In fact, difficulties arise from the fact that Bi is a transition metal with
d orbitals that are easily coupled to the surrounding environment, unlike the rare-earth elements that have well-isolated
f–f transitions. On the other hand, it is this coupling to the external environment that allows tailoring the formation of broadband emission defect sites using additional dopants such as aluminum, germanium, phosphorous, etc. As “the wonder metal,” bismuth can participate in numerous reduction reactions without other elements, producing a variety of products [
24]. With the increasing melting temperature of Bi-doped glasses, the following change in the valence state of bismuth takes place [
25]: Bi
3+→Bi
2+→Bi
+→Bi/Bi
2, Bi
2-, Bi
3, etc., →(Bi)
n, where Bi
2, Bi
2-, Bi
3, etc. are Bi clusters, and (Bi)
n is a metallic colloid. So far, a number of hypotheses have been suggested for the origin of the NIR emission from Bi-doped glasses [
10] like Bi
+, Bi
5+, Bi-clusters, BiO, Bi
2-, Bi
22- point defects, etc., but none of them have been experimentally confirmed. Sun et al. revealed that Bi
53+ [
26], Bi
82+ [
27], and Bi
22- [
28] in molecular crystals and Bi
+ in zeolites [
29] are NIR emitters, but whether these emitters exist in glassy systems such as bulk glasses or fibers remains an open question for further investigation. Even the latest experimental data obtained from Bi-doped germanosilicate fibers with a small concentration of Bi (≤0.1 at.%) can only confirm that BACs are clusters consisting of Bi ions and oxygen deficiency centers but not Bi ions themselves [
15]. However, the structural configurations and the type of Bi ions forming BACs are still unclear.