Fig. 5 shows the SEM micrographs of sealing interface between glass–ceramics and GDC held at 700 °C for 30 days, along with the elemental EDS line scans taken across the interface. The elemental compositions determined using EDS at selected points in the glass–ceramics are summarized in Table 3. The EDS line scans reveal the formation of an interdiffusion zone with ∼1 μm thickness at the interface between the glass–ceramics and GDC, due to the presence of an interaction layer. The Al/Zn ratio decreases from 2.33 for the interface between glass#2ZnO and GDC (point#1 in Fig. 5a) to 1.50 for the bulk of glass–ceramic of glass#2ZnO (point#3 in Fig. 5a); whereas, the Al/Zn ratio decreases from 2.33 for the interface between glass#10ZnO and GDC (point#5 in Fig. 5b) to 1.25 for the bulk of glass–ceramic (point#7 in Fig. 5b). This indicates that two hardystonite phases aggregate at the sealing interface, in particular for solid solution phase. In addition, the bismuth-enrichment can be observed in glass–ceramics (point#4 in Fig. 5a and point#8 in Fig. 5b), in agreement with the identified bismuth-containing phase, Bi4B2O9, in glass–ceramics (Fig. 1).