On the Mechanism of Crystal Growth from Undercooled Melts

The mechanism of crystal growth from undercooled melts is studied experimentally by means of a simple capillary technique. Thymol and Na₂S₂O₃ · 5 H₂O are used as model substances. Dislocation-free crystal faces of these compounds are obtained by continuous growth in the capillary or by prolonged annealing under appropriate conditions. Two-dimensional mechanism of growth was experimentally verified on such perfect crystal planes. The conditions under which the surface nuclation mechanism operates are described accounting for the supersaturation, the influence of impurities etc. Transition from two-dimensional into spiral growth of purposedly defected crystal faces is demonstrated and investigated. The mechanism of formation of perfect crystals is discussed and further experimental evidence on the possibility of two-dimensional growth from melts is given.     

A study of the rate-controlling stage of the process of crystal growth in solutions

The effective activation energies of crystal growth on the faces (100) of KCl, KBr, and KI crystals in water solutions have been determined. The values obtained have been compared with the respective crystal lattice energies according to Sletter and Mayer, as well as with the experimental ones. The activation energies of crystal growth are in strictest linear dependence on the heats of dehydration of the ions. This shows that the dehydration of the ions is most probably the rate-controlling stage of the process of crystal growth in solutions. This concept is supported by the results of an experimental study of the rates of growth on the faces (111) and (100) of NaClO₃ crystals.     

Determination of linear growth rates of crystals (I). Calculation of linear growth rates of individual crystal faces from overall rates

The presented paper (Part I and II) is devoted to the fundamental principles of the kinetics of crystal growth. The number of studies in this field has been increasing and some authors have introduced different terms and definitions. To avoid possible mistakes and confusions, the individual definitions of the rate of crystal growth are specified and their interrelations are discussed in detail. Basic methods for measuring the kinetics of crystal growth are described. The rate of growth of individual crystal faces or the average rate of crystal growth can be determined using the selected experimental method. The presented study demonstrates that the results obtained by either of the two measuring methods (linear rate of growth of crystal faces and average rate of crystal growth) can be interrelated. The established method of calculating linear growth rates of individual crystal faces is based on an analysis of the time dependence of the volume of a growing crystal. The relationship between the linear growth rates of individual crystal faces and the over-all crystal growth rate is presented and a method for assessing the linear growth rates of individual crystal faces from over-all growth rate data, which can be measured readily, is suggested.     

Distinction between topography of prism faces of natural and cultured quartz crystals

Results of detailed microtopographical studies on prism faces of a large number of natural and hydrothermally grown quartz crystals are described, illustrated and discussed. In the light of these results distinction between prism faces of the two varieties of quartz is assessed. Horizontal striations, parallelogram shaped and low angle tetrahedral growth pyramids are reported as characteristic features on prism faces of natural quartz crystals, whereas hexagonal and square shaped pyramids are shown to be characteristic features on prism faces of cultured quartz crystals. Evidence of independent growth on prism faces of both the varieties of quartz crystals are given. A wide variety of growth forms, like oriented snail shaped hillocks, pentagonal elevations, rectangular spiral patterns, screw ledges, missile-like structures, influence of impurities on advancing growth fronts, microdiscs patterns and etch patterns due to chemical etching are explained.     

Yb∶YAl3(BO3)4晶体形貌与生长速度的关系

观察测量了不同生长速度(相应于不同降温速度)自发成核生长的Yb∶YAl3(BO3)4晶体形貌。粒度较大(>2mm)的晶体不管降温速度快慢形态都很简单,只发育六方柱｛1120｝和菱面体｛1011｝;粒度较小(<2mm)的晶体形态随降温速度增快而变复杂,发育一些罕见的高指数晶面。说明在生长速度较快的条件下,在晶体生长早期,一些高能面发育,在晶体生长后期已尖灭了,晶体生长的大部分时间是在低能面｛1120｝和｛1011｝上进行的。对比了不同生长条件下晶面的粗糙度,随着降温速度的增快,六方柱面｛1120｝和菱面体面｛1011｝由光滑变粗糙,顶面｛0001｝永远是粗糙的。从晶体结构上定性地探讨了3种晶面的杰克逊因子α及生长机理     

