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.
The Origin of Mechanical Twins in (NH4)2SO4
We have found that, if in (NH 4 ) 2 SO 4 a hexagonal phase happened to exist somewhere on higher temperature side, we can explain group-theoretically various experimental results of mechanical twins observed in (NH 4 ) 2 SO 4 crystals as straightforward consequences of a hypothetical hexagonal-orthorhombic phase transition. The transition occurs through an instability at the M point of the Brillouin zone. The microscopic picture of the transition may be the ordering in the orientations of the tetrahedral SO 4 - ions.
Studies in the System BaO‐Al2O3−SiO2: V. The Ternary System Sanbornite‐Celsian‐Silica
Phase relations in the BaSi2O5−BaAl2Si2O8−SiO2 portion of the system BaO-Al2O3−SiO2 were reinvestigated by the standard quenching method. In contrast to the results of earlier studies, this portion is revealed as a simple ternary system; its features include a ternary eutectic at 1122°±7°C and a composition of BaO 35.2, Al2O3 9.5, and SiO2 55.3 wt%, involving the phases sanbornite, celsian, and silica. These results are compared with those of earlier quenching studies, with relations in analogous ternary systems, with data from hydrothermal experiments, and with observations of natural mineral assemblages.