Bismuth-containing
rare-earth codoped fluoride phosphate glasses are promising materials
for superbroadband near-infrared (NIR) emission with potential applications
in optical amplification. To elucidate their structural organization,
this contribution develops a comprehensive multinuclear solid-state
single and 31P/19F double resonance NMR strategy,
applied to glasses in the system (BiF3)
y
(50NaPO3–20Ba(PO3)2–20NaF–10BaF2)100–y
(y = 0, 5, 10, 20, 30, 40). To separately
assess the influence of bismuth and fluorine constituents on the network
structure, an additional set of (NaPO3)50–2x
(Ba(PO3)2)20–x
(NaF)20+2x
(BaF2)10+x
(x = 0, 2.5, 5.0,
7.5) glasses was also investigated. Introduction of alkali fluoride
ions into phosphate glasses is shown to follow a new network modification
scheme involving depolymerization of polyphosphate chains under the
formation of F-bonded fluorophosphate units. However, a significant
part of the fluoride stays separate, interacting exclusively with
metal-ion species. 23Na MAS and 23Na{31P} double resonance NMR results are consistent with a local sodium
ion environment dominated by phosphate ions in the Bi-free system,
whereas in the BiF3-containing glasses, the successive
reduction of the dipolar second moment M
2(Na–P) with increasing BiF3 content reflects the expected dilution
effect, accompanied by increasing competition of the fluoride ions
for ligation with the Na+ ions.