Crystallization of pseudo-orthorhombic anorthite on basal sapphire

Anorthite-glass films were grown on basal Al2O3 substrates using pulsed-laser deposition. The substrates were cleaned and annealed in air at 1400°C to produce crystallographically flat (0001) terraces. The films were deposited in an oxidizing environment. X-ray microanalysis confirmed the composition of the glass films to be close to that of anorthite (CaO·Al2O3·2SiO2). Although anorthite usually has triclinic symmetry, subsequent crystallization of these films in air at 1200°C resulted in the formation of pseudoorthorhombic CaAl2Si2O8 (o-anorthite), a known metastable form of the mineral. Microstructural characterization was performed using visible-light microscopy, scanning electron microscopy, and transmission electron microscopy. The films dewetted the substrate either before or after crystallization to form o-anorthite islands which had strong orientation relationships to the A12O3 substrate. The epitaxy of the o-anorthite islands was accompanied by a small lattice mismatch parallel to the substrate plane. The formation of three orientational variants is consistent with the symmetry of the basal Al2O3 surface. The dislocation network observed at the o-anorthite/Al2O3 interface indicates that nucleation and growth of the anorthite occurs directly on the substrate surface without an intervening interfacial amorphous layer. The study of anorthite-glass films is important because they are present in liquid-phasesintered A12O3, and may be devitrified by postsintering heat treatments.