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Abstract—Processes of nanocrystalline phase formation in transparent yttrium oxyfluoride nano-glass- ceramics doped with neodymium ions are studied.

ISSN 0030-400X, Optics and Spectroscopy, 2015, Vol. 118, No. 6, pp. 936–938. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.Yu. Bibik, R.K. Nuryev, V.A. Aseev, E.V. Kolobkova, N.V. Nikonorov, 2015, published in Optika i Spektroskopiya, 2015, Vol. 118, No. 6, pp. 968–970.

CONDENSED-MATTER SPECTROSCOPY

Study of Structural and Spectral Properties of Neodymium-Doped Lead–Yttrium Oxyfluoride Nano-Glass-Ceramics A. Yu. Bibik, R. K. Nuryev, V. A. Aseev, E. V. Kolobkova, and N. V. Nikonorov University of Information Technologies, Mechanics, and Optics, St. Petersburg, 197101 Russia e-mail: [email protected] Received November 7, 2014

Abstract—Processes of nanocrystalline phase formation in transparent yttrium oxyfluoride nano-glassceramics doped with neodymium ions are studied. An optimal heat treatment regime for a given glass composition is determined using differential thermal analysis (DTA). Glasses are heat-treated for 30, 60, and 120 min; the sizes of crystals are calculated, and the unit cell parameters are determined. The physicochemical and spectral properties of yttrium oxyfluoride glasses doped with neodymium ions, as well as of nanoglass-ceramics based on these glasses, are studied. DOI: 10.1134/S0030400X15060041

At present, it is a topical scientific problem to develop new optical materials with specific characteristics. It is of great interest to obtain nanostructured rare-earth-doped optical compositions based on oxyfluoride silicate glasses. Glass ceramics doped with neodymium are promising optical materials owing to characteristic absorption bands determined by f–f transitions in Nd3+ ions in the near-IR regions (1.06 and 1.3 μm) [1]. Excitation into the absorption range (the most efficient is the range of 570–590 nm) gives rise to intense luminescence in the region of 1.06 (4F3/2–4I11/2) and 1.35 μm (4F3/2–4I13/2). If a rare-earth dopant is incorporated into the crystalline phase, the spectral and luminescent characteristics of glass ceramics are close to the characteristics of corresponding single crystals. Such materials combine the optical parameters of low-phonon fluoride crystals with the high mechanical and chemical characteristics of silicate glasses [2, 3]. One of the main drawbacks of these materials is intense light scattering from the interface between the crystalline and glassy phases. Therefore, it is a key problem in the development of nano-glass-ceramics to decrease light scattering due to growth of nanosized crystals in the glass matrix. Thermal treatment allows one to vary the volume fraction of the crystalline phase and the size of crystals, thus improving the spectral characteristics of materials. A properly chosen regime ensures the formation of the maximum number of crystallization centers with required crystallization degree and phase composition [4]. The volume and size of formed crystals are determined using X-ray diffraction analysis.

This work is devoted to the study of the structural and spectral properties of lead–yttrium oxyfluoride nano-glass-ceramics doped with neodymium ions. EXPERIMENTAL As a matrix for nano-glass-ceramics, we chose glass of the composition 30SiO2–15AlO3/2–29CdF2– 18PbF2–5ZnF2–xNdF3–(3 – x)YF3, where x = 3.0, 2.9, 2.5, 1, 0.5, 0.2, 0.1, and 0. Synthesis occurred for 30 min at a temperature of Т = 1050°С in open corundum crucibles in air atmosphere. Glasses were then heat-treated for theat = 30, 60, and 120 min at crystallization initiation temperature Tcr = 500°C to obtain nano-glass-ceramics. The crystallization initiation temperature was determined by differential thermal analysis (DTA). Figure 1 shows the DTA curve for a sample with 1 mol % of NdF3. The DTA curve exhibits two crystallization peaks. The first peak is caused by separation of the PbYOF3 crystalline phase. The sizes of formed crystals were determined by X-ray diffraction analysis. Figure 2 shows the diffraction patterns of the initial and heattreated glasses. The initial glass contains no crystalline phases, while secondary heat treatment of glass with 0 mol % of NdF3 leads to the formation of a crystalline phase corresponding to yttrium lead oxyfluoride PbYOF3 [5]. The formed phase has a fluorite-like face-centered cubic unit cell with a constant of 5.74 Å. At a neodymium concentration of 3 mol %, i.e., in the case of complete substitution of yttrium by neodymium, a hexagonal crystalline phase is formed.

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DSC (mW/mg) exo

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Gas flow (mL/min)

200

Peak 547.4°C

0

Peak 635.4°C

−0.04 100

−0.12 0 100

300 500 Temperature, °C

700

Fig. 1. DTA curve (differential scanning calorimetry (DSC)) for initial sample with NdF3 concentration of 1 mol %. Glass transition temperatures are 413.5°C (onset), 425.9°C (midpoint), 429.8°C (inflexion), and 438.0°C (end). Tf = 424.7°C. The Cp change is 0.450 J/g⋅K. Dashed areas are 14.01 (peak at 547.4°С) and 28.37 J/g (peak at 635.4°С).

