Coordination state of Nb5+ ions in silicate and gallate glasses as studied by Raman spectroscopy

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“…The large bandwidth of this band is indicative of Nb-O stretching in distorted NbO 6 octahedra, and this may arise from their bonding to more silicate than niobate entities [45]. This proposed assignment is also supported by the available literature including reviews on Raman spectra of structurally well-defined niobate compounds [46,[51][52][53][54][55]. For example, Cardinal et al [51,52] showed that the introduction of Nb 2 O 5 to borophosphate glasses leads to the formation of distorted isolated NbO 6 octahedra (900 cm −1 ), and for higher and are known to exist at low concentrations in silicate glasses [45,55].…”

“…As for the origin of the broad Raman features at 1075 cm −1 and 855 cm −1 in Figure 6a, we note that Nb 5+ -containing silicate glasses exhibit Raman bands at around 1000 to 1100 cm −1 and 800-900 cm −1 , which have been associated with the stretching modes of Si-O and of Nb-O bonds, respectively [46]. [46][47][48][49][50].…”

“…[46][47][48][49][50]. The nomenclature Q n denotes SiO 4 -tetrahedra with n bridging and 4-n non-bridging oxygen atoms.…”

“…The second niobate crystal structure identified by XRD is Na 3 NbO 4 ( Figure 3), and therefore we discuss here its possible contribution to the Raman spectrum in Figure 7e. Since the Raman spectrum of the Na 3 NbO 4 crystal is not available, we consider reported Raman spec-tra of the isostructural Li 3 NbO 4 crystal [46,66]. Its structure consists of NbO 6 octahedra which have three nonbridging oxygen (NBO) atoms, while the other three oxygen atoms are shared by three other NbO 6 octahedra [66].…”

“…Its structure consists of NbO 6 octahedra which have three nonbridging oxygen (NBO) atoms, while the other three oxygen atoms are shared by three other NbO 6 octahedra [66]. The strongest Raman band of Li 3 NbO 4 was reported at 824 cm −1 [66] or 840 cm −1 [46], and originates from the symmetric stretching mode of such NbO 6 octahedra with NBOs. Along these lines, we tentatively associate the broad Raman band at 865 cm −1 (Figure 8a) in the amorphous phase and that at 860 cm −1 in the crystalline phase (Figure 8b) …”