The vibrational spectra of NbBO 4, TaBO 4, NaNb 3O 8 and NaTa 3O 8

Abstract

The infrared and Raman spectra of NbBO 4 have been interpreted in terms of the zircon structure in space group I4 1/ amd ( D 17 4 h ). Frequency shifts were observed in the Raman bands upon isotopic substitution of the boron atoms and when Nb was replaced by Ta in the crystal lattice. NaTa 3O 8 contains TaO 8 dodecahedra which are almost identical to the ones in TaBO 4 and its Raman spectra are compared with those of NaNb 3O 8 and of NbBO 4 amd TaBO 4 in order to assign metal—oxygen bands. The BO 5− 4 vibrational bands resemble those of the SiO 4− 4 groups rather than the B(OH) − 4 ones. However, the BO stretching force constant is much lower than the corresponding SiO one in SiO 4− 4 and it appears as if the metal—oxygen interactions are much stronger in NbBO 4 and TaBO 4 than in ZrSiO 4 and HfSiO 4, respectively.