Hot-stage transmission electron microscopy study of phase transformations in hexacelsian (BaAl2Si2O8)
Phase transformations in synthetic hexacelsian were investigated by hot-stage transmission electron microscopy. A second phase transformation from an orthorhombic to hexagonal structure was identified in the synthetic hexacelsian at approximately 700 °C. The hexacelsian was found to exhibit a sequence of phase transformations on heating of hexagonal (P6 3 /mcm)-orthorhombic (Immm)-hexagonal (P6/mmm). Antiphase domain boundaries, which were observed in P6 3 /mcm and Immm phases, were absent in the P6/mmm phase. Crystal symmetries of the three phases were determined by convergent beam electron diffraction, and space group symmetries were derived by comparison of experimental selected area electron diffraction patterns with computer-simulated patterns.
Investigation of a Phase Transition in Pure and Magnesia- and Titania-Substituted Hexacelsian
Hexacelsian, the hexagonal polymorph of BaAl2Si2O8, experiences a phase transition near 300oC with an accompanying large volume change, which can lead to cracking and thermal shock, limiting the potential applications of hexacelsian. It has previously been reported that certain additions, including MgO and TiO2, can suppress the undesirable phase transition. Room-temperature neutron and high-temperature X-ray diffraction have been used to study the structure and thermal behavior of hexacelsian, both pure and substituted with varying amounts of MgO and TiO2. The diffraction data of the pure sample was refined using the P-3 space group. Attempting to refine the substituted sample with the same structure led to significant error due to peak intensity mismatch and the presence of extra peaks. The most likely explanation for the poor fit may be that the addition of Mg and Ti cations to hexacelsian significantly alters the structure in a way that has not been accounted for in the structural model thus far. Additions up to 10mol% MgO and 6.67mol% TiO2 did not suppress the α↔β transition near 300oC in a solid-state synthesized sample. However, it has been previously claimed that additions of 5-25mol% MgO, 6-14mol% TiO2, 0-10mol% ZnO, and 0-8mol% ZrO2 would suppress the α↔β transition, when using melt-glass crystallization synthesis. Also, thermal expansion coefficients for solid-state synthesized samples were calculated and found to be lower than those previously reported for zeolitederived synthesis. It may be concluded that the method of synthesizing pure and substituted hexacelsian affects the thermal properties and phase relationships.
Phase Equilibrium in a Portion of the Ternary System Ba OA-I2,O3,-SiO2,
Phase-equilibrium relations on the liquidus surface in the system Ba0-A12O3-SiO2 have been investigated by the quenching method. The compositions investigated within the ternary area were those containing less than 30%, A12O3 and more than 20% SiO2 by weight. Petrographic and X-ray techniques were employed in the determination of the crystalline phases. 
 
 
 
The crystal phases that separate from melts within the area investigated are barium orthosilicate (2BaO. SiO2,), barium metasilicate (BaO 2SO2,), solid solutions, sanbornite (BaO 2SiO2), tridymite and cristobalite (SO2), mullite (3A12O3 2SiO2), and celsian (BaO A12O3.2SiO2). Diagrams show the isotherms and indices of refraction of the glasses. 
 
 
 
Five quintuple points and eleven boundary curves have been determined within = .5yo compositional variations. The liquidus-surface temperatures have been obtained within limits of ± 125°C.