Structural properties of lithium borate glasses doped with rare earth ions

Propriedades estruturais de vidros de borato de lítio dopados com íons de terras raras

Structural properties of lithium borate glasses doped with rare earth ions

(Propriedades estruturais de vidros de borato de lítio dopados com íons de terras raras)

D. Thomazini¹, F. Lanciotti Jr.², A. S. B. Sombra³

¹Laboratório de Novos Materiais - LNM, Centro de Ciências Tecnológicas, UNIFOR

Av. Washington Soares, 1321, Fortaleza, CE, Brazil, 60811-341

[e-mail]

²Grupo de Semicondutores, Departamento de Física, UFSCar

Rod. Washington Luiz, km. 235, S. Carlos, SP, Brazil, 13566-905

³Laboratório de Óptica Não-Linear e Ciência dos Materiais - LONLCM

Departamento de Física, UFC, C.P. 6030, 60455-760 , Fortaleza, CE, Brazil.

Abstract

This paper presents the study on lithium triborate glass (LBO) in the system (1-x)|3B₂O₃.Li₂O| (x)Nb₂O₅ yPr³⁺ zYb³⁺ wNd³⁺ with 0 £ x £ 20 mol% (y, z and w in mol%). The samples were studied by Raman spectroscopy, infrared absorption and differential thermal analysis. Pr³⁺-doped LBO and Pr³⁺/Yb³⁺-doped LBO samples show an increase of the glass transition and crystallization temperatures and a decrease of the fusion temperature associated with the increase of the praseodymium concentration in the LBO matrix. For the Nd³⁺-doped LBO and Pr³⁺/Yb³⁺-doped (LBO+Nb₂O₅) samples, a decrease of the glass transition temperature of the samples was observed. The increase of the rare earth doping leads to an increase of the difference between the glass transition and the crystallization temperatures. From infrared analysis it was possible to identify all the modes associated to the B-O structure. The NbO₆ octahedra was also identified by IR spectroscopy for samples with x=5, 10, 15 and 20 mol% and y=0.05, z=1.1 mol%. Raman spectroscopy shows the presence of boroxol rings, tetrahedral and triangular coordination for boron. For samples containing niobium, the Raman spectra show the vibrational mode associated with the Nb-O bond in the niobium octahedra (NbO₆).

Keywords: rare-earth, LBO, niobium, up-conversion, laser.

Resumo

Este trabalho apresenta um estudo de vidros de triborato de lítio (LBO) do sistema (1-x)|3B₂O₃.Li₂O| (x)Nb₂O₅ yPr³⁺ zYb³⁺ wNd³⁺ com 0 £ x £ 20 mol% (y, z e w em mol%). Foram feitos estudos por espectroscopia Raman, absorção no infravermelho e análise térmica diferencial. Amostras de LBO dopadas com Pr³⁺ e com Pr³⁺/Yb³⁺ mostram um aumento nas temperaturas de transição vítrea e de cristalização e uma diminuição na temperatura de fusão associada ao aumento da concentração de praseodímio ma matriz de LBO. Foi observada uma diminuição na temperatura de transição vítrea nas amostras de LBO dopadas com Nd³⁺ e nas de (LBO+Nb₂O₅) dopadas com Pr³⁺/Yb³⁺.O aumento no teor de dopagem de terras raras leva a um aumento na diferença das temperaturas de transição vítrea e de cristalização. A análise de espectroscopia na região do infravermelho possibilitou identificar todos os modos associados à estrutura B-O. O octaedro NbO₆ foi também identificado por espectroscopia de infravermelho nas amostras com x=5, 10, 15 e 20 mol% e y=0,05, z=1,1 mol%. Os resultados de espectroscopia Raman mostram a presença de anéis boroxol, e coordenação tetraédrica e triangular para o boro. Para as amostras contendo nióbio, os espectros Raman mostram o modo vibracional associado com a ligação Nb-O no octaedro de nióbio (NbO₆).

Palavras-chave: terras raras, LBO, nióbio, up-conversion, laser.

INTRODUCTION

Research on materials with non-linear electro-optical properties has been very intense since 1976 [1]. Anionic group theory [2] and experimental methods were used to predict and to grow many non-linear optical crystals of the borate family, like BaB₂O₄ (BBO) and LiB₃O₅ (LBO) [3].

