Fluorite Phase Transformations under Vacuum

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 CaF₂ crystals were possible on the moon in its very early period of formation.