The crystal structure of Al 7 O 3 N 5 was characterized by laboratory X-ray powder diffraction (CuKα 1 ). The title compound is trigonal with a space group R3̅ m (centrosymmetric). The hexagonal unit-cell dimensions (Z = 3) are a = 0.305 06(1) nm, c = 5.7216(1) nm, and V = 0.461 11(2) nm 3 . The initial structural model was derived by the charge-flipping method and refined by the Rietveld method. The final structural model showed the positional disordering of two of the four Al sites. The maximum-entropy method-based pattern fitting method was used to confirm the validity of the splitatom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The disordered crystal structure was successfully described by overlapping five types of domains with ordered atom arrangements. The distribution of atomic positions in one of the five types of domains can be achieved in the space group R3̅ m. The atom arrangements in the four other domains are non-centrosymmetric with the space group R3m. Two of the four types of domains are related by a pseudo-symmetrical inversion, and the two remaining domains also have each other in the inversion pseudo-symmetry.

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The structural chemistry of the double-layered perovskite-type gadolinium barium cobaltate, GdBaCo2O5+δ (0 < δ < 1/2), is not well-known, in comparison to the representative end-member phases α at δ = 0 and β at δ = 1/2. This study unveils the structural evolution of the room-temperature stable phase γ at δ = 7/18, using in situ single-crystal X-ray diffraction at elevated temperatures and electron microscopy. The γ phase is essentially charge-ordered with Co3+ and Co2+ in a ratio of 8:1 and is incommensurately modulated in the five-dimensional superspace. The approximant of the modulated structure reveals the presence of island-like charge-ordered square cell domains interleaved with a discommensurate zone. The γ phase underwent a reversible first-order phase transition at ∼380 K to the commensurate phase β, without any change in composition at δ = 7/18. The charge ordering of Co in γ ceased in β, in combination with a redistribution of oxygen atoms (O3) in the [GdOδ] layer and a change in the spin state of Co. Further heating of β induced partial oxygen detachment at ∼700 K, causing a change in δ from 7/18 (∼0.389) to 0.19 in the nitrogen flow atmosphere. The crystal irreversibly turned into a tetragonal prototypal phase α. A peristaltic oxygen transport mechanism in GdBaCo2O5+δ (0 ≤ δ ≤ 1/2) accompanied by electron transfer and resulting in polaronic local structure relaxation is also proposed. This mechanism underpins a usefulness of the compound for application to solid oxide fuel cells as revealed in recent years.

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We investigated the optical absorption properties of InP porous structures formed by the electrochemical process using photoelectric conversion (PC) devices formed on p-n junction substrates. The photocurrent measurements revealed that the current from PC devices changed in response to the incident light power and the thickness of the top layer on the p-n interface. Since the photocarriers contributing to the observed photocurrents are excited by the photons reaching the p-n interface through the top layer, the photocurrents give us information on the optical absorption properties of the top layer. The photocurrents observed on a porous device with a porous structure in the top layer were lower than that of a non-porous device, indicating that the absorption properties of InP were enhanced after the formation of porous structures. This phenomenon can be explained in terms of absorption coefficient, α, increased by the light scattering and the sub-bandgap absorption in the porous layer.