The main building units forming the amorphous network are BO3 (1270 and 1200 cm− 1), BO4 (930–880, 1050–1040 cm− 1), MoO4 (840–760 cm− 1) and BiO6 (470 cm− 1). It was proved that Bi2O3 favors the BO3 → BO4 transformations while MoO3 preserves BO3 units in the amorphous network.