Dendritic growth of CaO single crystals

Dendritic growth of CaO single crystals

603 Journal of Crystal Growth 57 (1982) 603-604 North-Holland Publishing Company LETTER TO THE EDITORS DENDRITIC GROWTH OF CaO SINGLE CRYSTALS H...

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603

Journal of Crystal Growth 57 (1982) 603-604 North-Holland Publishing Company

LETTER

TO THE EDITORS

DENDRITIC

GROWTH OF CaO SINGLE CRYSTALS

H. OHSATO

and T. SUGIMURA

Deparimenr of Inorganic Materials, Nagoya Institute of Technology, Gokiso-cho, Showa-ky Received

6 January

1982; manuscript

received

in final form 16 February

Nagoya 466, Japan

1982

Dendrites of CaO were grown at 1400°C from molten CaCl, in a wet atmosphere on the bottom of a Pt crucible. Some of them were like flowers with four petals, The elongation of the petals was in the [I I I] direction and the surface was shaped like { 100) step planes. Other dendritic crystals obtained are also described. Their crystal habits are explained by the relative growth rate.

Previously [l], we reported on the growth of CaO single crystals from molten CaCl, in a wet atmosphere. The habit of the crystals was explained by the relative growth rate along [ 11 l] and [loo] at different temperatures. At low temperatures octahedral crystals with { 11l} faces and at high temperatures cubic crystals with { lOO} faces were grown. At intermediate temperatures, around 1200°C, the crystal had a combination of the above two crystal habits. In this note, dendritic

Fig.

growth of CaO was reported and their crystal habits were also explained by the relative growth rate. The dendrites of CaO were grown at 14OO’C from molten CaCl, in a wet atmosphere on the bottom of the Pt crucible. Some of them were like flowers with four petals, as shown in fig. 1. The others were grown in contact with the Pt surface: elongation of the dendrites was parallel to the surface of the Pt crucible. An example is shown in

I. (a), (b), (c) Flowery crystals with four petals; (d) steps on the surface of petals

Fig. 2. (a) Dendrites

on surface

of platinum

0022-0248/82/0000-0000/$02.75

plate; (b) part of (a) magnified;

0 1982 North-Holland

(c) steps on the surface

of the dendrite.

fig. 2. The flowery crystals were confirmed to be single crystals by X-ray precession method. Elongation of a petal was along the crystallographic direction of [ll l] and three-fold symmetry was observed at the top of the petals as shown in figs. la- lc. On the surface of the petals, fine steps were found, as shown in fig. Id, which seemed to be composed of fine { lOO} faces of the cube. On the surfaces of the dendrites contacted with platinum, fine steps were also observed, as shown in fig. 2c, and the steps seemed to be composed of { lOO} faces, just as in the case of the flowery crystals. As reported previously [l], the condition of r[ 11 l]/r[ lOO]> fi was supposed to be fulfilled at 1300°C, where r[l 1 l] was the growth rate along the [ill] direction and r[lOO] that along the [loo] direction. Cubic crystals with { IOO} faces and parallel growth of these cubes along the [ll l] direction were found. At 14OO”C, it is reasonable to suppose that r[ 11 l] is very large in comparison with r[lOO]. Therefore, the dendritic growth along the [ 11 l] direction occurred and its rate was so fast that the { 100) faces developed only as steps on the surface of the dendrites. The flowery crystal seemed to grow from a nucleus on the bottom of the crucible along four-corner directions of the cube, i.e. the [ 11 l] direction. At 1250°C skeletal crystals were also obtained, an example being shown in fig. 3. The elongation of these crystals was along the [ 11 l] direction and the steps on the surfaces were composed of { lOO} faces as in the cases of the flowery crystals and the dendrites obtained at high temperatures. The

Fig. 3. Skeletal direction.

dendrite

with

(100)

steps elongated

to

[I I I]

skeletal crystals are much clearer. At this temperature, a combined crystal habit of { 111) and { 100) faces was found on single crystals and dendrites. However, the growth rates differed from place to place in the Pt crucible because of inhomogeneous distribution of the solute CaO. In some places, mostly at the surface of molten CaCl,, r[ll l] seemed to exceed r[ 1001 so remarkably that skeletal growth occurred. At this temperature, however, the ratio r[lll]/r[lOO] could not be as high as at 14OO’C. So the { lOO} faces developed more clearly on these skeletal crystals than the flowery crystals and the dendrites obtained at high temperatures.

Reference [I] H. Ohsato, Growth

T. Sugimura 48 (1980) 459.

and

K. Kageyama.

J. Crystal