Precipitation of Er3+-doped Na5Y9F32 crystals from fluoro-phosphate glasses: an advanced solid-state NMR spectroscopic study

摘要

A series of 16BaF2-(84 x) NaPO3-xYF3-yErF3 (x = 0, 12, 24, 36, and 48 mol%; y = 0.1, 0.5, and 1 mol%) glasses were prepared via the conventional melt-quenching method and subsequently heated to synthesize glass ceramics containing Er3+-doped Na5Y9F32 crystals. The structures of glasses and glass ceramics were characterized by multiple advanced one-and two-dimensional solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) techniques. At the atomic level, multiple phosphorus species Qn (n = 0, 1, and 2) were resolved. Here, the value n denotes the number of P-O-P bonds, which can be quantified by 2D J-resolved spectroscopy. The connectivity between F ions and cations was detected by the rotational echo double resonance (REDOR) and the J-coupling based heteronuclear multiple quantum coherence (HMQC) method. With the increase of YF3 content, the phosphate network depolymerized gradually. 31P and 19F MAS spectra, as well as 23Na {31P} and 23Na {19F} REDOR results, consistently proved that Y3+ ions, replacing partial Ba2+ and Na+ ions, aggregated around phosphorus tetrahedrons [ PO4] 3 , whereas partial Ba2+ and Na+ ions moved away from [ PO4] 3 to bond with F ions and formed Y-F Na and Ba-F Na linkages. All cations could exist as oxygen and fluorine complex ligand units M(OxFy), besides single oxygen ligand units M(Ox+ y), where M represents cations in glasses. When YF3 content reached 48% by mole, micro-regions enriched in Y-F Na linkages were formed, which promoted the crystallization. 31P MAS spectra indicated that the crystallization of Na5Y9F32 crystals has not affected the local structure of phosphorus species. Intensive upconversion luminescence can be observed in the fluoro-phosphate glass ceramics containing Er3+: Na5Y9F32 crystals. On the basis of these complementary evidences from solid-state NMR experiments and luminescence spectra, a novel structure evolution model during crystallization is developed.