Growth Morphology and Atomic Surface Topology by Hartman-Perdok Analysis: Application to ABCO4\text{}_{4} and YBa2\text{}_{2}Cu3\text{}_{3}O7−x\text{}_{7-x}

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 CaYAlO$\text{}_{4}$ as model for all other ABCO$\text{}_{4}$ compounds with a K$\text{}_{2}$NiF$\text{}_{4}$ crystal structure. F forms are {002}, {101}, {103}, {110}, {112}, {200}, {211} and {213}. The strongly anisotropic shape caused by the perovskite-like AlO$\text{}_{6}$ 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, q$\text{}_{O}$, the ordered forms are still tabular, while {110} and {112} are the only lateral faces. At still lesser negative q$\text{}_{O}$ {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}. YBa$\text{}_{2}$Cu$\text{}_{3}$O$\text{}_{7-x}$ 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$\text{}^{+}$ (x=1) or Cu$\text{}^{3+}$ and O$\text{}^{2-}$ (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

Measurement of specific fracture energy and surface tension of brittle materials in powder form

V článku uvedeno Libor M. Hlaváč.This article presents a method for the experimental measurement of specific fracture energy and surface tension of a brittle materials in a powder form. This work is focused on testing a method on the mineral, almandine. A hydraulic press was used in the experiment to crush powder particles, and statistical evaluation was used to analyze the change in the powder surface. The powder was subject to various conditions during crushing. The crushing was performed both in air and in water. It was done at three different compression speeds, namely 15.8 MPa/s, 3.95 MPa/s and 2.25 MPa/s. The experimental results showed measurable differences in the specific fracture energy values in the presented regimes