Silver halide face centered cubic (fcc) crystals can, besides in the common cubic habit under certain conditions also be grown with a tabular morphology. Tabular or platy silver halide crystals are produced in the photographic industry in large amounts. These tabular crystals, which are multiply twinned are used because photographic films containing tabular crystals need a lower amount of silver as compared to block-shape crystals. This is because around the surface of silver halide crystals the photographic process takes place and tabular crystals have a relatively large surface. Still the quality of the photographic emulsions used can be improved by producing batches wich are more homogeneous and by decreasing the number of other (twinned) crystals in the crystal batch. For this a better understanding of the growth mechanism and twin formation for silver halide crystals is needed. This thesis addresses both topics. Models to explain the growth mechanism and twin formation of twinned silver halide crystals are presented together with experiments to validate these. The developed models are not only relevant for silver halide crystals but can also be applied to other NaCl-type structures and fcc metals.

The side-face structure and the preferential growth mechanism of tabular silver bromide crystals grown from DMSO are studied in chapter 2. Tabular silver bromide crystals grown from DMSO are over a thousand times larger than micrometer-sized crystals grown by the industrial double-jet precipitation method in water. The larger size makes the determination of the fast growing side faces possible. The crystal sides consist of {100} and {111} faces separated by parallel {111} twin planes. The number of twin planes is deduced from the morphology and the proposed twinned cubo-octahedron model. The preferential lateral growth is explained by a substep model. The substep model is based on the twin-lamella theory of Ming Nai-ben which explains the increase of growth rate of fcc crystals with parallel (111)-type twin planes and stacking faults. Fast growing {100} faces present on the crystal sides are capable of increasing the growth rate of the slower growing {111} side faces owing to the twin plane. {100} faces are linked via a twin plane to a {111} side face. At the twin plane position the growing {100} side faces can produce substeps on the {111} side faces. Substeps will easily form steps on the {111} side face, a complete growth layer will be formed and the growth rate of the whole crystal side will be increased.

In the third chapter tabular silver bromide crystals are grown at various circumstances. Increasing the growth rate of the {100} faces (as compared to the {111} faces) leads to tabular crystals with higher aspect ratios containing side-face structures with smaller {100} side faces. For these tabular crystals the {100} side faces are present between the twin planes and adjacent to the tabular {111} faces. The {100} side faces adjacent to the tabular faces will grow out because of their higher growth rate. {100} faces between the twin planes can not grow out and will still increase the growth rate of the other {111} side faces owing to the substep mechanism. Thus, the relation between the relative growth rate of the {100} faces (as compared to the {111} faces) and the side-face structure and aspect ratio is revealed. At conditions where the {100} faces grow fast only one side-face geometry expresses preferential lateral growth. Tabular crystals bounded by other side-face geometries grow more slowly and will dissolve in the physical ripening process. Thus, homogeneously shaped crystals are formed after the physical ripening stage.

The step source on the crystal sides was made visible by optical microscopy as presented in the fourth chapter. Tabular crystals with relatively low aspect ratios (relatively large crystal sides) are examined on which growth patterns on the {100} and {111} side faces are clearly visible. The in-situ observation reveal that the (100) side faces (present between the twin planes) is the step source for both adjacent {111} side faces. The position of the parallel twin planes can be made visible by etch techniques.

In chapter 5 silver bromide and silver chloride crystals grown from the vapour phase are studied. Cubic, cubo-octahedral, needle-shaped and {111} and {100} tabular crystals are observed. The morphology of the crystals is dependent on the driving force. At low driving forces {100} faces are stable whereas at higher driving forces the {111} faces become stable. Twinning occurs above a threshold driving force. This threshold driving force is comparable for silver chloride and silver bromide. The observed vapour grown morphologies are the same as obtained for pure silver bromide grown from precipitation and with the extractive crystallization in DMSO. For the vapour grown crystals the driving force is constant during growth, nevertheless, twin planes are only formed in the nucleation stage. At larger crystal sizes the chance of forming twin planes decreases. The twin formation is probably caused by twinned two-dimensional nuclei which can grow out to form a twinned layer. This will only occur when the supersaturation is above a critical value and the crystal size is small enough. At larger crystal sizes the chance is high that also correctly stacked nuclei occur. These nuclei have a stronger binding energy and a higher growth rate. Therefore, the twinned stacked islands will have a high chance to dissolve again on larger crystals. This crystal size effect is studied in chapter 10.

In chapter 6, the position and the number of twin planes of micrometer-sized tabular silver halide crystals grown in water and in DMSO are studied using transmission electron microscopy (TEM). For the water grown tabular crystals also the side-face structures are examined. Tabular crystals contain two or three parallel {111}-type twin planes which is in agreement with the number of twin planes deduced from the twinned cubo-octahedron model presented in chapter 2. The side-face structure of the water grown tabular crystals is analogous to that of the DMSO grown ones.

Needle-shaped silver halide crystals grown from the vapour and from DMSO are examined in chapter 7. All needles studied contain non-parallel {111}-type twin planes. The needles grown from DMSO have the same morphology as needles grown in water observed by Goessens et al.. For the vapour grown needles a rough face is present on the needle top between the non-parallel twin planes. This face is capable of increasing the growth rate of the whole needle top. The growth mechanism of both needles is explained by the substep mechanism.

In chapter 8, atomic force microscopy (AFM) studies on the {111} and {100} faces of tabular crystals are presented. In-situ growth experiments of the polar {111} faces reveal monosteps with the height of the {111} interplanar distance (d111) and pinning of these steps. In-situ precipitation experiments show small nuclei on the tabular {111} face. The observed nuclei contain twin planes with a separation between the twin planes of 40 nm, which is comparable to the results of the TEM measurements of chapter 6.

Silver metal crystals grown from the vapour phase are studied in chapter 9. Silver also crystallises in an fcc-structure. The morphologies observed are the same as those found for AgBr crystals. The twinning mechanisms of (tabular) Ag and (tabular) AgBr and AgCl crystals are analogous. This gives new insight in the twinning mechanism and the role the charge plays.

In chapter 10, Monte Carlo simulations of the twinning event are presented. These simulations serve as a test for the hypothesis concerning the twin formation in chapter 5. The chance of forming twin planes is studied as a function of the supersaturation, the crystal size and the ratio of the vertical bond strength of a twinned unit and the vertical bond strength of a correctly stacked unit. Increasing the crystal size leads to a decrease in the chance of forming twinned layers. This is in agreement with the results obtained for vapour grown silver and silver halide crystals. Increasing the supersaturation leads to an increase in the chance of forming twin planes. Above a certain threshold supersaturation dependent on the stability of twinned growth units, less completely twinned layers are formed. This is owing to the splitting of the surface above a certain driving force. For multiply twinned silver and silver chloride crystals grown from the vapour phase the ratio between the numbers of parallel and non-parallel twinned crystals is in reasonable agreement with the results deduced from the presented model.