Fluorite Phase Transformations at Variety of Pressure – Temperature Crystal Growing Conditions

Abstract

Complex crystal aggregates from fluorspar vapour 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 are strongly dependent on the mass-transport inside an original, specially designed multicameral crucible, whose cameras (inserts) were loaded by several portions of fluorspar prepared from natural fluorite, being the inserts separated from the rest inner part of the crucible through lids fit out by appropriate narrow channels. After deep evacuation of crucible's interior, by changing the temperature heat over an already molten fluorspar portions as well as using lids’ channels with different gas-permeability, it is succeeded a control in the rate of gaseous-vapour diffusion, in a way to ensure super-saturated conditions inside the crucible interior wherein nucleation and crystal growing occurred simultaneously. In that way grown aggregates revealed a complicated behaviour accordingly three-stage distinguishable growing process provided by relevant thermodynamics and phases. Residual stresses were not found into the aggregates grown from vapour phase, whereas stresses were clearly distinguished into the crystal boules grown simultaneously from the non-vapourized melts’ parts inside the crucible inserts. At that, the optical transmittance spectra of the boules were obtained considerably better, especially in the UV (ultra-violet) range, comparing to those obtained from the crystal aggregates, as both type spectra showing several peaks of specific light-absorption due to enhanced presence of rare-earth elements (REEs) impurities. The aggregates manifest high absorption from VIS (visible), up to NIR (near infra-red) region. The vapour phase growth mechanisms, imposed when natural fluorite with some REEs contents has been used, were explained on thermodynamic grounds that revealed the right manner for reliable control on the phenomenon. The results were anticipated to help for improving and/or developing contemporary perspective techniques for growing from vapour phase several single or/and mixed alkaline-earth metal (AEMs) fluoride crystal systems with complex structure and composition and wide scope of application as promising materials for: photonics and electrochemical devices, fluoride type ionic conductors and others. It could be speculated as well that similar vapour growth mechanisms of CaF₂ crystal formations would be possible to occur on the bowels of the Moon in its very early period of its existence.