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 pseudo-orthorhombic 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 Al₂O₃ 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-phase-sintered Al₂O₃, and may be devitrified by postsintering heat treatments.