Bulk Crystallization in a SiO(2)/Al(2)O(3)/Y(2)O(3)/AlF(3)/B(2)O(3)/Na(2)O Glass: Fivefold Pseudo Symmetry due to Monoclinic Growth in a Glassy Matrix Containing Growth Barriers

A glass with the mol% composition 17 Y(2)O(3)·33 Al(2)O(3)·40 SiO(2)·2 AlF(3)·3 Na(2)O·2 CeF(3)·3 B(2)O(3) is heat treated at 1000 °C for 6–24 h. This results in the surface nucleation and growth of YAG. Nucleation and growth of star-shaped alumina and later of monoclinic β-Y(2)Si(2)O(7) and orthorhombic δ-Y(2)Si(2)O(7) are additionally observed in the bulk. Phase identification and localization are performed by electron backscatter diffraction (EBSD) as well as TEM analysis. The monoclinic β-Y(2)Si(2)O(7) observed in the bulk occurs in the form of large, crystal agglomerates which range from 50 to 120 μm in size. The individual crystals are aligned along the c-axis which is the fastest growing axis. Ten probability maxima are observed in the pole-figures illustrating the rotation of orientations around the c-axes indicating a fivefold symmetry. This symmetry is caused by multiple twinning which results in a high probability of specific orientation relationships with rotation angles of ~36°, ~108° (also referred to as the pentagon angle) and ~144° around the c-axis. All these rotation angles are close to the multiples of 36° which are required for an ideal fivefold symmetry. This is the first report of a fivefold symmetry triggered by the presence of barriers hindering crystal growth.