Abstract

Synthetic bromellite (BeO) macrocrystals grown from molten lead oxide, lead fluoride, and their equimolar mixtures using slow cooling techniques arc nearly always prismatic with one end terminated by a small pyramid. Crystals grown from molten alkali vanadates mo vbdates, and tungstates using a modified Kruger-Fincke method have essentially four basic habits, i.e., platy, prismatic, hemi-pyramidal, and dendritic, although intermediate developments also were observed. The important parameter determining whether a crystal be elther a plate (length/width<0.81) or a prism (length/width>0.81) is the temperature. Supersaturation has a smaller, nonsystematic effect on habit control, and is important in regulating nuclealion phenomenon and growth rates. Pyramids (length/width=0.81) and dendritic forms (length/width>>0.81) are rare and occur under rather narrow conditions of temperature and/or supersaturation. Growth habit relationships and positions of nudeation zones were sensitive to the presence of small concentrations of impurities, particularly tetrahcdrally coordinating cations with ionic radii <0.38 Å. The pattern of habit change from plates →prisms→pyramids, as the temperature changes from 1400 to 1000 (, was rationalized in terms of a surface diffusion mechanism similar to the one proposed for habit control in ice, but calculations of nucleation rates suggest that a nudeation phenomenon may also be important, particularly at high supersaturations. The techniques are described for growing large, high-quality crystals from molten lithium molybdate fluxes. Growth rates measured normal to basal surfaces as a function of temperature indicate that the enlargement process in molten lithium dimolybdate requires an act ivation energy of 15 kcal/mole. Overgrowth on BeO single crystals and on orientated, polycrystalhne substrates suggests that nucleation occurs at surface impurities and growth proceeds along the preferential directions (00·1]* and [11·0|*.

High-quality crystals, 1.5 cm in length, have been obtained within 24 hours by hydrolyzing BeF- vapor in air at 1000“ C. The high growth rates results in a somewhat more complex morphology than in other methods, but the crystals are still dominantly prismatic. Crystals oblained from hydrothermal experiments using 2-N NaOH solutions at 420-400 C and 2000 bars are in general of highest quality although much smaller in size and purity. All crystals show essentially the same prismatic habit.

Twinning which could be initiated by selective impurities was commonly observed in all the environments studied and obeys one twin law with (00.1) or (00.1) as the twin plane. The relationships of the structure across the twin boundaries for the two types of twins observed, contact and penetration, are discussed in detail. Etching studies, x-ray diffraction topography, and optical microscopy of penetration twins show the important imper ections to be screw dislocations with [00-1|* Burgers vectors in the central core. These data permitted the growth of penetration type twins to be rationalized in terms of the simultaneous operation of a dassical screw dislocation and twin boundary mechanism.

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