These crystals have been developed as a result of molecular engineering in which localized molecular orbital of the anionic group such as (B₃O₆)^3- e (B₃O₇)^5-. The distortions in the structures have been analyzed to determine the microscopic contributions of second order optical susceptibility of the group.

The LBO crystal is optically biaxial [4], in the group mm2 (C_2v), grown by high temperature self-flux, flux pulling and top-seeded solution growth. LBO is chemically stable, mechanically robust, and not hygroscopic. Its transparency window is larger than the one for BBO, which has a transparency range from 0.19 to 2.6 mm, extending down to the ultraviolet range. The basic structure of LBO is a unit from the anionic group (B₃O₇)^5-, with boron trigonally or tetrahedrally coordinated to oxygen.

Recently, ultraviolet (UV) light sources have been used in various applications such as high-density optical disk mastering, photolithography, etc. Therefore, an UV solid-state laser, which combines a high power infrared laser with a nonlinear optical crystal, has been desired as a replacement for the excimer lasers.

In recent years, glasses containing B₂O₃ have been of great interest due to their applications as nonlinear optical materials, and as lasers hosts having high nonlinear optical parameters [5].

In this work, a LiB₃O₅ (LBO) glass matrix was doped with rare earth ions: Nd³⁺ (Nd³⁺:LBO), Pr³⁺(Pr³⁺:LBO), and Pr³⁺/Yb³⁺(Pr³⁺/Yb ³⁺:LBO). The Pr³⁺/Yb³⁺:LBO samples were also doped with niobium oxide (Pr³⁺/Yb³⁺:LBO+Nb₂O₅).

EXPERIMENTAL PROCEDURE

LBO glass samples were prepared from mixtures of boron oxide and lithium carbonate in the followed stoichiometric proportion:

3 B₂O₃ + Li₂CO₃® 2 LiB₃O₅ + CO₂

Some samples were prepared with addition of niobium oxide (Nb₂O₅) to observe the influence of niobium as a network modifier. The earth rare raw materials are 99.99% pure. The samples were doped with Pr³⁺ (Pr³⁺:LBO), Nd³⁺ (Nd³⁺:LBO), and co-doped with Pr³⁺ and Yb³⁺ (Pr³⁺/Yb³⁺:LBO) and Nb₂O₅ (Pr³⁺/Yb³⁺:LBO+Nb). Table I shows the concentration of rare earth or niobium oxide.

To prevent an excessive boiling and consequent loss of mass, water was removed from B₂O₃ by pre-heating at 500 ^oC for three hours. The mixture of raw materials was heated in platinum crucibles, in an open electric furnace and melted at 950 ^oC for one hour to promote a uniform melt. The melt was poured into an aluminum mold, pre-heated at 350 ^oC, with 2 mm thickness and 20 mm diameter, and pressed with an aluminum plate that yielded the disk shaped pellets. To avoid breaking of the samples by residual internal strain, the materials were annealed at 350 ^oC for 30 minutes.

Powder samples were used for differential thermal analysis and infrared spectroscopy. Raman spectroscopy was preformed in pellets.

The infrared (IR) spectra were obtained at room temperature using KBr pellets mixed to powders for each glass composition. The pellet thickness varied between 0.5 and 0.6 mm. The IR spectra were measured from 400-2000 cm^-1 with a Nicolet 5ZPX FT-IR spectrometer.

Differential thermal analysis (DTA) was carried out to obtain glass transition (Tg), crystallization (Tx) and fusion (Tf) temperatures of the glasses using a STA-409 Netzsch apparatus. The analyses were performed in a platinum crucible with a heating rate of 10 ^oC/min under nitrogen. The weight for all samples was 40 mg.

Raman measurements were performed using a U-1000 Jobin-Yvon double monochromator coupled to a cooled GaAs photomultiplier and a conventional photon counting system, in air, in a 0-2000 cm^-1 range.

RESULTS AND DISCUSSION

Differential Thermal Analysis

Fig. 1 shows DTA curves for samples with different praseodymium concentrations. It can be seen that Tg, Tx, and Tf increase with the addition of Pr³⁺.

