TITLE:
Fluorite Phase Transformations under Vacuum
AUTHORS:
J. Mouhovski, O. Vitov, V. Dimov, B. Kostova, S. Gechev
KEYWORDS:
Vapour Phase, Fluorspar Purification, Crystal Growth, Optical Medium
JOURNAL NAME:
Journal of Crystallization Process and Technology,
Vol.4 No.3,
July
28,
2014
ABSTRACT:
Complex
crystal aggregates from fluorspar vapor phase were grown at specific low-pressure/high-temperature conditions. The quasi-equilibrium of initiated crystal-chemical
reactions at the proceeding vapour-crystal phase transformation was strongly
dependent on the mass-transport inside an originally designed multicameral
crucible, loaded by several portions of natural fluorite. By changing the
temperature pressure over the already molten fluorspar portions as well as the
gas-permeability of the channels connecting different sections in crucible
interior to vacuum ambient, one may control the rate of gaseous-vapour
diffusion and the degree of supper-saturation inside the peripheral crucible
compartment wherein nucleation and crystal growing occurred. In this way, grown
aggregates revealed a complicated habit formed during three-stage growing
process provided by relevant thermodynamic and phase. Residual stresses were
not observed in the aggregates whereas those in simultaneously grown boules
from the non-vaporized melts in crucible cameras were clearly distinguished. The
optical transmittance spectra of the boules were obtained considerably better,
especially in the UV, comparing to those for crystal aggregates, both showing
several peaks of specific light-absorption due to enhanced presence of
rare-earth (RE) impurities. The aggregates manifest nearly full reflectivity
from Vis to near IR region. The vapor phase growth
mechanisms, when natural fluorite with some RE contents has been used, were explained
on thermodynamic grounds that shown the manner of reliable control on the
phenomenon. The results were anticipated to help for developing new perspective
techniques for growth from vapor of several fluoride compounds with complex
structure and composition and wide application. It was speculated that similar growth mechanisms of CaF2crystals were possible on the moon in its very early period of formation.