Investigation of a phase transition in pure and magnesia- and titania-substituted hexacelsian

by Kirkham, Melanie Jean.

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 Mg2+ and Ti4+ 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 ??? 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 ??? 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. iii