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

Abstract

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(O_xF_y), besides single oxygen ligand units M(O_x+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.