Precipitation in LiF Crystals Doped with MgF(2)*

X-ray, and optical and electron microscopy studies have been made of precipitation in single crystals of LiF doped with MgF(2). MgF(2) was found to nucleate only with difficulty, which probably accounts for the large MgF(2) precipitates that were observed. However, a metastable phase was discovered which readily nucleated, and, at low concentrations (e. g. 0.045 mol % MgF(2)), it formed even on slow cooling. Its structure appears to be similar to that proposed by Suzuki for the NaCI/CdCI(2) system. The morphology of the metastable phase was seen by electron microscopy to be intersecting {100} platelets. On holding crystals containing the metastable phase at temperatures of about 300 degrees C, particles of MgF(2) nucleated at, and grew at, the expense of the metastable phase. The observation of precipitation of the metastable phase at low Mg(++) ion concentrations is notable, for precipitation has almost certainly occurred during other studies of ionic conductivity, dielectric loss, and colour centres, etc., where crystals have been held at temperatures below 250 degrees C. Solid-solubility measurements have been made to establish the MgF(2) solvus line at concentrations up to 6.5 mol % MgF(2) and the metastable phase solvus line up to 0.225 mol % MgF(2). Kinetic measurements indicate that the growth of the metastable phase is controlled by bulk diffusion of Mg(++) ions, whereas the growth of MgF(2) appears to be controlled by the motion of vacancy pairs.