Topotactic growth of α-alumina platelets on 2/1 mullite single crystal surfaces upon thermal decomposition of mullite in dry and wet atmospheres

Annealing experiments of Czochralski-grown 2/1 mullite single crystal coupons, oriented parallel to (1 0 0), (0 1 0), and (0 0 1) were performed in the temperature range 1200–1650 °C for 4–16 h in different atmospheres (dry versus wet air) at normal pressure. Upon thermal decomposition of the Al 2O 3-supersaturated 2/1 mullite, metastable in the two-phase region “mullite plus Al 2O 3”, the simultaneous release of alumina and silica gave rise to nucleation and growth of thin, perfect (0 0 · 1) α-alumina platelets which were completely wetted by a glassy phase. The chemical composition of the initially siliceous glassy phase at the mullite surfaces was shifted to a Na–Mg–Ca bearing alumosilicate composition via uptake of network modifying cations from external impurity sources (kiln furnace material, alumina reaction tube), independent of the nature of the atmosphere. It is this modified glassy phase which filled the mullite dissolution paths driven forward by the growing platelets and provided a build-in flux. Growth faces of α-alumina included only special forms of the trigonal point group 3 ¯ 2 / m . The habit was very similar to that of sapphire crystals grown from molten fluxes, in agreement with the Wulff shape of undoped α-alumina. Three different types of α-alumina platelets could be distinguished including (i) type I platelets parallel, (ii) type II platelets perpendicular, and (iii) type III platelets inclined to the mullite surfaces. While type I showed on all mullite substrates investigated, type II platelets did predominantly occur on (0 0 1) and were rarely observed on (1 0 0) and (0 1 0) mullite surfaces. Type III platelets were restricted to (1 0 0) and (0 1 0) mullite surfaces. Platelet types II and III were not independent from each other. Furthermore, for each type of platelets the azimuthal rotation around [0 0 · 1] was restricted to well defined snap-in conditions. A total of 14 symmetrical non-equivalent, topotactic α-alumina/mullite orientation relationships was derived unambiguously by the combined approach of optical microscopy, X-ray diffraction, and focused-ion-beam assisted transmission electron microscopy confirming previous solutions as a sub-assembly group. Possible composition planes characteristic of platelet nucleation on mullite were identified and discussed in terms of coherency of cation sublattices. Short-term wet annealing of 2/1 mullite at 1650 °C characterized by a low gas flow rate and a water partial pressure of P H