). Bi5+ (BiOs") can oxidize Mn2+ to Mn04", while Cus+ is not a strong enough oxidizing agent to accomplish this task. 

Among the procedures described in this chapter, reductive thermogravimetric analysis is the simplest, but not very accurate. For accurate analyses of slightly oxidized superconductors, the citrate-complexed copper titration is recommended. The difficult problem of assessing oxidation states of individual elements in Bi- and Tl-con-taining superconductors has not been addressed, and remains a significant challenge to analytical chemists. 

The second difference between the nickel and copper compounds is that La2Cu04 is a superconductor, being the first of the Cu02 layer compounds in which superconductivity was observed. The lattice-induced strain is a necessary condition for superconductivity since it stabilizes the higher oxidation state needed to provide the superconducting carriers as discussed in Section 13.3.2. 

Many layer-perovskites with Cu as the B cation are superconductors at relatively high temperatures ( 100 K). Although the mechanism of superconductivity is not well understood, a necessary condition is that the oxidation state of the copper ion be around +2.2. In many compounds, such as the well-known YBa2Cu307 (63324) this is achieved naturally through the relaxation of... 

---