Polyhedral tilting: A common type of pure displacive phase transition and its relationship to analcite at high pressure

Polyhedral tilt transformations are a subgroup of pure displacive solid-solid phase transitions that occur in many ionic compounds and have all the following characteristics: (1) the transitions occur in compounds with structures composed of corner-linked, rigid polyhedral elements; (2) transitions are between a higher-symmetry or less-distorted form (stable at higher temperature or lower pressure) and a lower-symmetry or more-distorted form (stable at lower temperature or higher pressure); (3) transitions are nonquenchable, and single crystals are preserved through the transition; (4) twinning is common in the low-symmetry form, with the twin law governed by a symmetry operator lost in the high-to-low transition; (5) the value of dP/dT is always positive and is similar to the ratio of large-site thermal expansivity to compressibility. Analcite, (NaAl)x Si2-x O6 · H2O with 0.9 > x < 1.0, is a zeolite mineral that undergoes several polyhedral tilt transitions. Under room conditions analcites are pseudo-cubic; seven dimensionally distinct varieties are cubic, tetragonal c < a and c > a, orthorhombic, monoclinic with b parallel to pseudo-cubic [110] or [100], and triclinic. Several phase transitions have been detected from unit-cell measurements at high pressure on single crystals. At 4 kbar, orthorhombic and tetragonal analcites become monoclinic (C centered) with b parallel to pseudo-cubic [101]. No volume change is observed at this transition. At 8 kbar a volume discontinuity greater than 1 percent is observed. At 12 kbar dimensionally-monoclinic analcite becomes triclinic with no apparent volume change. At about 18 kbar a second volume discontinuity of 0.25 percent is observed, indicating a fourth transition. All these transitions conform to the criteria of polyhedral tilt transitions.