Distribution of Nd3+ ions in oxyfluoride glass ceramics
© Zhao et al.; licensee Springer. 2012
Received: 16 April 2012
Accepted: 18 May 2012
Published: 30 May 2012
It has been an open question whether Nd3+ ions are incorporated into the crystalline phase in oxyfluoride glass ceramics or not. Moreover, relative research has indicated that spectra characters display minor differences between before and after heat treatment in oxyfluoride glass compared to similar Er3+-, Yb3+-, Tm3+-, Eu3+-, etc.-doped materials. Here, we have studied the distribution of Nd3+ ions in oxyfluoride glass ceramics by X-ray diffraction quantitative analysis and found that almost none of the Nd3+ ions can be incorporated into the crystalline phase. In order to confirm the rationality of the process, the conventional mathematical calculation and energy-dispersive spectrometry line scanning are employed, which show good consistency. The distribution of Nd3+ ions in oxyfluoride glass ceramics reported here is significant for further optical investigations and applications of rare-earth doped oxyfluoride glass ceramics.
Nd3+ is always considered to be one of the most efficient rare-earth (RE) ions to generate laser operation around 1.06 μm in different hosts, such as crystals and glasses. In the 1970s, transparent glass ceramics (GCs) with nanocrystals (NCs) of about 50 nm were originally suggested to be used as laser host materials [1, 2]. In subsequent oxide glass system studies, the fluorescence lifetimes and absorption spectra of Nd3+ ions were measured in neodymium-doped glasses and GCs to investigate the distribution of the Nd3+ ions . The results indicated that Nd3+ ions were excluded from the crystalline phase and accumulated into the residual glass matrix in the GCs. Also, in a similar glass system, Dymnikov et al.  and Kang et al. , respectively, found that Nd3+ ions exhibited fairly different distribution tendencies when the crystalline phase varied in glass ceramics by performing fluorescence, absorption, and Judd-Ofelt analyses.
In 1993, Wang and Ohwaki, for the first time, reported the fabrication of transparent oxyfluoride GCs which combined the advantages of fluoride glasses with efficient frequency upconversion and oxide glasses with good chemical and mechanical stability . RE ions, specifically Er3+ and Yb3+ ions, could be dissolved into the crystalline phase with lower phonon energy, generating a remarkable increase of luminescence, which made the transparent GCs potential outstanding laser host materials. Afterwards, extensive studies on RE-doped (Er\Eu\Yb\Ho) oxyfluoride GCs were carried out with various compositions and proportions [7–9]. Nevertheless, fewer studies were performed on Nd3+-doped oxyfluoride GCs, different from Er3+\Yb3+\Tm3+\Eu3+ ions, as Nd3+ ions were found to be difficultly incorporated into the crystalline phase [5, 10–12]. Pisarska et al. [10, 11] revealed that in oxyfluoroborate glass compositions, the 4 F3/2 fluorescence lifetime didn't change after thermal treatment, showing that Nd3+ ions did not incorporate into the crystalline phase. Abril et al.  focused on different preparation methods with various Nd3+ ions sources and the corresponding distributions of Nd3+ ions in GCs. NdF3 was thought to be more helpful for Nd3+ ions to be incorporated into the crystalline phase using analysis of fluorescence, absorption, and the Judd-Ofelt theory [13, 14]. From the previous research mentioned above, whether in the Nd3+-doped oxide glass system, oxyfluoroborate glass compositions, or similar oxyfluoride glass system, Nd3+ ions were difficultly incorporated into the crystalline phase, while Wang et al. [15–18] investigated the thermal and optical properties of different kinds of Nd3+-doped GCs, which suggested that most of Nd3+ ions were doped in the crystalline phase.
While most of the previous research focused on incorporating the RE ions into the nanocrystals, such as Er3+\Yb3+\Tm3+\Eu3+ ions, the distribution of Nd3+ ions in GCs is somewhat ambiguous and exhibits some peculiar results, which much influences the materials' fluorescence properties. As previously reported about the distribution of Nd3+ in various oxyfluoride glass ceramics, indirect characterization techniques, for example, fluorescence analysis, X-ray diffraction (XRD), Judd-Ofelt theory calculation, and the like, were used. The properties of samples doped with Nd3+ ions show small changes after thermal treatment. Even with the slight changes caused by the distribution of Nd3+ ions in the nanocrystalline phase or glass phase, the Nd3+ ion segregation in the glass matrix also leads to the same results, that is to say that most of the results obtained from the indirect characterization technique cannot distinguish between Nd3+ ions entering into the nanocrystalline phase and locating on the interface between the crystalline and glassy phase. Nowadays, direct characterization technique energy dispersive X-ray spectroscopy (EDS, conventional fixed point measurement) was used to study the distribution of Nd3+ ions in glass ceramics. Since the crystal size is smaller than the probe beam, the analyzed volume in GC always includes both crystalline and glassy phases; the results of EDS do not seem to be entirely reliable.
