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Complexcrystal aggregates from fluorspar vapor phase were grown at specific low-pressure-high-temperature conditions. The quasi-equilibrium of initiated crystal-chemicalreactions at the proceeding vapour-crystal phase transformation was stronglydependent on the mass-transport inside an originally designed multicameralcrucible, loaded by several portions of natural fluorite. By changing thetemperature pressure over the already molten fluorspar portions as well as thegas-permeability of the channels connecting different sections in crucibleinterior to vacuum ambient, one may control the rate of gaseous-vapourdiffusion and the degree of supper-saturation inside the peripheral cruciblecompartment wherein nucleation and crystal growing occurred. In this way, grownaggregates revealed a complicated habit formed during three-stage growingprocess provided by relevant thermodynamic and phase. Residual stresses werenot observed in the aggregates whereas those in simultaneously grown boulesfrom the non-vaporized melts in crucible cameras were clearly distinguished. Theoptical transmittance spectra of the boules were obtained considerably better,especially in the UV, comparing to those for crystal aggregates, both showingseveral peaks of specific light-absorption due to enhanced presence ofrare-earth RE impurities. The aggregates manifest nearly full reflectivityfrom Vis to near IR region. The vapor phase growthmechanisms, when natural fluorite with some RE contents has been used, were explainedon thermodynamic grounds that shown the manner of reliable control on thephenomenon. The results were anticipated to help for developing new perspectivetechniques for growth from vapor of several fluoride compounds with complexstructure and composition and wide application. It was speculated that similar growth mechanisms of CaF2 crystals were possible on the moon in its very early period of formation.

KEYWORDS

Vapour Phase, Fluorspar Purification, Crystal Growth, Optical Medium

Cite this paper

Mouhovski, J. , Vitov, O. , Dimov, V. , Kostova, B. and Gechev, S. 2014 Fluorite Phase Transformations under Vacuum. Journal of Crystallization Process and Technology, 4, 156-167. doi: 10.4236-jcpt.2014.43020.





Author: J. Mouhovski, O. Vitov, V. Dimov, B. Kostova, S. Gechev

Source: http://www.scirp.org/



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