X-ray Diffraction Study of Superstructure in GdBaCo2O5.5

TL;DR

This study uses X-ray diffraction to analyze the superstructure transition in GdBaCo2O5.5, revealing a continuous phase change with atomic displacements and implications for cobalt ion spin states.

Contribution

It provides detailed experimental evidence of the superstructure transition and atomic displacements in GdBaCo2O5.5, including symmetry change and critical behavior analysis.

Findings

Superstructure reflections appear at 341.5 K indicating a phase transition.

The transition involves a symmetry change from Pmmm to Pmma.

Atomic displacements suggest spin-state/orbital ordering of Co3+ ions.

Abstract

A single crystal of GdBaCo2O5.47(2) has been studied by means of X-ray diffraction. Appearance of superstructure reflections at T = 341.5(7) K gives an evidence of continuous transition to the phase with unit cell doubled along the shortest edge a1. Critical exponent for the order parameter is found to be beta=0.33(1). The superstructure reflections are about 2-4 orders of magnitude weaker than the basic ones. Their systematic extinction indicates the crystal symmetry change from Pmmm to Pmma. The integrated intensities allow to calculate displacements of atoms from the positions in the high-temperature phase. The cobalt-ligand distances in the ordered phase are discussed in terms of the spin-state/orbital ordering of Co3+ ions.