Collective Processes in the Nucleation and Growth of Ferrite Crystals in a Molten Solution

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

During the crystallization of yttrium and lead ferrites, observations of crystal nucleation revealed features indicating collective phenomena in the formation of embryonic films in the surface layer of the solution. In the films, an orientation was observed of individual needle-shaped crystallites corresponding to the crystalline structure of the future single crystal, which may indicate a mechanism of sequential aggregation and orientation of “prenucleation” molecular associations.

The existence of aggregates of molecular complexes in saturated and unsaturated solutions is generally recognized [1–6]. The concept of associated complexes has been broadened by theoretical conclusions regarding the existence in molten solutions of crystallizable and anticrystallizable molecular associations—clusters [5]. It is acknowledged that in multicomponent systems the emergence and growth of nuclei of the crystallizing phase occur through the successive enlargement of “prenuclei” to critical dimensions [2, 4]. The studies of A. Ubbelohde, in particular, have shown that crystal nuclei are formed only from ordered crystallizable clusters [5]. In the precrystallization period, as the density of the melt increases, one may expect the appearance of microregions with an oriented arrangement of crystallizable clusters among chaotically arranged anticrystallizable molecular associations. It is precisely in the indicated microregions that fluctuation regions with high supersaturations should be expected, which are necessary for the nucleation and growth of crystals according to molecular-kinetic theory.

In the study of the spontaneous nucleation of lead and yttrium ferrites, data were obtained that confirm the above-described mechanism of crystal nucleation and growth. The crystallization of the indicated ferrites was carried out in a molten solution prepared from oxides of iron, yttrium, lead, and lead fluoride, taken in the following proportion: \(8Y_2O_3 \cdot 22Fe_2O_3 \cdot 30PbO \cdot 40PbF_2\) (in weight percent). Owing to gravitational differentiation and evaporation of part of the solution in the indicated system, the first crystals always appeared in the surface film of the melt, where the supersaturation necessary for crystal nucleation was created first of all.

The studies were conducted by direct visual observation, with a microscope, of the initial growth of crystals in the surface film of the melt, followed by microscopic study of the stru-

…of the grown crystals in the plane of their initial growth. Crystallization was carried out in open platinum crucibles with a volume of about 40 cubic cm, filled with molten solution to 0.2 of their volume. To conduct the experiments, the furnace was first preheated; the crucible was then introduced into it and heating was carried out to the temperature of complete dissolution \((1220—1250^\circ\mathrm{C})\).

When the melt was cooled in the switched-off furnace at a temperature close to the crystallization point, instantaneous local fluctuations of luminescence were observed on the surface of the melt. At the supersaturation temperature, crystals usually appeared in the “dimming” regions in the form of point centers and clusters ranging in size from fractions of a millimeter to the first millimeters. The clusters—subcrystals—seemed to “develop” on the surface of the melt like a photographic print in a developer. Owing to the fact that ferrite crystals have a lower density than the solution, they float up, partially protruding above the surface of the melt; as a result, it is possible to observe the initially formed embryonic structures, which have varying degrees of perfection.

The structure and stages of formation of the embryo could be studied most clearly in the growth of magnetoplumbite crystals. The embryonic part of the crystals is characterized by microroughness (Fig. 1) and is composed of individual, very fine needle-like crystallites or their skeletal six-rayed aggregates, the former being subject to the hexagonal outline common to the given crystal (Fig. 2). Consequently, it may be assumed that the crystallites of the seed film are formed in regions of oriented accumulations of pre-embryonic molecular complexes which, in the precrystallization period, themselves arrange themselves according to the crystallochemical type of the lattice structure of the future solid phase.

Aggregation of the crystallizing clusters and growth of the crystallites probably proceed by a collective route, the mechanism of which can be illustrated by the example, studied by us, of the growth of closely situated crystals of potassium bichromate. With parallel placement of two chromium potassium alum crystals of equal size in a supersaturated solution, their simultaneous and rapid growth was observed predominantly from the inner side, up to the formation of a single crystal. Placement of two identical crystals, disoriented relative to one another by \(180^\circ\), led to preferential growth of their outer faces. In the final result a twin was formed. With parallel placement of small crystals among large ones, equal growth of all crystals was observed with

Fig. 1. Microrough relief of the embryonic surface of a magnetoplumbite crystal. Magnification 10×.

Fig. 2. Hexagonal orientation of needle-like crystallites and their aggregates. Magnification 40×.

with the formation of a single large crystal. Small crystals situated obliquely between parallel-oriented large crystals were inhibited in growth and even dissolved. Hence it may be supposed that, in collective processes, only those clusters and crystallites whose orientation does not coincide with the common crystallographic direction of larger, parallel-arranged neighbors undergo dissolution and destruction.

The foregoing makes it possible to draw the following obvious conclusions.

1. In molten solutions, the growth of ferrite crystals and their nucleation are controlled by collective processes, which take place both in the precrystallization period and after it.

2. In the precrystallization period, the crystallizing clusters form the proto-skeleton of the future skeletal structure of the solid phase.

3. The growth of the skeletal structures of the solid phase occurs predominantly at the expense of those crystallites that have a crystallographic orientation common with the skeleton.

REFERENCES

1. G. Blum and J. O’M. Bockris. The Structure of Molten Salts, “Mir,” Moscow, 1966, p. 7.

2. R. O. Grisdale. Theory and Practice of Crystal Growth. Metallurgiya Publishers, Moscow, 1968, p. 176.

3. A. M. Kuz’min. Izv. Sib. Otd. AN SSSR, 6, 1958, p. 10.

4. N. N. Sirota. Crystallization and Phase Transitions. Publishing House of the Academy of Sciences of the BSSR, Minsk, 1962, p. 11.

5. A. Ubbelohde. Melting and Crystal Structure. “Mir,” Moscow, 1969, p. 311.

6. Ya. I. Frenkel. Kinetic Theory of Liquids. Moscow–Leningrad, 1945.