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

Infrared spectra of 20La2O3–xNb2O5–(80 − x)B2O3, 24La2O3–xNb2O5–(76 − x)B2O3 and 28La2O3–xNb2O5–(72 − x)B2O3 glasses, with x = 1, 5, 10, 15 and 20, have been obtained. Also, the luminescence spectrum of the 28La2O3–10Nb2O5–62B2O3 sample has been obtained. Density functional theory (DFT-B3LYP) calculations on clusters containing BO3, BO4, NbO6, edge-sharing NbO6–NbO6, edge-sharing NbO6–NbO6 (niobyl), and corner-sharing NbO6–NbO6 groups have been used to aid the infrared spectra assignments. From these data it was possible to propose a structural model for the La2O3–Nb2O5–B2O3 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 NbO6 octahedral groups replacing the BO4 tetrahedral groups giving rise to non-bridging oxygen ions and distorted NbO6 chains. These chains change from edge-sharing to corner-sharing distorted NbO6 octahedra depending upon the La2O3 concentration. For La2O3 concentration larger than 28 mol% there seems to be a segregation of La(III) to another domain, restoring the structure observed for samples with La2O3 concentrations lower than 20 mol%.