Evolution of ferroelectric LiNbO3 phase in a reactive glass matrix (LiBO2–Nb2O5)

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Abstract

Transparent glasses in the system (100  x)LiBO2xNb2O5 (5  x  25, in molar ratio) were fabricated by the conventional melt quenching technique. The as-quenched samples were amorphous as established via X-ray powder diffraction (XRD) studies. Differential thermal analyses (DTA) confirmed their glassy nature. The glass transition temperature (Tg) and the crystallization temperature were found to be strongly composition (x) dependent. Lithium niobate (LiNbO3) nanocrystals were produced within the glass by heat-treating it at 500 °C/3 h (for x = 15). Two-stage heat-treatment process has improved the optical transmission characteristics. Impedance analysis was done to rationalize the electrical behavior of these glasses embedded with 100 nm sized LiNbO3 crystallites. The observed pyroelectric response and ferroelectric (P vs E) hysteresis loop at room temperature confirmed the polar nature of these composites.

Introduction

Transparent ferroelectric glass–ceramics have been of increasing interest over the single crystals owing to their potential applications for multifunctional devices. The optical and polar properties could be appreciably changed by varying the volume fraction of the active phase dispersed and its nano/microstructure. The transparent composites embedded with ferroelectric crystals could be tailored to exhibit either Kerr or Pockels effects depending on the crystallite size. Yet another important factor is that these could be obtained with relative ease. These are also of technological prominence because of the flexibility that this route offers in fabricating into intricate sizes and shapes depending on the requirement. A number of glass–ceramics comprising well-known ferroelectric crystalline phases (BaTiO3, LiNbO3, LaBGeO5, Sr0.5Ba0.5Nb2O6, SrBi2Nb2O9, etc.) have been fabricated and investigated for their polar and electro-optic properties [1], [2], [3], [4], [5], [6].

Single crystals of Lithium Niobate (LiNbO3) are important non-linear optic (NLO) materials because of their efficient second harmonic generation (SHG) as well as high electro-optic and elasto-optic coefficients [7], [8], [9]. It is because of these promising properties, LiNbO3 has occupied a prominent place in the fabrication of optical waveguides, modulators, switches and optical circuits [10]. Keeping these applications in view, many researches around the globe have been making attempts to fabricate transparent glasses comprising LiNbO3 crystallites. As a first attempt Imaoka and Yamazaki [11] reported the glass-forming region in the system TeO2–Li2O–Nb2O5. Komatsu et al. [3] fabricated transparent telluride glasses containing ferroelectric LiNbO3 crystals. In crystallizing ferroelectric LiNbO3 phase in these glasses, one invariably encounters an intermediate pyrochlore phase precipitating during the heat-treatment [3], [12]. However, the direct crystallization of LiNbO3 was achieved in telluride glasses with high Li content through a two-stage heat-treatment [13]. The surface crystallization of LiNbO3 in the TeO2–LiNbO3 glass system was demonstrated by one of the authors through single-stage heat-treatment [14].

The event of the pyrochlore and centrosymmetric LiNb3O8 impurity phase formation along with the desired LiNbO3 in TeO2 glass matrix is a nuisance in achieving monophasic transparent LiNbO3 phase. Recently, transparent LiNbO3 glass–ceramics were fabricated in the crystallization of LiNbO3–SiO2–Al2O3 glass system [15], [16]. In all the processes, that were reported so far in the literature, either pre reacted LiNbO3 or constituent oxides were taken in suitable ratios and recrystallized or allowed to react in glass matrices wherein it was difficult to exercise strict control over the crystallite size which is very crucial for obtaining transparent glass–ceramics especially in optically incompatible glass matrices. Therefore, we thought it was worth attempting to crystallize the desired phase as a result of the in situ chemical reaction. For this purpose we have chosen LiBO2 as the matrix, which is rich in Li (as compared to the other borates known in the literature) which is essential to overcome the formation of impurity phase, LiNb3O8. We were successful in preparing the LiBO2–Nb2O5 glasses in a binary system with different compositions from which transparent LiNbO3 crystallites were grown as a result of the reaction between Li and Nb2O5.

In this paper, we report the results concerning the glass formation and evolution of the nanocrystalline LiNbO3 phase in the glass system (100  x)LiBO2xNb2O5 (5  x  25, in molar ratio). The structural, microstructural, ferroelectric, pyroelectric and dielectric characteristics of the glass composite, embedded with LiNbO3 nanocrystals, are elucidated.

Section snippets

Experimental

Glasses in the system (100  x)LiBO2xNb2O5 (5  x  25, in molar ratio) were prepared by conventional melt quenching technique. All the glasses under investigation were prepared from reagent grade LiBO2 and Nb2O5. Well mixed batches were melted in a platinum crucible at 1100–1250 °C for 1 h to yield 15 g of the glass. Melts were quenched by pouring on a steel plate that was maintained at 150 °C (to prevent the glass samples from cracking) and pressed with another plate to obtain 1–1.5 mm thick glass

Results and discussion

Transparent colorless glasses, of varied dimensions without any visible inclusions were fabricated in the system (100  x)LiBO2xNb2O5 (5  x  25, in molar ratio). Glasses of compositions higher than x = 25 could not be quenched into glasses. Therefore the present investigations have been confined to the compositions which fall within the glass-forming region.

Conclusions

Nanocrystallization of LiNbO3 has been demonstrated in a reactive glass system LiBO2–Nb2O5. Compositional dependence of dielectric properties of these glass nanocomposites indicates an increase in LiNbO3 volume fraction with increase in Nb2O5 content. Based on the crystallization kinetic studies, two-stage heating technique has been employed and achieved the improvement in optical transmission characteristics. The most interesting aspect of the present investigations has been the observation of

Acknowledgments

The authors thank the Council of Scientific and Industrial Research (CSIR), Government of India for financial grant. One of the authors (Syam Prasad N.) also acknowledges the University Grants Commission (UGC), Government of India, for a research fellowship.

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