Fig. 2 shows DTA curves for Pr³⁺/Yb³⁺:LBO glasses. One can notice that Tg and Tx increase and Tf decreases as a function of doping. This behavior could be associated to the rare earth ions acting as network modifiers in the LBO matrix, leading to a reduction of the melting temperature and viscosity.

Fig. 3 exhibits DTA curves for Pr³⁺/Yb³⁺:LBO+Nb₂O₅ samples. The reduction of Tg, Tx, and Tf with the addition of niobium was also observed.

Fig. 4 shows DTA curves for Nd³⁺: LBO glasses. The Tg and Tx temperatures increase and Tf is almost constant with the addition of neodymium ions.

Fig. 5 shows the behavior of Tg, Tx, and Tf for samples: a) Pr³⁺:LBO, b) Pr³⁺/Yb³⁺:LBO, c) Pr³⁺/Yb³⁺:LBO+ Nb₂O₅, and d) Nd³⁺:LBO. A better thermal stability of the glass is related to a larger difference between Tg and Tx [6]. Table II shows the values of Tg, Tx, and Tf for the LBO samples. It can be observed that the samples Pr³⁺/Yb³⁺:LBO+ Nb₂O₅ haVE better thermal stability than the other samples due to the higher difference between Tg and Tx. The Nd³⁺:LBO glasses show the lower thermal stability of all series.

Infrared Spectroscopy

In previous studies it was observed that the B₂O₃ glass structure is a random network composed by boroxol rings and BO₃ triangles, linked with B-O-B [7]. The addition of alkaline oxides modifies the boroxol rings, forming complexes groups with one or two boron coordination [8].

Massot and co-workers [9] studied lithium-boron glasses by infrared spectroscopy and observed the influence of the lithium content in the boron network. For the 3B₂O₃.Li₂O composition, they concluded that the band at 1380 cm^-1 is due to the stretching vibration of trigonal units of BO₃; the band around 850-1100 cm^-1 is ascribed to the stretching vibration of B-O; the band at 700 cm^-1 is attributed to scissor vibrations of B-O-B bridges in the boron-oxygen network.

Figs. 6, 7, and ⁹ show the infrared spectra of Pr³⁺:LBO, Pr³⁺/Yb³⁺:LBO, and Nd³⁺:LBO, respectively. In these figures bands at 1380 cm^-1, one in the range of 850-1100 cm^-1, and one close to 700 cm^-1 are observed, in good agreement with the literature [9, 10]. No absorption associated to the rare earth doping was observed. The small amount of these ions, when compared to other elements of LBO glass, should be responsible for this result.

Tatsumisago and co-workers [11] showed that the infrared band in the 600-545 cm^-1 range is associated to the presence of the NbO₆ octahedra in niobate glasses. This observation was also reported by Araújo [12] and Andrade [13]. Fig. 8 shows the IR spectra of the Pr³⁺/Yb³⁺:LBO+ Nb₂O₅ samples. By increasing the concentration of Nb⁵⁺, an increase of the intensity related to the band around 600-440 cm^-1 is noticed.

Raman Spectroscopy

The Raman spectra of a pure B₂O₃ glass is characterized by a highly polarized and well defined band at 805 cm^-1 [14]. The dissociation of lithium oxide in O^- and Li⁺ ions results in the transformation of boroxol rings in a complex network, involving the coupling of the ring with at least one boron of four-coordination. The band at 770 cm^-1 is associated to the vibration of the ring formed by six elements that contains BO₃ triangles and BO₄ tetrahedrons.

Fig. 10 shows Raman spectra of LBO samples. A single band at about 770 cm^-1, corresponding to boron scattering was observed. No Raman band associated to the rare earth ions was observed, probably due to the small amount of these elements in the matrix.

Jazouli [15] measured the vibrational mode of Nb-O in the NbO₆ octahedra. The Raman band associated to this mode was detected at about 900 cm^-1 . The band observed in Raman spectra has been attributed to the high disorder in the NbO₆ octahedra in the absence of oxygen bridges, and further a low order to the NbO₆ octahedra in the presence of oxygen bridge [16, 17]. Fig. 11 shows Raman spectra of Pr³⁺/Yb³⁺:LBO+ Nb₂O₅ samples, showing a band around 890 cm^-1, due to the vibrational mode of Nb-O in the NbO₆ octahedra.

