Fig. 4(c) shows the deconvoluted Raman spectra in the high scattering wave number region and the excitation modes of each peak correspond to, (P13)B–O stretching of pyroborate B2O4 5 [21,23],(P14) stretching of B–O in BO3 trigonal with one or two BO4 tetrahedra (P15) stretching of B–O in
BO trigonal [24,31].