P. Hartman
Utrecht University
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Featured researches published by P. Hartman.
Journal of Crystal Growth | 1989
P. Hartman
Abstract Theoretical growth forms are derived from surface energies of relaxed surfaces of corundum (α-Al2O3) and hematite (α-Fe2O3), as calculated by Mackrodt et al. [J. Crystal Growth 80 (1987) 441]. This growth habit is rhombohedral {011} with small {111} faces. The habits observed on natural and on synthetic crystals mostly have a tabular {111} habit that does not agree with either the relaxed equilibrium or the relaxed growth habit. The observation can only the understood by assuming specific solvent adsorption on {111} for which there are structural arguments.
Journal of Crystal Growth | 1991
W.M.M. Heijnen; P. Hartman
A uniform description of the crystal structures of gypsum (CaSO4·2H2O), brushite (CaHPO4·2H2O) and pharmacolite (CaHAsO4·2H2O) is presented. From these structures, a PBC analysis leads to the following F forms: {020}, {011}, {⦶1}11 {120} and {⦶1}22 (for brushite and pharmacolite the counterfaces (⦶h⦶k⦶l) are included). Attachment energies of the gypsum F faces have been calculated in an electrostatic point charge model for various charge distributions in the water molecule and in the sulphate ion. The theoretical habits are all platy or tabular {020} with {120}, {011} and {⦶1}11, the latter being smaller than {011}, in disagreement with observation. Arguments are given that {011} can grow in layers built12d011, and therefore faster, at a supersaturation for which {⦶1}11 cannot grow in half layers, thus removing the discrepancy between theory and experiment. The curious triangular or trapeziumlike habit of brushite has been related to its crystal structure. The polar habit is ascribed to different adsorption of water and cosolutes on opposite faces. The most important forms are {020} with {⦶1}20, 11{⦶1} and {111}, the latter being an S form. Its appearance is ascribed to a habit change.
Journal of Crystal Growth | 1990
Bing Nan Sun; P. Hartman; C.F. Woensdregt; Hans Schmid
The structural morphol. of YBa2Cu3O7-x (YBCO) was investigated by application of the periodic bond chain (PBC) theory. For x = 1, the F forms are {001}, {011}, {013}, {112} and {114}. Attachment energies were calcd. in a broken bond model and in an electrostatic point charge model. For x = 1 the theor. growth habit is tabular to platy {001} with {011} as side faces. For x = 0 {010} also becomes an F form. The habit is isometric with large {001} and {011} and small {010} faces. The outermost layer of {001} contains half of the Cu+ (x = 1) or Cu3+ and O2- (x = 0) ions in an ordered arrangement based on a c(2 * 2) quadratic lattice. For the outermost layer of (010) (x = 0) an ordering scheme of the Cu and O ions is proposed. The occurrence of {010} rather than {011} on grown crystals has to be ascribed to external factors.
Journal of Crystal Growth | 1986
R.A. Terpstra; P. Bennema; P. Hartman; C.F. Woensdregt; W.G. Perdok; Marjorie Senechal
Eighteen connected nets have been determined for hydroxyapatite, and 12 different F faces run parallel to at least one of these connected nets. The order of morphological importance of these F faces is determined by the slice energy, Eslicehkl and compared to the morphological data for natural apatites. It is found that there is a close agreement between predicted and observed morphology. It follows that the lowering of the apatite symmetry (P63/m → P21/b) due to the ordering of OH is not likely to be the cause of the occurence of plate like apatitic crystals in calcified tissues.
Journal of Crystal Growth | 1991
Elly van der Voort; P. Hartman
A qualitative explanation for the discrepancy between the theoretical and observed habits of gypsum, CaSO4 · 2H2O, is presented in terms of solvent interaction. The theoretical models predict {011} faces to be large and {111} face to be small, whereas the observed habit shows large {111} faces and {011} absent or small. It is argued that the hydrated surfaces will behave differently in the growth process. Two kinds of water molecules can be present on the faces. One kind is part of the crystal structure and does not have to be removed before growth can continue. This occurs for [011]. The other kind does not belong to the crystal structure and has to be removed, thereby lowering the growth rate, which is the case for [111]. When organic impurities are present the growth rate of the {111} and {011} faces are equally reduced, which is in agreement with the observed habit from solutions with organic impurities. A growth rate reduction is also expected for {120} faces, leading to an elongation into the c axis direction.
