Structure and peculiar luminescence of Eu3+-doped sodium/alkaline earths phosphate glasses
Introduction
Phosphate glasses are attractive vitreous matrices in diverse fields considering their thermal, chemical, and optical properties [[1], [2], [3], [4]]. These attributes make them candidates as bioactive materials, sealants for planar Solid Oxide Fuel Cells (SOFCs) [5], solid-state electrolytes [6,7], seals for nuclear waste [8], and radiation shielding materials [9,10]. Additionally, photonic devices based on phosphate glasses are proposed as waveguides, LEDs, and fluorophores [[11], [12], [13], [14]].
Among the various phosphate compositions, metaphosphate glasses are intensively studied in High Power High Energy solid-state lasers considering their large storage energy, efficient energy extraction, resistance to laser-induced damages, and mature manufacturing technology [4]. These glasses are formed by quasi-infinite chains of phosphate tetrahedron linked together via P–O–P bridges [1]. This type of structure is obtained by keeping the oxygen over phosphorus ratio (O/P) fixed at 3 in the glass composition [1,4]. Nonetheless, despite the apparent simple structure of these materials, previous investigations suggest the presence of two packing behaviours in mixed (strontium or calcium) sodium metaphosphate glasses having the composition xSrO/CaO-(50-x)Na2O–50P2O5 [15,16]. The variations of mixing and dissolution enthalpies indicate two ranges of interactions. The first one is located between 0 and 20 mol% of strontium or calcium oxides and shows a negative heat of mixing. Whereas, the second range, situated between 25 and 50 mol% of SrO or CaO, shows a positive heat of mixing. The switching between these packing schemes is progressive and for the compositions containing 20 mol% and 25 mol% of SrO or CaO, the two packing behaviours seem to coexist in the metaphosphate network. Thus, to probe this variation, investigate the potential effect of these packing schemes on the luminescence of Eu3+, and try to propose these compositions as fluorophores, five metaphosphate glasses have been doped with a fixed amount of Eu2O3.
Trivalent europium is one of the most investigated rare earths ions, as a probe or as an active ion, owning to its non-degenerate ground state 7F0 and excited state 5D0, which give simple luminescence and absorption spectra [17]. Its intense red emission line 5D0→7F2 around 615 nm possesses an extended lifetime together with high colour purity and quantum efficiency [17,18]. These attributes make the trivalent europium an extensively studied ion, as fluorophores for white LEDs, optical thermometry, pH sensor and as a probe for biochemical or biomedical applications [17,[19], [20], [21]]. Moreover, the similarity of ionic radii of Eu3+ and Ca2+ is also used for the design of new bioactive implants [22,23].
This paper reports the study of the structure and luminescence of europium oxide-doped mixed sodium strontium/calcium metaphosphate glasses. The structure of the samples was preliminary checked using X-ray Diffraction and Raman spectroscopy. The density and thermal properties of the doped glasses were compared with the undoped ones. The absorption coefficient together with the steady-state, time-resolved luminescence, and phonon sidebands spectroscopy were recorded to collect more insights about the local structure of the dopants, the phonon energy of the glasses, and the electron-phonon coupling strength of the host matrix.
Section snippets
Materials and methods
Sodium dihydrogen phosphate monohydrate (AppliChem, 98%), calcium nitrate tetrahydrate (PRS, 98%), strontium nitrate (Sigma Aldrich, 98%), ammonium dihydrogen phosphate (Fluka, 99%), and europium oxide nanopowder (Sigma Aldrich 99.5%) were used as a starting materials for the preparation of the glasses. The desired proportion of the precursors was ground in a porcelain mortar and transferred to an alumina crucible. The mixture was dried at 250 °C and 400 °C for 24 h; to evaporate the volatile
X-ray diffraction and Raman spectroscopy
The X-ray diffraction patterns of the doped glasses are reported in Fig. 2. As expected, the vitreous state persists in these compositions thanks to the large amount of P2O5. Indeed, the glass-forming ability of phosphate melts can be related to the average chain length and the viscosity of the melt [1,24].
The Raman spectra of the doped glasses are presented in Fig. 3. The bands ranging from 240 cm−1 to 440 cm−1 can be assigned to the bending of the P–O–P bridges between the phosphate
Conclusion
This investigation aimed to study the effect of the packing schemes, on the luminescence of Eu3+-doped mixed metaphosphate glasses. The first step was devoted to analysing the structure, physical and thermal properties of the doped glasses in comparison to the undoped compositions. The spectroscopic analyses show that Q2 phosphate species are the major building blocks in the doped glasses and a close similarity with the parent glasses was also detected using the density and DSC measurements.
Author statement
M.A. Cherbib: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors want to thank Mrs. Nadja Buchert for the X-ray diffraction, density, and differential scanning calorimetry analyses. The authors want also to thank Mr. Christian Zeidler for the Raman, transmission, and luminescence analyses.
The authors want to thank the Tunisian Ministry of Higher Education and Research for funding this work.
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