Growth Morphology and Atomic Surface Topology by Hartman-Perdok Analysis: Application to ABCO4 and YBa2Cu3O7-x

C.F. Woensdregt Faculty of Earth Sciences, Geodynamical Research Institute, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands

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The Hartman-Perdok theory explains the relation between crystal structure and morphology and provides the atomic surface topology of the crystal-melt interface. Hartman-Perdok theory has been applied to CaYAlO4 as model for all other ABCO4 compounds with a K2NiF4 crystal structure. F forms are {002}, {101}, {103}, {110}, {112}, {200}, {211} and {213}. The strongly anisotropic shape caused by the perovskite-like AlO6 layers || {001} is very distinct in all theoretical growth forms. The form with formal charges is planar following {001} with {101} and {110} as lateral forms. Disordering of the boundary ions results in the disappearance of {110}. At lower effective charge on oxygen ions, qO, the ordered forms are still tabular, while {110} and {112} are the only lateral faces. At still lesser negative qO {112} appears as well. On the disordered models {112} replaces {110}. Crystals show often variations in colour parallel to the {110} interface due to the surface topology of {110}. YBa2Cu3O7-x has, for x=1, the following F forms: {001}, {101}, {103}, {112} and {114}. The theoretical growth form of this tetragonal phase is tabular following {001} with {101} as lateral form. For x=0 the growth form shows important {101} and minor {103} and {001}. When the boundary ions on (001) are ordered, the outermost layer of {001} contains half of the Cu+ (x=1) or Cu3+ and O2- (x=0) ions in a c(2×2) quadratic lattice which reduces the {001} growth rate significantly. An (1×2) reconstructed {010} surface can be traced for the orthorhombic polymorph which results into the appearance of {010} on the ordered growth form. Otherwise the presence of {010} on as-grown crystals must be due to external factors.

DOI: 10.12693/APhysPolA.92.35 PACS numbers: 68.35.Bs, 68.35.-p, 81.10.Aj