X-Ray Absorption Fine Structure Applied to the Study of Systems with Lattice Instabilities

Abstract

We present a simplified model of electrons and phonons in a three-site cluster as a paradigm of a system exhibiting a lattice instability. We point out the utility of X-ray absorption fine structure (XAFS) in the study of materials where the coupling between electrons and phonons leads to the appearance of such lattice instabilities. As examples of these systems, we present X-ray absorption fine structure (XAFS) measurements on magnetic manganese oxide materials and II-VI semiconductors. Both of these systems exhibit local lattice instabilities which are reflected in the transport properties. In the case of the manganese oxide La0.67Ca0.33MnO3 we observe a change in the Mn-O local structure accompanying the ferromagnetic and metal-insulator transitions. For In doped CdTe we observe the appearance of a lattice distortion centered at the Cd atoms as the In concentration is increased. This distortion is associated with the trapping of free charge carriers, leading to the saturation of the conductivity as the In concentration increases.

Local lattice distortions and dynamics in extremely overdoped superconducting YSr 2 Cu 2.75 Mo 0.25 O 7.54

“…XAFS probes the local order of each element separately without the restrictions imposed by the space-group symmetry, so that it is not constrained to the average structure of the periodic fraction of the material. In addition, its structure factor, S(q, ω) (or S[q, t = 0]) (34)(35)(36), includes time and energy scales in the range of dynamical processes coupled to collective phenomena. XAFS therefore often provides key insights into the structure-property relationships of complex correlated materials.…”

“…The associated double-well potentials have been ascribed to tunneling polarons that are components of the dynamic structure. In contrast to a conventional polaron that transfers its excess charge and lattice distortion to a neighboring site by hopping, a tunneling polaron is a fixed set of atoms that oscillate between two conformations denoted by distinct geometries and charge distributions that in chemical terms would involve changes in the valence and resulting speciation of the affected atoms (34)(35)(36)46). The signature for a tunneling polaron is that the two sites give individual signals in the EXAFS of only the Cu atom with that correlation for the Cu-O pair distribution, in this case Cu2 (SI Appendix, Fig.…”

“…They also inform about the tunneling polarons that are coupled to the superconductivity (46,(55)(56)(57)(58)(59)(60)(61). The model that was used for the original theoretical analysis was the simplest one utilizing a symmetric cluster with a degenerate double well (34)(35)(36). This approximation may be valid in materials with a single CuO 2 layer or when the polaron is on the chain Cu of YBCO.…”

Nonadiabatic coupling of the dynamical structure to the superconductivity in YSr 2 Cu 2.75 Mo 0.25 O 7.54 and Sr 2 CuO 3.3

“…One candidate is the dynamical structures of cuprates, S ( Q , E ) or experimentally S ( Q , t = 0), specifically their internal quantum tunneling polarons (iqtp). An iqtp is a set of atoms oscillating between two structures that possess different geometries, energy levels, and charge distributions ( 17 – 19 ). A chemist would describe these endpoints as separate species, adapting this term that applies more intuitively to solutions to the atoms in crystalline solids.…”

“…A chemist would describe these endpoints as separate species, adapting this term that applies more intuitively to solutions to the atoms in crystalline solids. Neutron scattering and X-ray absorption fine structure (XAFS) measurements identified O-centered iqtps and their correlation with the superconductivity 30 y ago ( 17 – 25 ). We now present Cu K edge extended XAFS (EXAFS) results from “overdoped” YSr 2 Cu 2.75 Mo 0.25 O 7.54 (YSCO-Mo) that is isostructural with YBa 2 Cu 3 O 7 ( Fig.…”