Buguo Wang

Twinning Morphologies and Mechanisms of ZnO Crystallites under Hydrothermal Conditions

Two kinds of twinning morphologies of ZnO crystallites prepared under hydrothermal conditions were obtained. The twin relations of the crystallites could be influenced by additives. The twinned crystallites in the pure H 2 O or weak basic solutions (1 N KOH) are bipyramidal and take (0001) as the twin and composition plane, whereas the twinning morphology of the crystallites obtained from 4 N KBr or 3 N NaNO 2 solutions is dumbbell-like and takes (0001) as the composition plane. Various twinning mechanisms based on the linkage of the growth units are suggested. The formation of twin morphologies of ZnO crystallites clearly demonstrate that twinning of crystals is either a result of differences between symmetrical and energetic most favourable structure arrangements or due to the consequence of oriented intergrowths.

On the Morphological Changes and Twinning of ZnS (Sphalerite) Crystallites under Hydrothermal Conditions

Morphological characteristics and twinning mechanism of ZnS crystals under hydrothermal conditions have been investigated in this paper. It was shown that under hydrothermal conditions the morphology of ZnS crystallites changes along the four-fold axis directions, and the crystals are observed in a positive or negative tetrahedron, or in a combination of positive and negative tetrahedra depending on the growth conditions. The positive tetrahedral areas on the crystallites get larger with increase of the concentrations of OH - and S 2- in solutions, whereas the twinned crystallites of ZnS taking an elliptic shape with (111) as composition plane are easily formed in weak basic solutions. It can be found that the morphologies of ZnS crystals are in accordance with the crystallization orientations of positive or negative coordination tetrahedra ([S-Zn 4 ] 6+ , [Zn-S 4 ] 6- ) in the crystal although, in some cases, the practical morphology could be greatly affected by growth conditions, and the twinning mechanism can be suggested based on the linkage of growth units of positive and negative coordination tetrahedra, which were formed in the solution. The present investigations further indicated that the crystal chemistry approach based on the linkage/incorporation of growth units previously proposed by us can be sucessfully applied to interpret the growth mechanisms of the crystals and to control a desirable morphology.

Formation mechanism of barium titanate nanocrystals under hydrothermal conditions

Growth units and the crystallization habit of BaTiO3 nanocrystals have been investigated. It has been proposed that the growth units of BaTiO3 were surface hydroxylated Ti(OH)62− octahedra. The relationship between crystal morphology and the variation of the current intensity in the solution has been obtained through the measurement of the OH− in the solution and the hydrothermal experiment with superimposed direct electric field. Based on the computation of the stability energy of the growth units, the relationship between the crystallized morphology of crystallites and the most favorable growth units under different conditions has been derived. It has been found that there is structure similarity between anatase (TiO2) and BaTiO3 from the crystal chemistry viewpoint, so they are soluble in each other, which can serve as a reasonable explanation for the abnormal phenomenon of the retention of cubic substable phase at room temperature.

Hydrothermal preparation and formation mechanism of titanium dioxide microcrystallites

Hydrothermal preparation experiments of TiO2 microcrystallites were carried out. The formation of TiO2 microcrystallites of polymorphic forms was related to the precursor types and the basicity or acidity of reaction mediums under hydrothermal conditions. Based on the “growth unit” model, the space lattice graphs of growth units responding to different polymorphs were established. The stable energy of growth unit was calculated for different basicity or acidity of reaction mediums. The formation of polymorphic TiO2 microcrystallites under hydrothermal conditions was discussed based on the experimental results.

Twinning morphologies and mechanisms of β-BaB2O4(BBO) crystal grown by TSSG method

The twinning morphology of β-BaB 2 O 4 crystal grown by TSSG has been studied on the basis of crystallography. It was found that the faces R{1012} and r{0114} are often intergrown together by either r{0114} intergrown in R{1012} or in reverse, two individuals mostly take X(1120) and occasionally take Y(0110) as twin boundary; and that two individuals in the twin could take the same or opposite crystallographic polarity, this was identified by surface etched figures and the piezoelectric measurements. The formation of the twinned crystals are discussed from the crystallographic structural characteristics and the energetic considerations. The experimental factors such as crystal seed quality are suggested to reduce or eliminate the twin formation and to improve the crystal quality.

Morphology of β‐BaB2O4 (BBO) Crystals in Relation to its Structure and Growth Conditions

β-BaB 2 O 4 (BBO) crystals with well-defined morphology have been grown from Na 2 O solutions using the top seeded solution growth (TSSG) method. The crystal morphology in relation to its structure and growth conditions has been studied in detail on the basis of crystallography and crystal chemistry. It is found that the morphological characteristics are related to the orientations of structural unit (B 3 O 6 ) 3- anion rings in the crystal. On the other hand, the growth parameters may greatly affect the appearance of faces of the crystal, but the crystals still generally take trigonal in outline and have a diagnostic character of point group 3m. The observed morphology is in disagreement with BFDH and PBC analyses and is explained from the incorporation of the growth units on the faces and facets. Since the incorporation rates of the growth units are different on different faces, the boule habits with well-defined morphology are formed.

Solvothermal preparations of α-alumina and boehmite crystallites and investigations on the formation of their morphologies

The solvothermal preparations of α-alumina and boehmite crystallites have been investigated, and the effects of solvothermal conditions such as reactive medium and temperature on the phase, crystallization morphology and size of the crystallites have been discussed. The calculation of the stability energies of the growth units of the crystallites has been performed based on the model of anion coordinate polyhedrons growth unit. The results could explain the formation mechanisms of the crystallites.

Morphological characteristics of α-Al2O3 crystallites obtained direct1y from hydro /solvothermal solvents

The morphological characteristics of α-Al2O3 crystallites obtained directly from hydro/solv othermal solvents are reported and the formation mechanisms of corundum morphology are discussed from crystal growth and crystal chemistry principles. The crystal growth process is considered as a process of incorporation of growth units on the growth interfaces, and the crystal morphology is determined by the linkage of the coordinated polyhedra.