Solid transformations and premelting phenomena in fluorite structures and the system SrCl₂BaCl₂

<p>"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.</p> <p>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.</p> <p>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.</p>

Name thesis:degree_name

Ph. D. in Ceramic Engineering

Grantor

University of Missouri--Rolla

Year dc:date.available

2016

Author dc:creator

• Sukhotanang, Damri

• Ceramic Materials

Repository record dc:identifier

https://scholarsmine.mst.edu/doctoral_dissertations/291

OAI identifier oai:identifier

oai:scholarsmine.mst.edu:doctoral_dissertations-1293