P. Bennema

The structure of different phases of pure C70 crystals

Abstract Single crystal of pure C70 are grown from the vapour phase and the structure and morphology of these crystals is studied. By means of X-ray diffraction and TEM measurements five different phases are observed. The observed phases are (from high to low temperatures) fcc, rhombohedral, ideal hcp (c/a=1.63), deformed hcp (c/a=1.82) and a monoclinic phase. The occurrence of these different phases and the phase transitions is accounted for in a simple model. For the monoclinic structure a model for the stacking of the orientationally ordered molecules in the lattice is proposed. For both the hcp and fcc phases a Lennard-Jones type interaction potential is used to calculate bond strengths, lattice energies and the theoretical morphology.

Growth and morphology of c60 crystals

Abstract Single crystals of pure C 60 are grown from the vapour phase. The crystal morphology is compared to the theoretical equilibrium form, predicted on the basis of a periodic bond chain analysis and a statistical mechanical approach. Some interesting growth phenomena are observed and discussed.

Morphology of orthorhombic long chain normal alkanes : theory and observations

Abstract This paper describes a theoretical and experimental investigation of the growth and especially the morphology of paraffin crystals of the odd n -alkanes (C n H 2 n +2 , n ≠ 2 n ) over a range of chain lengths with 15 ≤ n ≤ 41, for growth from solution, under pure conditions and under the influence of additives. Three strong and one weak interactions between the bodies of paraffin molecules in a crystal and four weak interactions between the head and the tails of the paraffin molecules are identified and the crystal graph is derived. The elementary cell of this crystal graph is characterized by a pseudo F cell with first nearest neighbour interactions between the points. From this crystal graph ten connected nets giving rise to eight (or nine) different F faces are identified. Using different quantities like E att ( hkl ) , E slice ( hkl ) , Ising temperatures (θ c ( hkl ) , interpl anar distance, etc. for the connected nets of paraffin crystals, growth and equilibrium forms are constructed. All crystal forms show large {001}, small thin faces {110} and often smallest faces {010}. It is shown that predicted and observed morphologies, agree quite well. In passing the sometimes dramatic change in the expected habit of crystals, caused by tailor made additives is discussed. The theory of roughening transition is applied to paraffin and it is shown how the roughening temperature depends on n , the length of the paraffin chain. An actual roughening temperature of 10.65±0.50°C is measured for faces of the form {110} of paraffin with 23 C atoms, growing from a slightly supersaturated solution of hexane.

Theory of growth and morphology applied to organic crystals; possible applications to protein crystals

Abstract A brief survey is given of an integrated crystal growth and morphology theory based on surface roughening and the crystallographic morphological theory of Hartman and Perdok. Phenomena as surface roughening and kinetical roughening are discussed and the experimental verification on growth of organic crystals is mentioned. Work on and possibilities to apply the theory to protein crystals are discussed briefly.

The equilibrium state of solid-liquid interfaces of aliphatic compounds

The ordering and orientation of fluid units at interfaces of the aliphatic crystal solution system are the theme of this paper. Based on the idea that the bulk phase and the interfacial phase are two different phases, appropriate relations between the interfacial bond energies and the bulk bond energies are developed by introducing a surface scaling factor Cl. As a description of the interfacial structure of aliphatic compounds, it is assumed that a high degree of ordering is stored in the interfacial fluid phase. Then the so‐called α molecule approach is used to estimate the values of bond energies in the interfacial phase. This leads to the result of Cl≤1, meaning that the the interfaces of n‐paraffin crystals and the solutions will be extra wetting. This is consistent with the experimental data obtained from the roughening transitions occurring in several n‐paraffin–n‐hexane solution systems. Finally, a roughening phase transition diagram of the n‐C23H48–n‐hexane system is presented, which indicates th...

Morphology of synthetic and natural garnets: Theory and observations

Abstract The approach described in the previous paper is in this paper applied to the calculation of the advance velocities of steps in the (211) 1,2 slices and the (220) slice in different crystallographic directions for two synthetic garnets, YIG and YAG, and four natural garnets. Due to different chemical compositions, the anisotropy of the growth fronts depends on the chemical composition. Also, the relative growth rates of the slices (220) 1 and (211) 1,2 depend on the chemical composition of the garnets of the mother phase and the growth temperature. It is shown that the calculated habits of garnet crystals, concerning the relative morphological importance of faces {211} and {110} is in satisfactory agreement with a variety of observed growth forms of synthetic and natural garnets.

Ordering of paraffin-like molecules at the solid-fluid interface.

Abstract Structures of the liquid molecules at the solid—fluid interface for paraffin-like molecule solution systems are studied via calculations of self-consistent field lattice models. Periodic oscillations of the segmental density are directly associated with the length of molecular chains. Molecules in the first and sometimes the second liquid layers lie preferentially parallel to the surface, due to interfacial effects. Comparisons between the calculated data and the results obtained from surface roughening experiments are made for several n-paraffin systems.

Advance velocities of steps on {211} and {110} faces of yttrium iron garnet: Theory and observations

Abstract In a previous paper it was shown how a kinetical statistical mechanical sublattice model and the integrated Hartman-Perdok roughening theory, in which the concept of connected net plays a key role, could be integrated. This approach was applied to the complex connected net (220) 1 of garnet. Carrying out computer calculations, advance velocities of steps in different crystallographic directions in dependence of supersaturation were calculated and phenomena of interlaced steps could be explained. In this paper the approach described above is applied to the two alternative connected complex nets (211) 1 and (211) 2 of garnet. The kinetic equations of the sublattice model are derived and the structures of the complex connected nets (211) 1 and (211) 2 of garnet are presented. Advance velocities of steps in different crystallographic directions are calculated in dependence of supersaturation for YIG crystals growing from a lead oxide flux. Results are compared with calculated advance velocities of steps in (220) connected nets. Bond energies were calculated from the presupposition that the faces of the form {322} are for YIG growing from a PbO flux close to their roughening temperature. It is shown that the agreement between calculated and observed anisotropy on both (220) 1 and (211) 2 are in agreement with each other and that the freedom to choose attachment frequencies for Fe 3+ and Y 3+ ions is very limited. The anisotropy of growth fronts in dependence of temperature is calculated and it is shown that the anisotropy decrease if the temperature increases.

Imperfections in KDP crystals caused by organic impurities and a method for eliminating the effect

The authors report the result of a preliminary study concerning the elimination of organic impurities such as microbes or bacteria in KDP growth solution. A chemical preservative (antiseptic mercury, Hg), was successfully employed for the sterilization of bacteria and microbes, which can be eliminated from the beginning to the end of the growth process. The preservative does not contaminate the solution or the as-grown KDP crystals.

A study on the morphology and stability of incommensurately modulated structures; a case study of ((CH3)4N)2ZnCl4−xBrx and AuTe2

In this paper we investigate the stability of crystal forms of modulated structures. First, in a classical analytical approach the forms on AuTe2 are investigated. Of the experimentally observed forms almost all important forms can be characterized as being stable. Secondly the forms on ((CH3)4N)2ZnCl4−xBrx are investigated, using a simple Ising model and various parameters to determine the stability of forms. Here we find that satellite forms {hklm} are clearly less stable than the main forms {hkl0}.