Features of the Initial Growth of Yttrium Ferrite Crystals in a Complex Solvent

A. N. Kondakov, A. M. Kuzmin, B. F. Nifantov

Observations of the growth of yttrium ferrite crystals in a molten solution consisting of oxides of yttrium, iron, lead, and lead fluoride have established a substantial dependence of the growth forms and skeletal character of the crystals on the ratio between lead fluoride and lead oxide, which develops during the growth process.

Crystallization of yttrium ferrite was carried out in open crucibles with a volume of about 40 cm³. Crystal growth was studied by direct visual observation with the aid of a microscope. Subsequently, the grown crystals were subjected to microscopic examination and photographed in the plane of their initial growth in order to study the mechanism of crystal enlargement. In the course of the observations it was found that nucleation of crystals occurs primarily in the surface film of the melt, which is explained by gravitational differentiation of the melt by specific gravity during the precrystallization period, as well as by additional supersaturation of the solution as a result of intensive evaporation of the solvent from its near-surface part.

Two growth forms were observed on the yttrium ferrite crystals studied: “pyramidal” and “boat-shaped.” The development of each of these forms depended on the content of lead fluoride in the composition of the complex solvent.

At an initial lead fluoride content in the complex solvent equal to 42 mol. %, a predominantly skeletal-pyramidal growth form was observed (Fig. 1). As a rule, crystal growth began with the appearance of an almost ideal crystallite in the form of a rhombododecahedron with an edge size of about 0.05 mm, which was found at the center of any of the crystals studied. The enlargement of the embryonic rhombododecahedron proceeded mainly in the direction of the edges, with the ultimate formation of an edge rhombododecahedron. Deep cavities replacing the faces (110), in the form of negative stepped truncated rhombic pyramids or their fragments, impart a sharply pronounced skeletal character to the crystals. On the surface of the crystallized melt, the edge rhombododecahedron protrudes in the form of a “little pyramid,” and the cavities in its upper part contain no inclusions of solvent. The latter indicates that during its growth the crystal floats up, breaking away from the feeding region, owing to which, according to the terminology of I. I. Shafranovskii [4], internal open edge forms of skeletons develop.

In a solvent containing 50 mol.% lead fluoride, analogously to crystallization in the above-mentioned solvent, crystal growth begins with the appearance of a regular rhombododecahedral crystallite; however, the subsequent growth of the latter proceeds with less pronounced, or practically absent, internal rib growth (Fig. 2). Developing as separate steps parallel to the upper face of the rhombododecahedron and simultaneously floating upward above the melt surface, the upper part of the crystal acquires a false stepped-pyramidal growth form, sometimes complicated by rib striation. In the final stage of crystal growth there occurs

Fig. 1. Growth forms of yttrium ferrite crystals in the surface film of the melt when the initial solvent composition contains lead fluoride, 50 mol.%. Magnification 20×.

Fig. 2. Growth forms of yttrium ferrite crystals in the surface film of the melt when the initial solvent composition contains lead fluoride, 42 mol.%. Magnification 20×.

the formation of an external closed ribbed funnel-shaped skeleton, with replacement of the upper face of the rhombododecahedron by a concave rhombic pyramid. Consequently, in the course of growth of the upper part of the crystal, the buoyant force acting on the crystal was balanced by its weight, and for some time the crystal grew in the surface film of the melt in the form of a boat [1].

When the lead fluoride content in the complex solvent is 56 mol.%, in the crystal, at the site of the rhombododecahedron face, a deep concave truncated rhombic pyramid formed in the surface film of the melt (Fig. 3). Ultimately the upward-facing part of the crystal assumes the form of an external closed ribbed skeleton. Usually the central nucleation part of the latter is located below the melt level, whereas the outer upper frame of the skeleton protrudes above the melt surface. The noted feature of the crystal structure indicates that they grow analogously to the growth of the well-known crystalline “boats” of sodium chloride.

It follows from the foregoing that the “boat” and “pyramidal” growth forms of yttrium ferrite crystals in the surface part of the melt are caused by a change in the density of the surrounding nutrient solution, associated with a change in the ratio of the light and heavy components of the complex solvent. As a result, the crystals either float above the melt surface, breaking away from the nutrient medium, or sink, bending the surface film of the melt with the formation of boats.

As is known, molten solutions undergo gravitational differentiation by specific weight. In particular, the fact of differentiation of the solution has also been established for systems used for obtain-

…of yttrium ferrite crystals in the molten solution [3]. As we showed earlier in a report at the Second Conference on Mineralogical Crystallography (Sverdlovsk), the magnitude of differentiation of multicomponent solutions in a gravitational field during the pre-crystallization period reaches considerable values, giving rise to conical symmetry of the crystal-forming medium. In this connection, the differentiation was studied of a complex solvent containing equal molar fractions of lead oxide and lead fluoride, since it is precisely at the indicated initial ratio of solvent components that the least pronounced skeletal growth of the upper part of the crystal was experimentally established.

In the melt with the indicated initial ratio of components, using a method developed at the Tomsk Polytechnic Institute for the direct determination of the concentrations of components in a solution–melt by gamma transillumination, it was established that in the investigated melt layer, about 25 mm thick, at a temperature close to the crystallization nucleation of ferrite, the fluctuationally varying concentration of lead fluoride in the upper part of the melt reaches 70–80 mol. %, whereas in the lower part of the melt lead fluoride is practically absent. Thus, directly in the zone of spontaneous crystallization, as a result of the action of the gravitational factor in the solvent, which provides optimal conditions for nonskeletal growth of crystals, a considerable excess of lead fluoride is established, promoting the preferential growth of the planes of edges normal to the faces \(\{211\}\), and thereby the limitation of the crystal by the faces of the rhombododecahedron [1, 3].

Fig. 3. Growth forms of yttrium ferrite crystals in the upper part of the melt at a content of lead fluoride in the initial solvent composition of 50 mol. %. Magnification 20×.

It follows from the foregoing that, when choosing the ratio of components of a complex solvent, it is necessary to be guided not only by the condition of improving the solubility of the useful phase and lowering the temperature of its crystallization nucleation, but also by the density of the mother solution that develops by the beginning of crystallization. It is especially important to observe this condition when choosing systems for growing single-crystal films on the surface of a molten solution. In this case it should be borne in mind that the complex solvent undergoes substantial differentiation by specific gravity, which sharply changes the initial ratio of solvent components in the crystallization zone and, consequently, can substantially alter the facial morphology of the crystals being grown.

REFERENCES

1. S. Sh. Gendelev. Kristallografiya, Vol. 8, No. 3, pp. 431–436, Moscow, 1963.

2. S. Sh. Gendelev, A. G. Titova. In: Growth of Crystals, Vol. VI, p. 98. Izd. “Nauka,” Moscow, 1965.

3. V. A. Timofeeva, R. A. Voskanyan. Kristallografiya, No. 5, Vol. 6, Moscow, 1961.

4. I. I. Shafranovskii. Lectures on Crystallomorphology. Izd. “Vysshaya …”