CONCLUSIONS

For Pr³⁺:LBO and Pr³⁺/Yb³⁺:LBO glasses, an increase of the glass transition and crystallization temperatures, and a decrease of fusion temperature as a function of the increase of the rare earth doping content ware observed. For Nd³⁺:LBO and Pr³⁺/Yb³⁺:LBO+ Nb₂O₅ glasses, a decrease of the glass transition was noted. The difference between Tg and Tx increases with the addition of rare earth elements, promoting a higher thermal stability of these glasses.

The infrared analyses show that the B-O link is configured in trigonal form. With the addition of rare earth ions, no change related to B-O link or trigonal configuration of boron was observed. The presence of NbO₆ octahedra in the Pr³⁺/Yb³⁺:LBO+Nb samples was confirmed by infrared analysis.

Boroxol rings were detected by Raman spectroscopy, confirming the presence of triangles and tetrahedrons of boron. In these samples, vibrations modes associated to rare earth ions were not observed, probably due to the small amount of these elements in the matrix. In Pr³⁺/Yb³⁺:LBO+ Nb₂O₅, the vibrational mode of Nb-O in the NbO₆ octahedra was observed.

(Rec. 18/09/2000, Rev. 16/04/2001, Ac. 20/04/2001)

• [1] C. T. Chen, G. Liu, Ann. Ver. Mater. Sci. 16 (1976) 16.
• [2] C. T. Chen, S. X. Chen, Acta Phys. Sin. 29 (1980) 1000 .
• [3] R. H. French, J. W. Ling, F. S. Ohuchi, C. T. Chen, Phys. Rev. B 44, 16 (1991) 8496.
• [4] C. Chen, Y. Wu, A. Jiang, B. Wu, G. You, R. Li, S. Lin, J. Opt Soc. Am. B6 (1989) 616.
• [5] A. Renuka Devi, C. K. Jayasanky, J. Non Crys. Sol. 197 (1996) 111.
• [6] J. Zarzycki, Glasses and the Vitreous State, Cambridge University Press, Cambridge (1991).
• [7] J. Krogh-Moe; Phys. Chem. Glasses 6 (1966) 46.
• [8] T. Minami, Y. Ikeda, M. Tanaka, J. Non-Cryst. Solids 52 (1982) 159.
• [9] M. Massot, C. Julien, M. Balkanski, Infrared Phys. 29 (1989) 775.
• [10] E. I. Kamitsos, A. P. Patsis, M. A. Karakassides, G. D. Chryssikos, J. Non-Cryst. Solids 126 (1990) 52.
• [11] M. Tatssumisago, A. Hamada, T. Minami, M. Tanaka, J. Non-Cryst. Solids 56 (1983) 423.
• [12] E. B. de Araújo, J. A. C. de Paiva, J. A. Freitas Jr., A. S. B. Sombra, J. Phys. Chem. Solids 59 (1998) 689.
• [13] J. S. Andrade, A. G. Pinheiro, I. F. Vasconcelos, J. M. Sasaki, J. A. C de Paiva. M. A. Valente, A. S. B. Sombra, J. Phys.: Condens. Matter 11 (1999) 4451.
• [14] B. P. Dwivedi, B. N. Khanna, J. Phys. Chem. Solids 56 (1995) 39.
• [15] A. El Jazouli, R. Brochu, J. C. Viala, R. Olazcuaga, C. Delmas, G. Le Flem, Ann. Chim. Fr. 7 (1982) 285.
• [16] K. Fukumi, S. Sakka, J. Mat. Sci. 23 (1988) 2819.
• [17] K. Fukumi, T. Kokubo, K. Kamiya, S. Sakka, J. Non-Cryst. Solids 84 (1986) 100.

Publication Dates

• Publication in this collection
  05 July 2001
• Date of issue
  June 2001

History

• Received
  18 Sept 2000
• Reviewed
  16 Apr 2001
• Accepted
  20 Apr 2001