Journal of Crystal Growth | 1990
Elly van der Voort; P. Hartman
Abstract The electrostatic field near the cube and octahedron faces of alkali halide crystals with the rocksalt structure was calculated. The results were used to recover the growth mechanisms of these faces. The solvent interaction with the kinks was assumed to determine the rate of integration of growth units in the kinks. With this assumption the habit change from cube to octahedron, which occurs when the supersaturation is increased or when one of the ions is present in excess, could be explained.
Pharmaceutical Research | 1993
P. Hartman; Hak-Kim Chan
The habit (external morphology) of a crystal is controlled by both the external (environmental) conditions of crystallization and the internal (structural) factors of the crystal. In order to separate the effects of the crystal structure and of the solvent and other external factors on the experimentally observed growth habit, the theoretical habit can be derived from the crystal structure using the periodic bond chain (PBC) theory and attachment energy considerations. According to the PBC theory the crystal habit is governed by a set of uninterrupted chains of strong bonds formed in the crystal lattice. In addition, the attachment energy (Eatt) is defined as the energy released per mole when a new layer is deposited on a crystal face. Since the habit of a crystal is determined by the relative growth rate (R) of the various faces, by taking R proportional to Eatt, the theoretical habit can thus be derived from Eatt. Using this approach, we obtained the theoretical crystal habit of an antitumor drug, hexamethylmelamine (HMM). The possible effect of solvents on the habit modification of HMM is discussed. This technique, based purely on the knowledge of the crystal structure, is directly applicable to other pharmaceuticals in deriving their theoretical crystal habit.
Journal of Crystal Growth | 1988
C.F. Woensdregt; P. Hartman
Abstract The structural morphology of compounds having the PbCl 2 and the closely related SbSI structures has been determined. Based upon the nine-coordination of the Pb atoms the F forms of the PbCl 2 structure are {110}, {020}, {120}, {011}, {200}, {111} , {201}, {121} and {211}. These forms are arranged in an order of increasing attachment energies, that were calculated using a broken bond model. In the SbSI structure type the Sb atom has a seven-coordination with the consequence that {020} becomes a different surface structure and that {120} is an S face. The theoretical habit of PbCl 2 and Pb(OH)Cl is short prismatic, elongated along the c axis, with {011} as terminal form. The appearance of {211} as main form on PbCl 2 when growth takes place from pure aqueous solution is ascribed to the preferential adsorption of OH - ions on that face. The predominance of {020} and {121} on PbCl 2 from solutions containing HCl is explained by adsorption of H 3 O + on these faces. The theoretical habit of the SbSI structure type is slender prismatic {110} with {011} as terminal form.
Journal of Crystal Growth | 1988
M. Van Panhuys-Sigler; P. Hartman; C.F. Woensdregt
Abstract Crystallization of lead chloride, PbCl 2 , from pure aqueous solution produces crystals with dominant {211} and smaller {010} and {100} faces at low supersaturation. Increase of supersaturation yields crystals elongated along the c axis. Growth from a previously prepared saturated solution gives crystals elongated along the c axis with {011} as main terminal faces. This habit also occurs when KCl, NH 4 Cl or CdCl 2 or mixtures of these are added to a freshly prepared supersaturated solution. Only at low additive concentration and at low supersaturation {211} is dominant. The latter habit is supposed to be caused by adsorption of OH - ions on {211}. A deposit of Pb(OH)Cl has been observed. Crystallization from HCl containing solutions enhances the {010} and {121} forms, presumably through preferred adsorption of H + and possibly PbCl - 3 ions. Increasing supersaturation diminishes the effect of the habit modifying process until the stage where dendrites appear.
Journal of Crystal Growth | 1988
Elly van der Voort; P. Hartman
Abstract The interfacial tension for the calcite cleavage rhombohedron and water has been calculated from Coulomb potentials of lattice slices, taking into account the polarizability of water. The water layer is considered as a crystalline structure. It appears that the interfacial tension as measured from nucleation experiments does not refer to the calcite-water, but to the interface between water and a calcite surface covered with an either complete or incomplete monolayer of water. In the models used the calcite-water interface has a negative interfacial tension of -556 mJ m -2 , whereas the completely hydrated surface-water interface has an interfacial tension of 49 mJ m -2 .