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Definition of way of crystallization at polymorphic transformations

Definition of a primary way of change of structure liquid phase after beginning of crystallization

At crystallization from melt of one firm phase the structure of a liquid phase (melt) changes (i. e. the way of crystallization passes) from a point of structure initial melt on continuation of the straight line which has been carried out through a point of structure of crystallizes connection and a point of structure of initial melt in the party of downturn of temperature.

For initial melt a₁ (see fig. 1) the structure of a liquid phase at crystallization of connection АС changes (i.е. the way of crystallization passes) on continuation of a straight line АC- a₁ connecting point АС and a₁, i.е. till a straight line a₁b in the party shown by arrows. If the point of structure of initial melt (for example, a₂) gets on congruous boundary curve, the change of structure of a liquid phase (way of crystallization) at simultaneous crystallization of two firm phases (in this case A and АС) will occur in the party of downturn of temperature on this curve. If the point of structure of initial melt gets on incongruous boundary curve, the way of crystallization from it will leave and in this case works described corrected definitions of a primary way of change of structure of a liquid phase, i.e. the way of crystallization passes on continuation of a straight line connecting a point of structure of crystallizes phase and a point of structure of initial melt.

The way of crystallization at presence in system of polymorphic transformations does not change the direction and is defined by general rules. At crossing isotherm appropriate temperature of polymorphic transformation, there is a transition (at constant temperature) one polymorphic modification of crystal substance in another.

In a fig. 3 the type of the three-component diagram of a condition with threefold chemical connection (ABC) is submitted melting congruous (on it the cases of polymorphic transformations and immiscible liquids) are represented. The component B in system A-B-C has three polymorphic forms designated B', B" and B'". At cooling of melt of structure а₁ the crystals of modification B' will drop out at temperature t₁, appropriate temperature of polymorphic transformation B'↔В", these crystals will proceed in updating B" between temperatures t₁ and t₂ from melt the crystals of modification B" will be allocated which at temperature t₂ will proceed in modification B'".

Figure 3- The diagrams of a condition of three-component system with threefold chemical connection ABC congruous melting and immiscible liquids

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