Modulated Distribution of Differently Ordered Tetrahedral Chains in the Brownmillerite Structure

Abstract

Two crystal structures Ca2Co2-xAlxO5 (x ≈ 0.75) of brownmillerite type, have been investigated by single-crystal X-ray diffraction methods. The X-ray and electron diffraction patterns show satellite reflections, with commmensurate positions along [100]* (a = 5.509 Å; b = 5.278 Å; c = 14.703 Å; q* = αa* with α = 5/8 and 2/3). The 4D superspace formalism was used for the refinement of these modulated structures. The average framework can be described as an alternate stacking along [001] of tetrahedral and octahedral layers formed from the metal (Co, Al) and interconnected via apical oxygen atoms. Two kinds of orientation of the tetrahedral chains, noted L (left) and R (right-hand), are evidenced, running along the [010] direction. It is shown that the L and R chains are ordered with various sequences along the modulation direction [100].