Diamagnetic transition

Eighteen-electron rule — An electron-counting rule to which an overwhelming majority of stable diamagnetic transition metal complexes adhere. The number of non-bonded electrons at the metal plus the number of electrons in the metal-ligand bonds should be 18. The 18-electron rule in transition metal chemistry is a full analogue of the Lewis octet rule . 

In summary, we have shown that samples of YBa2Cu3Ox can be prepared with x > 7.0. For x = 7.1, the material exhibits a sharp diamagnetic transition with an onset temperature of 90 K and a transition width of 10 K (10% - 90%). When x is increased to 7.2, the transition is broadened substantially, and the magnitude of the Meissner effect at 5 K is decreased. 

This is normally found for Be +, Mg + and higher charge cations including some of the electronically diamagnetic transition metal ions, lanthanides and actinides. Paramagnetic transition metal cations can be studied using or D resonance. 

The use of NMR spectroscopy to characterize diamagnetic transition metal and posttransition metal complexes has increased tremendously in the past decade (111, 129). Mercury has two NMR-active isotopes, Hg (natural abundance = 13.22%) and (natural abundance = 16.84%). The former is quadrupolar (nuclear spin, / = ), and consequently, much more difficult to observe, due to line broadening. The latter, on the other hand, is one of approximately 20 nuclides in the periodic table with the preferred spin I = i. Its receptivity, a measure of relative signal strength, is five times that of C for comparison, " Cd, the most widely studied of the posttransition nuclei, has a receptivity of only 8 relative to C. Magnetic resonance frequencies are intimately connected with the quantity and dis-... 

Readily soluble poly(2,9-o-phenanthroline-(2, 5 -dihexyl)-4,4"-p-ter-phenylene)s with a degree of polymerization up to 39 have been prepared via a Pd-catalyzed polycondensation reaction of 2,9-bis-(p-bromophenyl)-o-phenanthroline and 2,5-dihexylbenzene-l,4-diboronic acid [50]. Some cyclic oligomers could be separated from the polymer. Diamagnetic transition metal complexes of well-defined constitution as shown in 26 were obtained with Cu(I) and Ag(I) without branching and cross-linking. 

Diamagnetic transition A7 (10%-90%) = 15 K Oxygen-poor 123-0,. x 6.1 with vacuum annealing... 

Polycrystalline, stoichiometric samples (x=6.912) prepared with the above methods have d.c. susceptibilities with sharp diamagnetic transitions, as shown in fig. 8 (Conder et al. 1994a) for the optimally doped material with A7 c( 10—50%)=2.8 K (cf. sect. 7.3, figs. 86, 87). 

As the resistivity onset of the diamagnetic transition coincides with Tc2 it is clear that the long-range order of superconductivity starts at this temperature. As the resistive transition is finished at T > Td the authors conclude that the long-range order is fully established at this temperature. 

The splitting of the diamagnetic transition as a function of the exact oxygen content indication for phase separation... 

Fig. 85. Splitting of the diamagnetic transition of 123-0, at a > 6.950, indicating the phase separation of the overdoped phase. = 71 p, remains practically unchanged with x, whereas decreases with x. The onset of the splitting coincides with the displacive martensitic transformation (sect. 6.4) at the onset of the overdoped phase. After data of Conder et al. (1994a) and Zech et al. (1995a,b).

Fig. 86. Morphology of the magnetization curve of an equilibrium sample due to the splitting of the diamagnetic transition. The arrow shows the change of curvature. Open circles, YBa2CuJ 0, 9a. Crosses, the same sample after site-selective isotope exchange with O which slightly increased the O content to 6.991. After data of Zech et al. (1995a) and Conder et al. (1994a).

The splitting of the diamagnetic transition shows the existence of physical phase separation in the overdoped phase. Roughly, the stability of the optimally doped phase is such that the material tends to retain it in cluster form even in the overdoped range. Thus the Tc2,onset Stays constant near the value of the Tc,optimal- We will see that in the much wider Ca-123 overdoped range the effect is present to a much larger extent. 

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