Microstructures of Tin Iodide Single Crystals Grown in Silica Gels

Single crystals of tin iodide (SnI₂) have been grown in silica gels. A detailed microtopographical study of {100} faces have been described. Horizontal striations are predominant on these faces for most of the crystals, while few of them show vertical striations. The horizontal striations are associated with the two-dimensional nucleation theory whereas the vertical striations relate to the growth fronts. Growth layers modified by the presence of misaligned microcystals have been illustrated. The natural etch pits on {100} faces of the crystal are attributed to the dissolution of crystals in the acid set gel. In the light of these observations, the mechanism of the development and growth of these faces have been assessed and the implications are discussed.     

Studies on Surface Structures and Etch Patterns on Habit Faces of Gel-grown Crystals of Iodates of Barium,Strontium, and Calcium

Microtopographical studies on habit faces of gel grown crystals (of different habits) of iodates of Ba, Sr, and Ca are illustrated and described. Etch patterns on these faces are illustrated and correlated to the observed growth patterns on the respective faces. Growth mechanism of the crystals is explained in light of the observed surface structures and etch pits suitably produced.     

Influence of the flow velocity,supersaturation and temperature on the crystal growth from solutions

This paper is a review of our experimental research on the influence of the supersaturation, flow velocity and temperature on the linear crystallization rate of different faces of model crystal systems. The obtained experimental results are discussed in the light of the new theoretical treatments on crystal growth from low temperature solutions.     

Growth and selective etching studies on NaBrO3 crystals

Surface studies on (111) faces of sodium bromate crystals grown from aqueous solution show the presence of growth steps. A new etchant consisting of a mixture of 8 parts of glacial acetic acid and 2 parts of formic acid, to which 3 mg/cc of cupric nitrate is added, reveals the dislocation sites on (111) faces of these crystals. The etching action on (111) and (111 ) faces is different. Different microhardness values are recorded on (111) and (111 ) faces of these crystals. The difference in etching action as well as in microhardness values on (111 ) and (111) faces is discussed.     

Polyhedral stability ‐ on the growth of flat crystal faces on small molecule and protein crystals

The old standing problem of face morphology is discussed. A special emphasis is put on the macroscopically flat faces, appearing on small molecule crystals mostly during calm growth, under low supersaturations. As distinct, protein crystals are growing with macroscopically flat faces even under surprisingly high supersaturations. Explanations of these facts are suggested by considering the surface micro‐profile of crystal faces which growth is driven by screw dislocations. It is shown that, due to kinetic reasons, the tips of the growth hillocks and the valley between them have to be levelled to some extent (at least on a quasi‐atomic scale) under low enough supersaturations. The amplitude of the surface roughness has to be suppressed also due to the surface energy gain, especially under quasi‐equilibrium. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphological Lattice and Reciprocal Crystal. Remarks on Morphology of Barite and Cerussite

The relationship between partitions of the three-dimensional space into Dirichlet domains of morphological points and the face forms observed on crystals will here be examined. Dirichlet domains, the central points of which are to be found on the exterior net plane, can be connected two or one-dimensionally through face-sharing or they occur as isolated groups. If the crystal faces are arranged according to the proportion of these surface polyeder on the total surface of the net plane, the order of rank of barite and cerussite agree excellently with the experimental observations. The packings of space filling polyeder allow conclusions on growth mechanisms.     

利用全息相衬干涉显微术研究EDTA对KDP晶体生长习性的影响

利用全息相衬干涉显微术(HPCIM)研究了EDTA对KDP晶体(100)柱面和(101)锥面生长习性的影响.通过界面边界层宽度的变化,可以直观地观测到在溶液中添加少量的EDTA后,促使KDP晶体沿柱面和锥面的生长,特别是对柱面的影响更为显著,并对此作了解释.最佳的EDTA浓度范围有待于进一步研究.     

