Crystallization processes in europium-doped Bi4Ge3O12 glass materials
Graphical abstract
Introduction
Recent applications of glass and glass-ceramic materials like the magnetic sensor for phosphotellurite glasses [1], the optical fiber for oxyfluoride glasses [2] or even photovoltaic applications based on the up-conversion mechanisms [3] becomes a new emergent domain in such materials nowadays. Several methods for obtaining glasses and glass-ceramic materials were developed, the most popular being the melt quenching method [4,5] but also sol-gel processing [[6], [7], [8]], solid state reaction, hydrothermal, dismantled cathode ray tubes (CRT) and ionic gelation methods [9].
Further processing methods result in glass-ceramic materials or directly in nanoparticles with other applications like nanothermometry, biological applications based on up-conversion mechanisms.
Most of these glass materials, mainly based on oxides, are generally regarded as excellent glass formers having electronegativity on the Pauling scale between 1.7 and 2.1. There are just a few elements used as glass formers like boron, silicon, phosphorus and germanium with electronegativity values in this range which lead to borate, silicate, phosphate and germanate glasses [10]. According to the Pauling scale, there are also other elements known as glass formers like arsenic, antimony and tellurium with an electronegativity value of 2.1 which theoretically forms specific glasses like tellurite.
There are specific methods of characterization for these glasses, the most common being X-ray diffraction for crystallization behaviors, optical and spectroscopic characterization, thermal analysis, Raman, Infrared Spectroscopy and X-ray Photoelectron Spectroscopy together with specific measurements of density and coordination. Most of these techniques are useful for evaluating the structural, thermodynamic and compositional analysis of these materials beside of crystallization processes in the case of glass-ceramic materials.
A way to understand the crystallization process of such materials is to use different rare earth ions as probes which are very sensitive to the environment and electronic transitions of these ions. These rare-earth elements introduce magnetic moments associated with the uncompensated spin of 4 f-electrons into these glasses [11]. One of these rare earth ions is europium used as a sensitive probe employed to investigate the coordination and the local environment around them [12]. The Eu3+ ions can be involved in crystallization processes since the 5D0 and 7F0 are not subjected to Crystal Field splitting while the split of the emission (absorption) transitions involving the 2S+1LJ multiplets with J > 0 are symmetry-dependent [13].
The aim of this paper is a deeper understanding of the crystallization processes in bismuth germanate glasses, studied by using Eu3+ ions as probes. The measurements combine the optical spectroscopy, including absorption, photoluminescence, cathodoluminescence, lifetime and magnetic circular dichroism on these Eu3+ ions in closed relation with coordination and local environment around these cations. In this context, the Magnetic Circular Dichroism (MCD) is an advantageous method for the detection of the overlapping transitions in the absorption spectra [14,15].
Section snippets
Experimental setup
The samples were obtained by melt quenching technique accordingly with molar composition of Bi4Ge3O12 having as starting materials: Bi2O3, GeO2 and Eu2O3. For undoped samples, 40% and 60% molar percentage is enough to obtain the bismuth germanate glasses, but in the case of doped samples, the mixing of powders was done as follow: 59% Bi2O3+39% GeO2+2%Eu2O3 [[16], [17], [18]]. It is important to note the annealing at 750–780C in air, which ensure a solid state reaction between oxides. The
Results and discussions
The optical absorption and MCD spectra of the rare-earth ions have complex characteristics due to the splitting of the ground states in the magnetic field, which induces C terms and due to the superposition between different ED and MD transitions of the RE ions. An exception is the Eu3+ ions in the optical materials, which show pure magnetic-dipole (MD) and electric-dipole (ED) transitions [20]. Eu3+ has 4f6 electronic configuration with 7F0 non-degenerate ground state and 5DJ (J = 0,1,2) first
Conclusions
The Eu3+ ions can be successfully used to reveal the crystallization processes of the inorganic materials, especially those with low energy phonons, because their unique characteristics compared with other rare-earth ions. In the case of germanate glasses, the main advantage is the substitution of Bi3+ which makes them easier to study. Both absorption and photoluminescence measurements have shown a gradual crystallization process for the glass samples annealed at 560C. After thermal annealing,
Acknowledgements
This work was supported by a grant from the Romanian National Authority for Scientific Research Core Programme CNCS-UEFISCDI PN19-03 (contract no. 21 N/08.02.2019).
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