Structure of Nb2O5–NaPO3 glasses by X-ray and neutron diffraction

Abstract

X-ray and neutron diffraction of excellent resolving power are used to determine the atomic structure of niobium phosphate glasses. These studies complement the results of earlier ³¹P and ⁹³Nb nuclear magnetic resonance, Raman scattering and O1s X-ray photoelectron spectroscopy of the (Nb₂O₅)_x–(NaPO₃)_1−x glasses (0 ≤ x ≤ 0.40). The Nb–O distances behave uniformly for glasses of 0.08 ≤ x ≤ 0.20 with distorted NbO₆ octahedra that are characterized by a corner with a short Nb–O bond of 0.174 nm, four bonds of ∼0.20 nm and a longer bond. The concomitant change of PO₄ units from chain to end groups extends likewise to glasses of x = ∼0.20, i.e. only one O atom per Nb contributes to the rupture of phosphate chains. This behaviour differs from that of related crystal structures and phosphate glasses of other oxide additions. Asymmetric Nb–O–Nb bridges that include the short Nb–O bond are formed for glasses of x ≤ 0.20 while symmetric bridges with two Nb–O bonds of ∼0.190 nm appear for x > 0.2. A pre-peak at 8 nm⁻¹ is found in the S(Q) for glasses of 0.08 ≤ x ≤ 0.20 which is interpreted as the presence of niobate and sodium phosphate micro-domains. The weakness of this pre-peak if it is compared with that of similar GeO₂–KPO₃ glasses is explained with modifications of the micro-domain structure by a fraction of non-bridging O in Nb–O⋯Na⁺ sites. The pre-peak vanishes for the glass of x = 0.40.