The fine powders were melted in a platinum crucible for 40min at 900 °C and the molten glass poured on to a pre-heated brass mold at 280 °C (glass transition temperature, Tg). The samples were annealed for 10 h at the same temperature to release thermal stress/strains and allowed to cool to room temperature. The samples were subsequently cut and polished (dimensions: 10_10_2mm3) with optical quality for the measurements.