X-ray powder diffraction studies of sample P1 series after DSC experiment in argon showed that it consists of cristobalite together with γ and y-modification of yttrium disilicate (Table 5). While in sample P2, the YAG (Y3Al5O12) phase was found together with β and γ-modification of yttrium disilicate (Table 8). Therefore, the second exothermic peak observed in P2 sample (Fig. 2) is due to the crystallisation of the YAG from the glass phase and/or due to the phase transformation of the y-modification of yttrium disilicate to the β or the γ-modification.
The qualitative analyses showed that the P2 samples have a nitrogen-containing phase at ≥1200 °C in Spermine NONOate to P1. This phase was identified as the D-phase having the stoichiometry YSi2AlO4N2 based on the literature . The BSE images showed that this phase exists in the regions between the yttrium disilicate dendrites. This phase was also observed in the centre as well as at the edges of all samples annealed at temperature ≥1200 °C, while it was not found in the samples treated at 1100 °C. The nitrogen content of this D-phase was relatively high and exceeds by more than half the initial starting glass composition. This leads to the conclusion that even after the first crystallisation period (<1200 °C), the residual glass still has a relatively higher nitrogen content that allows the crystallisation of this phase to occur.