Microstructures on {111} faces of stannic iodide single crystals

Single crystals of stannic iodide (SnI₄) havebeen grown using the controlled reaction between SnCl₂ and KI by diffusion process in silica gel medium. Orange to reddish octahedral stannic iodide crystals up to 3–4 mm in size have been grown at room temperature. Optical studies have been made on the various surface structures of {111} faces of the asgrown crystals. On octahedral faces of these crystals, triangular-shaped hillocks with growth layers in the 〈110〉 directions have been observed. Occasionally, growth spirals on octahedral faces have also been reported. Close loops of growth fronts have been investigated and have been interpreted. It has been suggested that two-diemensional nucleation, spreading and pilling up of triangular growth layers is mainly responsible for the growth and occasionally the growth is due to screw dislocations. The implications are discussed.     

Understanding and Controlling the Morphology of ZnO Crystallites under Hydrothermal Conditions

The morphological features of ZnO crystallites influenced by solution basicity under hydrothermal conditions have been studied from the standpoint of the incorporation of the growth units. A crystal chemistry approach is developed to understand and thus to control a desirable morphology of the crystallites. The effects of the additive OH⁻ on the crystal morphology of ZnO crystallites are qualitatively but satisfactorily explained by a mechanism considering two aspects: (1) solution structures and the structural forms of growth units under a certain growth condition, i.e. the interactions between the solvent molecules and the growth units; and (2) the influence of the solvent molecules or additives on growth interfaces, particularly on two polar faces of ZnO crystallites in terms of the inhibition or promotion of the incorporation of various growth units depending on the solution basicity. Since the incorporation rates of the growth units are different on positive and negative polar faces, the relative growth rates of these faces are different and thus lead to the habit modifications. The approach clearly demonstrates that the hindrance of crystal growth is a consequence of surface adsorption processes.     

2D modeling of the regeneration surface growth on crystals

A physical model is proposed to describe the growth of regeneration surfaces (flat crystal surfaces that are not parallel to any possible faces). According to this model, the change in the growth rate of a regeneration surface during its evolution and the decrease in the number of subindividuals forming the growth front can be explained by the implementation of two types of geometric selection: within each subindividual (the absorption of rapidly growing faces by slowly growing ones) and between subindividuals (when subindividuals absorb each other). A numerical modeling of the growth of the regeneration surface (30.30.19) of potassium alum crystals showed quantitative agreement between the model proposed and the experimental data.     

Effect of Mn(II) ions on the growth of ammonium oxalate monohydrate crystals from aqueous solutions: I. Growth habit and surface morphology

The effect of concentration of Mn(II) ions on the growth habit and the surface micromorphology of different as‐grown faces of ammonium oxalate monohydrate (AO) single crystals grown from aqueous solutions was studied at a constant temperature of 30 °C and predefined supersaturations up to 20%. It was observed that the growth habit and the surface morphology of the crystals strongly depend on the supersaturation used for growth and the impurity concentration in the solution. The experimental results were analysed in terms of connected nets determined from different projections of the structure of AO crystals. Analysis of the observations revealed that: (1) the directions of connected nets corresponding to basic growth units composed of single (NH₄)₂C₂O₄ · H₂O molecules are in excellent agreement with the low‐index crystallographic directions of the orientations of growth layers, (2) all faces appearing in the growth morphology of AO crystals are F faces, and (3) the {001} face growing from pure aqueous solutions is essentially a kinetically rough face but the presence of Mn(II) impurity leads to their appearance in the morphology due to increase in the strength of bonds of the connected nets composing the surface graph.     

