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2016
Journal Article
Titel
Bulk crystallization in a SiO2/Al2O3/Y2O3/AlF3/B2O3/Na2O glass
Titel Supplements
Fivefold pseudo symmetry due to monoclinic growth in a glassy matrix containing growth barriers
Abstract
A glass with the mol% composition 17 Y2O3 center dot 33 Al2O3 center dot 40 SiO2 center dot 2 AlF3 center dot 3 Na2O center dot 2 CeF3 center dot 3 B2O3 is heat treated at 1000 degrees 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 beta-Y2Si2O7 and orthorhombic delta-Y2Si2O7 are additionally observed in the bulk. Phase identification and localization are performed by electron backscatter diffraction (EBSD) as well as TEM analysis. The monoclinic beta-Y2Si2O7 observed in the bulk occurs in the form of large, crystal agglomerates which range from 50 to 120 mu m 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 similar to 36 degrees, similar to 108 degrees (also referred to as the pentagon angle) and similar to 144 degrees around the c-axis. All these rotation angles are close to the multiples of 36 degrees 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.