Solid transformations and premelting phenomena in fluorite structures and the system SrCl₂BaCl₂
"Thermal expansion and phase transformations in CaF2, BaF2, SrCl2, and BaCl2 were studied by high temperature x-ray diffractometry, differential thermal analysis, and thermodilatometric analysis. Thermal expansion characteristics of CaF2, BaF2 and SrCl2 are very similar. The structures expand normally at lower temperatures and depart substantially from linearity at higher temperatures, because of formation of defects and disordering of anions. At high temperatures CaF2 and BaF2 react with water vapor, even at low humidities, forming oxides and emitting HF, leading to difficulty in observation of phase transformations. SrCl2 undergoes the transformation from the cubic fluorite to a hexagonal structure in the temperature range 660°-750°C, with nucleation and grain growth. BaCl2 occurs in three forms: cubic, orthorhombic, and hexagonal. The cubic form is stable below about 165°C and transforms irreversibly to the orthorhombic form; at 920°C the orthorhombic form is changed to a highly disordered hexagonal structure, similar to the hexagonal form of SrCl2. Cubic fluorite compounds can change to the orthorhombic PbCl2 structure, to a hexagonal structure, or to a disordered structure, depending on pressure, temperature, and the chemistry of compound. The mechanism of the transformation is related to movement of anions in the open fluorite structure, followed by changes in first-coordination. The study of phase relationships of the system SrCl2-BaCl2 shows that at high temperatures SrCl2 forms a complete solid solution series with BaCl2, having hexagonal structure, with a freezing point minimum of 845°C at about 28 mole percent BaCl2. At lower temperatures the samples with more than 35 mole percent BaCl2 form orthorhombic solid solutions and those with less than 25 mole percent BaCl2 form cubic solid solutions. Orthorhombic solid solutions invariably persist at room temperature rather than inverting to the stable cubic structure"--Abstract, pages ii-iii.
Missouri University of Science and Technology
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