In this paper, a direct and quantitative investigation on the distribution of Nd3+ ions in GCs was performed. Transparent GCs doped with Nd3+ ions were prepared, and subsequently, the glass matrix was etched off for releasing the fluoride nanoparticles (NPs) into the water phase for further study. In contrast, investigating the EDS of NPs rather than that of GCs is a more scientific and more convenient method to obtain the chemical composition of NP. The Nd3+ ion distribution model was built in GCs and NPs. EDS line scan by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) on the concentrations of concerned elements and the Rietveld full-pattern fitting algorithm were employed for quantitatively clarifying the distribution of Nd3+ ions in GCs. The results will be of great significance for the RE distribution investigations and further applications of Nd3+-doped glass ceramics.
Precursor glasses (PGs) with the composition of 44SiO2-5Al2O3-40PbF2-10CdF2-1NdF3 (mole ratio) were fabricated with a traditional melting-quenching route. Transparent GCs were obtained by following thermal treatment at 440 °C under the guidance of differential scanning calorimetry result of as-quenched precursor samples. The GCs were then ground into powder, immersed into hydrofluoric acid solutions, and stirred with a magnetic stirrer so as to thoroughly release the nanocrystals from the glass matrix. Afterwards, nanocrystal powder was obtained by vacuum drying at 80 °C. This fabrication method has been reported in detail in the previous work .
All XRD data were obtained with a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan), using CuKα as the radiation. Quantitative analysis of the phases in NCs was carried out with material analysis using diffraction (MAUD) program (Luca Lutterotti, University of Trento, Trento, Italy) [20, 21] applying the RITA/RISTA method based on the Rietveld full-pattern fitting algorithm with XRD data in the range of 10° to 125° acquired in step-scan mode with a step of 0.02° (2θ) at a counting time of 1 s per step. The program was developed to analyze diffraction spectra and obtain crystal structures, quantity, and microstructure of phases along with the texture and residual stresses. High-resolution transmission electron microscope (HRTEM) analysis was performed to observe the morphology of samples on a Philips TZOST TEM (FEI Co., Hillsboro, OR, USA) operating at 200 kV. STEM and EDS line scan were performed on a Tecnai G2F30 field-emission transmission electron microscope (FE-TEM; FEI Co., Hillsboro, OR, USA) using HAADF as imaging mode. All the measurements proceeded under the same condition.
Results and discussion
As a result from that, due to Nd3+ ions aggregating into the glass matrix rather than incorporating into the β-PbF2 crystalline phase during thermal treatment, Nd3+ ion-doped oxyfluoride glass exhibits a weak difference in fluorescence and other optical properties before and after heat treatment. On one hand, the location of Nd3+ ions is the oxide environment with a higher coupled phonon energy, which will boost nonradiative relaxation process, compared with fluoride nanocrystal phase; on the other hand, aggregation of Nd3+ ions can also bring some much smaller effects on fluorescence and optical properties than that of GCs doped with Nd3+ ions into the fluoride nanocrystal phase. This is in agreement with some previous research, such as what was discussed by Rapp et al. in ; the fluorescence lifetimes of Nd3+ ions were much shorter than those in glasses, and the absorption and emission spectra of Nd3+ ions were identical both in neodymium-doped glasses and GCs, which indicated that neodymium ions were excluded from the crystalline phase of GCs and were entirely accumulated in the glass matrix. However, the most noteworthy thing is that the segregation of Nd3+ ions in the glass matrix also leads to the same results; in other words, most of the results obtained from indirect characterization techniques cannot distinguish between Nd3+ ions entering into the crystalline phase and locating on the interface between the crystalline and glassy phase. Therefore, unlike those indirect characterization techniques used by most of previous research, a direct and quantitative investigation method was employed to present a clear result that almost none of the Nd3+ ions can be incorporated into the crystalline phase but reside in the glass matrix.
Transparent Nd3+-doped GCs were prepared and corrosion-treated to release NCs from the glass matrix for a direct study on their composition and structure information. Unlike the former results, β-PbF2:RE crystalline phase was obtained, and pure β-PbF2 crystalline phase was generated after thermal treatment. Especially after etching off the glass matrix, massive NdF3 crystals simultaneously generate in free NCs. Through the XRD Rietveld refining method, almost the whole of Nd3+ ions reside in the glass matrix despite that the samples undergo the thermal treatment. For further demonstration, the models of Nd3+ ions existing in the glass matrix in GCs and the core-shell-like structure of pure β-PbF2 surface absorbing NdF3 crystals in NCs were built. Then, HRTEM, EDS line scan in HAADF mode, and conventional mathematical analysis were used to verify the models' rationality. The results of experimental characterization well coincide with those of the simulation. Our work explains the previous arguments about whether Nd3+ ions doped into the crystalline phase or not and removes the puzzle about minor differences on spectral properties. The study has paved a way to more comprehensively understand the properties of the Nd3+-doped oxyfluoride glass. The results here would benefit further research on the optical properties and applications of such materials.
LjZ is a professor in the School of Physics and TEDA Applied Physics School of Nankai University. HY is an associate professor in the School of Physics. HG, MZ, and YL are master students. ML is a doctoral candidate.
The authors would like to acknowledge the financial support of the Key Natural Scientific Foundation of Tianjin under Grant No. 9JCZDJC16700 and the Fundamental Research Funds for the Central Universities. The authors are also grateful to the doctoral candidate Nan Hu who is studying in the Institute of Physical Chemistry, Johannes Gutenberg-Universität Mainz for his enthusiastic help and kind guidance.
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