Orthorhombic YBaCo4O8.4 crystals as a result of saturation of hexagonal YBaCo4O7 crystals with oxygen

Hexagonal YBaCo4O7 crystals (sp. gr. P63 mc, a h = 6.3058(4) A, c h = 10.2442(7) A, Z = 2) are saturated with oxygen to the YBaCo4O8.4 composition and studied by X-ray diffraction (XRD) analysis. The saturation is completed by a structural first-order phase transition to orthorhombic crystals (sp. gr. Pbc21, a o = 31.8419(2) A, b o = 10.9239(5) A, c o = 10.0960(5) A, Z = 20). The connection of two lattices is expressed in terms of the action of matrix (500/120/001) on the hexagonal basis. Five structural fragments of the same type but with different degrees of order alternate along the long axis of the oxygen-saturated orthorhombic structure. The XRD data on single crystals differ from the results obtained by other researchers on ceramic samples; possible causes of these differences are discussed.