A local structure study of ZnS nanocrystals, doped with very low concentrations of Cu, was carried out using the EXAFS technique to better understand how Cu substitutes into the host lattice and forms Cu luminescence centers. We show that a large fraction of the Cu have three nearest neighbor S atoms and the Cu-S bond is significantly shortened compared to Zn-S, by ∼0.08 Å. In addition, the second neighbor Cu-Cu peak is extremely small. We propose that Cu occupies an interior site next to a S(2-) vacancy, with the Cu displaced towards the remaining S(2-) and away from the vacancy; such a displacement immediately explains the lack of a significant Cu-Cu peak in the data. There is no evidence for interstitial Cu sites (Cu(i)), indicating that no more than 2% of the Cu are Cu(i.) This study provides new insights into the local structure of the Cu dopant in ZnS without the presence of CuS nanoprecipitates that are present at higher Cu doping levels.
We present extended x-ray-absorption fine structure (EXAFS) data as a function of temperature (10-300 K) at the Zn and Nb K edges for Zn-doped LiNbO 3. The focus is on higher Zn concentrations (7-11 mol %) for which there is disagreement in the literature as to the substitution site for Zn. Our data show that Zn substitutes only on the Li site; we find no evidence for Zn on the Nb site. However, uncertainties result in an upper bound of at most 5% of the Zn dopants being Zn Nb. In addition, the Zn Li defect produces a significant distortion in the lattice out to at least 4Å; the O atoms are attracted toward Zn while the Nb neighbors are repulsed. The Nb EXAFS agree well with the structure from diffraction for the main Nb-X peaks out to about 3.7Å. However, there appears to be a weak third Nb-O peak in the first O shell, which has a low amplitude and a longer bond length. For Nb, the shortest Nb-O bond is extremely stiff (correlated Debye temperature, θ cD ∼ 1100 K), while the longer Nb-O bond is a little weaker (θ cD ∼ 725 K) and of comparable strength to the shortest Zn-O bond (θ cD ∼ 600 K); consequently, substituting Zn at the Li site will stiffen the structure as the Li-O bonds are weak. We discuss implications of a dominant Zn Li defect.
The local structure of two skutterudite families -CeM4As12 (M = Fe, Ru, Os) and LnCu3Ru4O12 (Ln = La, Pr, and Nd) -have been studied using the Extended X-Ray Absorption Fine Structure (EXAFS) technique with a focus on the lattice vibrations about the rare earth "rattler atoms", and the extent to which these vibrations can be considered local modes, with the rattler vibrating inside a nearly rigid cage. X-ray absorption data at all the metal edges were collected over a temperature range of 4 to 300 K and analyzed using standard procedures. The pair-distances from EXAFS results agree quite well with the average structure obtained from diffraction. The cage structure is formed by the M and As atoms in CeM4As12 and by Cu, O, and Ru atoms in LnCu3Ru4O12. Although some of the bonds within the cage are quite stiff (Correlated Debye temperatures, θcD, are ∼ 500 K for CeM4As12 and above 800 K for LnCu3Ru4O12) we show the structure is not completely rigid. For the rattler atom the nearest neighbor pairs have a relatively low Einstein temperature, θE; ∼ 100-120 K for Ce-As and ∼ 130 K for Ln-O. Surprisingly, the behavior of the second neighbor pairs are quite different; for CeM4As12 the second neighbor pairs (Ce-M have a weaker bond while for LnCu3Ru4O12 the Ln-Ru second neighbor pair has a stiffer effective spring constant than the first neighbor pair. In addition, we show that the As4 or CuO4 rings are relatively rigid units and that their vibrations are anisotropic within these cubic structures, with stiff restoring forces perpendicular to the rings and much weaker restoring forces in directions parallel to the rings. Consequently vibrations of the rings may also act as "rattlers" and help suppress thermal conductivity. In general neither the rigid cage approximation nor the simple reduced mass approximation are sufficient for describing rattler behavior.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.