Crystallization of pseudo-orthorhombic anorthite on basal sapphire

Anorthite-glass films mere grown on basal Al2O3 substrates using pulsed-las er deposition. The substrates were cleaned and annealed in air at 1400 degr ees C to produce crystallographically flat (0001) terraces. The films mere deposited in an oxidizing environment. X-ray microanalysis confirmed the co mposition of the glass films to be close to that of anorthite (CaO . Al2O3. 2SiO(2)). Although anorthite usually has triclinic symmetry, subsequent cr ystallization of these films in air at 1200 degrees C resulted in the forma tion of pseudoorthorhombic CaAl2Si2O8 (o-anorthite), a known metastable for m of the mineral. Microstructural characterization was performed using visi ble-light microscopy, scanning electron microscopy, and transmission electr on microscopy, The films dewetted the substrate either before or after crys tallization to form o-anorthite islands which had strong orientation relati onships to the Al2O3 substrate. The epitaxy of the o-anorthite islands was accompanied by a small lattice mismatch parallel to the substrate plane. Th e formation of three orientational variants is consistent with the symmetry of the basal Al2O3 surface. The dislocation network observed at the o-anor thite/Al2O3 interface indicates that nucleation and growth of the anorthite occurs directly on the substrate surface without an intervening interfacia l amorphous layer. The study of anorthite-glass films is important because they are present in Liquid-phase-sintered Al2O3, and may be devitrified by postsintering heat treatments.