Phase co-existence and structured diffuse scattering seen by X-ray 3D mapping of reciprocal space for Cs0.8Fe1.6Se2

TL;DR

This study uses 3D X-ray reciprocal space mapping to reveal complex phase coexistence, diffuse scattering, and structural disorder in Cs0.8Fe1.6Se2, providing insights into its microscopic phase behavior and electronic structure.

Contribution

It presents a comprehensive 3D reciprocal space mapping revealing phase coexistence, diffuse scattering, and disorder in Cs0.8Fe1.6Se2, advancing understanding of its structural complexity.

Findings

Diffuse scattering from Cs occupational disorder observed

Minor phase identified as monoclinic with 90.7° distortion

Structured diffuse scattering linked to Fermi surface nesting

Abstract

AxFe2-ySe2 (A=K, Rb, Cs) superconductors may show a complex mixture of structural phases even in a single crystal form. A full sphere of the diffraction intensity has been collected, with the help of synchrotron radiation, for Cs0.8Fe1.6Se2 . In addition to the expected pattern for the tetragonal phase with ordered Fe vacancies, a diffuse scattering from Cs occupational disorder has been observed, together with an extra Bragg contribution from a minor phase. The minor phase, in agreement with previous findings, is compressed in the tetragonal a-b plane and expanded along the c-direction; a set of modulated Bragg rods evidences a planar disorder. Fourfold splitting of the rods as well as the main Bragg peaks for L\neq0 imply that symmetry of the minor phase is not higher than monoclinic. The monoclinic distortion was estimated to be 90.7 degrees. Structured diffuse scattering, observed on top of the Bragg component, relates to the major phase and is attributed to a correlated distribution of Cs ions. Elastic nature of the observed scattering was confirmed by additional inelastic X-ray scattering experiment. Diffuse scattering forms 3D objects that have topological similarity with what one would expect, under assumptions discussed in the text, for a Fermi surface nesting.