A structural model of La2O3–Nb2O5–B2O3 glasses based upon infrared and luminescence spectroscopy and quantum chemical calculations

Abstract

Infrared spectra of 20La 2 O 3 -xNb 2 O 5 -(80 À x)B 2 O 3 , 24La 2 O 3 -xNb 2 O 5 -(76 À x)B 2 O 3 and 28La 2 O 3 -xNb 2 O 5 -(72 À x)B 2 O 3 glasses, with x = 1, 5, 10, 15 and 20, have been obtained. Also, the luminescence spectrum of the 28La 2 O 3 -10Nb 2 O 5 -62B 2 O 3 sample has been obtained. Density functional theory (DFT-B3LYP) calculations on clusters containing BO 3 , BO 4 , NbO 6 , edge-sharing NbO 6 -NbO 6 , edge-sharing NbO 6 -NbO 6 (niobyl), and corner-sharing NbO 6 -NbO 6 groups have been used to aid the infrared spectra assignments. From these data it was possible to propose a structural model for the La 2 O 3 -Nb 2 O 5 -B 2 O 3 glasses that is consistent with all the spectroscopic and theoretical results as well with previous luminescence study of similar samples. Briefly, this model consists of distorted NbO 6 octahedral groups replacing the BO 4 tetrahedral groups giving rise to non-bridging oxygen ions and distorted NbO 6 chains. These chains change from edge-sharing to corner-sharing distorted NbO 6 octahedra depending upon the La 2 O 3 concentration. For La 2 O 3 concentration larger than 28 mol% there seems to be a segregation of La(III) to another domain, restoring the structure observed for samples with La 2 O 3 concentrations lower than 20 mol%.

Preparation and spectroscopic properties of Nb2O5 nanorods

“…Occasionally, NbO 7 and NbO 8 structures can also be found [19]. The niobate green luminescence upon UV excitation because of distorted NbO 6 octahedral groups has been observed in crystalline materials and glasses [20][21][22]. Thus, we attributed the emission at 496 nm of the Nb 2 O 5 nanrods to the slightly distorted NbO 6 groups.…”

Study on optical properties of La2O3–TiO2–Nb2O5 glasses prepared by containerless processing

“…With the decrease of TiO 2 , the intensity of peak is weakening, and shifts to lower frequency. The peaks at 630 cm À 1 and 780 cm À 1 gradually shift to higher frequency, and the intensities are enhanced with the increase of Nb 2 O 5 [8,33,34]. The peak position at 630 cm À 1 may be associated with the vibrations of NbO n polyhedra.…”

An ab initio cluster-in-lattice model for the luminescence of K2NbOF5 crystal

“…Configuration interaction with single replacements (CIS) [14], time-dependent (TD) or polarization propagator (RPA) with Hartree-Fock (TD-HF) or hybrid functional density (TD-B3LYP) reference functions [15][16][17][18][19], and complete active space (CAS) [20][21][22] approaches have been used to calculate the electronic structure of the ground and lowest singlet and triplet excited states. The active space of the CAS method consisted of the three highest doubly occupied (HOMO, HOMO À 1, HOMO À 2) and two unoccupied (LUMO, LUMO + 2) molecular orbitals CAS (6,5). This active space was selected based upon the orbital symmetry, the optimization of the CAS wavefunction and practical viability of performing these calculations on larger systems.…”

“…Particularly, these materials usually have well ordered crystalline structures so that the quenching of the intrinsic luminescence by extrinsic centers is avoided and thus increasing their luminescence efficiencies [1][2][3]. It was thus very surprising when the luminescence of niobates were observed, for the first time, in borate-lanthanum glasses [4][5][6]. These observations have prompted us to develop a structural model for these glasses, which could explain their luminescence.…”