Role of microcrystals in the growth and development of prism faces of cultured quartz (I). Attached microcrystals

Microcrystals attached in random orientations to growing host prism faces of cultured quartz grown by the hydrothermal crystallization technique are described. The microcrystals are established to be quartz. Growth layers and pyramids initiated by the attached guest microcrystals of quartz on the growing host prism faces of bulk quartz crystals are illustrated. Also described are striations, parallel, perpendicular and inclined to the c-axis, and polygonal growth pyramids on the prism surfaces of attached guest microcrystals. Formation of the microcrystals of quartz is linked to the possibilities of creation of quartz micronuclei in the autoclaves which further establishes and solidly supports the explanation put forward by KOTRU . Effect of unoriented attached tiny crystals of quartz and impurities on the progress of growth layers on the host prism faces is dealt with. Role of attached guest microcrystals in the growth and development of the host prism faces in particular and the cultured quartz in general is discussed. Possibilities of the incorporation of quartz micronuclei inclusions and impurity inclusions in the quartz structure and its effect on the crystalline quality of cultured quartz is briefly dealth with.     

Role of microcrystals in the growth and development of prism faces of cultured quartz (II.) Detached microcrystals

Results of microtopographical investigations on prism faces of cultured quartz crystals grown by hydrothermal crystallization techniques, undertaken with a view to determine the influence of microcrystals which get attached to the growing host prism face and then get detached, at some stage of growth, from or still remain attached to the host face, with special reference to the role played by them in the growth and development of host prism faces in particular and the quartz crystals in general, are discussed. The study is an extension of, and obviously to be dealt in continuation with, the earlier one (Part-I, see JOSHI , KOTRU ). Polygonal cavities of various shapes and structures observed on the prism faces are described. Mechanism of formation of the polygonal cavities is explained. Also described are pyramidal depressions, striations, growth hillocks, and etch pits within the cavities on the growing host prism faces. Some of the observed structures within the cavities are attributed to the imprints of the surface structures on the faces of detached microcrystals of quartz. Evidence of growth within the cavities is offered and the implications of such a growth are discussed. Evidence of micro-etching in the cavities is suggestive of the possibilities of some of the grown cultured quartz crystals to get etched in the autoclave. Two different suggestions are made for the interpretation of the origin of etch pits within the cavities, and there is support for the view that they are as a result of etching, only at a time when the supply quartz gets exhausted, of some of the grown crystals where the growing chamber is the hottest.     

Explanation of some peculiarities of crystal morphology deduced from the BFDH law

The morphological variety of crystal habits is due to differences in relative growth rates of faces of which the crystal is composed. For equilibrium, the growth rates of faces are proportional to the distances from the centre of the crystal to the respective hkl faces. According to the BFDH law, such distances are inversely proportional to the interplanar distances, therefore the observed crystal faces are those with the largest interplanar distances. This paper tries to explain some peculiarities of the crystal morphology deduced from the BFDH law and shows that the crystal geometry influences morphological importance of faces and because of crystal geometry, the faces of the largest interplanar distances are not necessarily the largest faces in the BFDH morphology.     

Promotion of crystal growth rate in aqueous solution by direct contact with gas corona discharge

A new technique to promote crystal growth in aqueous solution using gas plasma is proposed. In this method, short‐lived radical species produced in solution which is contacted with gas corona discharge play a role to increase chemical potential of inorganic solute. In an experimental examination, single crystal of KDP was grown in a supersaturated solution which receives oxygen ions and radicals from adjacent corona discharge in air. KDP crystal has two unique growth faces (100) and (101), and the growth rates of both faces were increased considerably by generating the corona discharge. The both growth rates with and without corona discharge were well converged by one function based on chemical potential supersaturation. This result revealed that the solution in contact with gas corona discharge has a larger capacity of chemical potential than that without the discharge. Short‐lived species induced by gas corona discharge are considered to be anti‐solvents to cause this effect. The crystal growth process proposed here is considered to be an excellent method in terms of low impurity inclusion because such short‐lived species do not remain in the final crystal products and solution. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)