We have shown recently that the degradation of AC electroluminescence (EL) in ZnS:Cu,Cl and similar phosphors can be significantly reversed by a short anneal near 200°C. To better understand the degradation/rejuvenation processes, we investigated EL degradation and rejuvenation under different conditions. To probe for changes in the local atomic structure about the Cu sites, we collected Extended X-ray Absorption Fine Structure (EXAFS) data at the Cu K-edge EXAFS data for several as-made, thermally degraded (240°C anneal), and EL degraded powders.

Cu(II) doped ZnSe nanoparticles were synthesized using molecular cluster precursors. The Cu(II) dopant had the effect of quenching the ZnSe band edge emission, yet only weak emission from Cu(II) centers was observed. An X-ray Absorption Fine Structure (XAFS) experiment was performed on the Cu(II) doped ZnSe nanoparticles. It was determined that the Cu(II) occupies a tetrahedral site in the lattice in an environment similar to that of Cu in bulk Cu2Se, and is likely to have substituted for Zn(II) in the lattice.

In many systems for which there are several atoms in the unit cell, the displacements of the atoms may be locally correlated even though there is no long range coherence. Such displacements can play an important role in determining various macroscopic properties. We consider several examples to demonstrate this phenomena - the local distortions in the colossal magneto-resistive (CMR) and charge-ordered manganites, magnetic field induced distortions in CMR materials that are connected to macroscopic magnetostriction, and correlated displacements of tetrahedral units within the negative thermal expansion material ZrW2O8. The distortions in the CMR materials observed using XAFS change rapidly just below Tc and are attributed to the formation of polarons as the temperature is increased through Tc. In the ferromagnetic state, the lattice is more ordered for CMR systems; consequently applying a magnetic field for T ∼ Tc should decrease the local distortions. Such an effect has been observed and is a much larger effect than the measured macroscopic magnetostriction. Finally, in ZrW2O8 the tetrahedral and octahedral units are found to be very rigid as expected. More surprising is that the width of the pair-distance distribution for the W-Zr atom-pair is also quite small, indicating that the W-Zr linkage is stiff. In contrast, for the W-W pair, the distribution width grows rapidly with T, indicating correlated displacements of two WO4 tetrahedral units.

In constrast to other rare earths, replacement of Y with Pr in the high Tc superconductor YBa2Cu3O7 has a pronounced effect on Tc, with Tc going to zero at ≈ 55% Pr. We present Pr XAFS K-edge data for various concentrations of Pr in Y1−xPrxBa2Cu3O7. The Pr K-edge data show that the amplitude of the first Pr-O peak is greatly reduced when compared to the first Y-O peak in pure YBa2Cu3O7. In constrast, for the Y K-edge data in these alloys, there is little if any change in the oxygen environment, while the Cu K-edge data show at most a 10% reduction in the first Cu-0 peak. Fits to the Pr data suggest either that oxygen atoms in the CuO2 planes are missing or that some oxygen atoms about the Pr become highly disordered. Either result could help explain the reduction of Tc with increasing concentration of Pr.

Epitaxial Fe-substituted YBCO films on LaAIO3 substrates prepared by pulsed laser deposition exhibit a large variation in Tc with the substrate temperature and the cool-down rate of the films in an O2 environment, immediately after deposition. For 9% Fe films deposited at 700°C, a rapid cool-down yields a Tc of 60 K while a slow cool-down of 10°C/minute gives 18 K; Tc goes to 0 at 11% Fe for the slow cool-down films. For 9% Fe films deposited at 770°C, Tc > 80K and is much less sensitive to the cool-down rate or additional anneals.

We present data on thin films of YBa2Cu3O7 (YBCO) and YBa2(Cu1−xFex)3O7 prepared by laser ablation using a sequence of targets, including Cu-deficient YBa2Cu2.4O7, CuO, and (CuO)1−y(FeO)y. This technique achieves mixing on an atomic scale. We find that stochiometric films, made using a combination of the Cudeficient target and CuO, have a sharp transition (width < 1K) near 90K although the Cu-deficient material alone has a broad transition at a lower Tc. We have achieved substitution of Fe on the Cu sites, using a sequence of the three targets for a range of Fe concentrations. Tc decreases nearly linearly with concentration and Tc → 0 near 15% Fe. The Tc suppression for the films is slightly less than the corresponding value obtained for bulk samples.