Chemical Control of Hole Distribution and Superconductivity in (Cu,Mo)Sr₂(Ce,R)_sCu₂O_5+2s+δ (s = 2, 3; R = Y, La−Yb)

Abstract

In the crystal structure of members of the (Cu,Mo)Sr2(Ce,R) s Cu2O5+2 s + δ or (Cu,Mo)-12s2 homologous series of superconductive copper oxides, adjacent CuO2 planes are separated by a fluorite-structured (Ce,R)-[O2-(Ce,R)] s -1 blocking block. Here we utilize the fluorite block as a “chemical-pressure medium” for the s = 2 and 3 members of the series. The (Ce,R) sites readily accommodate rare earth elements (R) ranging in size from La down to Yb. With decreasing size of the R constituent, T c first monotonically increases and then decreases, having a maximum around R = Y for both s = 2 and s = 3. At the same time the average valence of copper, derived for the s = 2 samples from Cu L3-edge XANES spectra, is found to remain constant at 2.20 ± 0.01. An explanation for the unique chemical-pressure effect is revealed from O K-edge XANES spectra, the resolution of which allows us to distinguish the CuO2 plane holes from those residing in the (Cu,Mo)O1+ δ charge reservoir. The result shows that the T c value and the density of CuO2 plane holes follow parallel trends with respect to the size of R.

Extremely Overdoped Superconducting Cuprates via High Pressure Oxygenation Methods

“…This has been checked by combined O K and Cu L XAS and Mo K XAFS for YSCO-Mo [60]. The XAFS indicated the Mo to be Mo(VI).…”

“…The superconductivity is much more sensitive to substitutions in the CuO 2 planes than in other sites [59], with many substituting metals quenching it at only a few percent. In contrast, the Cu chains in the charge reservoir block remain stable with substitution and the materials remain superconducting; for example Cr, Ga, Ge, Fe and Mo have been incorporated, often at ratios around 10-20% [23,54,56] and as high as 25% for Mo [26,28,32,44,60]. The Mo solubility limit of 25% could be explained by a bond strain model: the differing bonding preferences of Mo and Cu drive nanoscale domain formation, which might be shifting into grain-level phase separation due to nanodomain percolation.…”

“…KCl can be washed away, at the expense of risking sample degradation, while Ag residues are practically inseparable from the cuprate product. An external oxidizer can be used in HPO synthesis [50,58,73,74], but is more often employed in HPO treatment of pre-formed but oxygen-deficient products [23,55,57,58,60,61]. Intermediate APO steps are commonly used to minimize the amount of oxidizer needed at the HPO step, thus minimizing residue and maximizing the often very limited amount of the HPO sample [30,50,56,57,61,75].…”

“…[23,55,57,58,60,61]. Intermediate APO steps are commonly used to minimize the amount of oxidizer needed at the HPO step, thus minimizing residue and maximizing the often very limited amount of the HPO sample[30,50,56,57,61,75].…”