Crystallographically, the formed phase structure corresponds to the NdF3 crystal (Fig. 3). The unit-cell constant is 5.84 Å. Based on the intensity and half-width of diffraction reflections, we concluded that the process of bulk crystallization in neodymium-doped nano-glassceramics finishes after two hours of heat treatment. In the entire studied range of NdF3 concentrations, the volume of the crystalline phase formed as a result of heat treatment does not depend on the concentration of dopant ions. At neodymium fluoride concentrations from 0.1 to 2.9 mol %, heat treatment results in the formation of the face-centered cubic unit cell PbY(1 – x)NdxОF3. This is caused by the fact that neodymium ions are embedded into the crystal and substitute yttrium,

l = Rλ / β cos θ ,

(1)

where R = 0.94 is the sphericity coefficient, λ = 1.5418 Å (CuKα) is the X-ray source wavelength, β is the peak width at half maximum (in radians), and θ is the peak position on the 2Θ scale. The size of crystals increases with increasing time of heat treatment, but, as is seen from the plot, this increase is insignificant. For example, for a NdF3 con-

0.4

NdF3(1, 1, 1)

NdF3(1, 1, 0)

1

0.8 NdF3(0, 0, 2)

2

PbYOF3(1, 1, 1)

0.6

The sizes of crystals (Fig. 4) were calculated by the Scherrer formula

Intensity, rel. units PbYOF3(1, 1, 1)

Intensity, rel. units 1.0

since the ionic radii of yttrium and neodymium are close to each other. The unit cell constant depends on the concentration of neodymium fluoride and varies from 5.74 to 5.83 Å.

0.2 20

25

30

35

40 2θ, deg

Fig. 2. Diffraction patterns of (1) glass and (2) nano-glassceramics heat-treated for 60 min; NdF3 concentration is 0 mol %. OPTICS AND SPECTROSCOPY

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0 22

24

26

28

30 2θ, deg

Fig. 3. Diffraction pattern of nano-glass-ceramics heattreated for 60 min; NdF3 concentration is 3 mol %. 2015

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Size of crystals, Å 280

With increasing heat treatment time, the absorption spectrum deforms and exhibits a Stark structure typical for crystalline materials (Fig. 5). These changes in the absorption spectrum testify to the incorporation of rare-earth ions into the crystalline phase.

1 2 3

240

4

CONCLUSIONS In this work, the spectral properties of lead−yttrium oxyfl uoride nano-glass-ceramics doped with neodymium ions are studied. The DTA performed in this study allowed us to determine the optimal heat treatment regime for these samples. The heat treatment leads to the formation of crystalline phases depending on the neodymium ant yttrium fluoride concentrations. The unit cell constants vary from 5.74 to 5.84 Å depending on the concentration of neodymium fluoride in the initial glass. The size of crystals changes within the range of 200−300 Å as a function of the heat treatment time. It is shown that an increase in the heat treatment time for nano-glass-ceramics causes a change in the shape of the absorption spectrum, which confirms incorporation of rare-earth ions into the crystal.

200 5 160 20

60

100 Time, min

Fig. 4. Dependence of size of crystals on heat treatment time (30, 60, and 120 min) for glass ceramics with NdF3 concentrations of (1) 0.2, (2) 0.5, (3) 1.0, (4) 2.5, and (5) 3 mol %.

Absorption coefficient, cm−1 14 12 4

10

I9/2→4F5/2

ACKNOWLEDGMENTS This work was supported by the Ministry of Education and Science of the Russian Federation, PNIER identifier RFMEFI58114X0006.

8 6 4

4

4

4

I9/2→2G7/2

I9/2→2F7/2

I9/2→4G7/2

4

2 0 500

1

I9/2→4F3/2

REFERENCES

2 700

900 Wavelength, nm

Fig. 5. Absorption spectra of (1) glass and (2) nano-glassceramics with NdF3 concentration of 1 mol % heat-treated for 30 min.

centration of 0.2 mol %, the size of crystals changes from 238 to 282 Å. The incorporation of rare-earth ions into the crystalline phase is confirmed by the absorption spectra.

1. Wei Xu, Hua Zhao, Zhiguo Zhang, and Wenwu Cao, Sensors and Actuators B 178, 520 (2013). 2. Hua Yu, Kaidi Zhou, Kai Chen, Jie Song, Chunxiao Hou, and Lijuan Zhao, Non-Cryst. Sol. 30, 3649 (2008). 3. W. G. Wybourne, Spectroscopic Properties of Rare Earth (Wiley, New York, 1965). 4. S. F. Wang, J. Zhang, D. W. Luo, F. Gu, D. Y. Tang, Z. L. Dong, G. E. B. Tan, W. X. Que, T. S. Zhang, S. Li, and L. B. Kong, Prog. Sol. St. Chem. 1–2, 20 (2013). 5. V. A. Aseev, V. V. Golubkov, E. V. Kolobkova, and N. V. Nikonorov, Glass Phys. Chem. 2, 212 (2012).

OPTICS AND SPECTROSCOPY

Translated by M. Basieva

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2015

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