Modulated Distribution of Differently Ordered Tetrahedral Chains in the Brownmillerite Structure

Two crystal structures Ca 2 Co 2 - x Al x O 5 ( 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* = α a* 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].