Eu3+ doped ferroelectric CaBi2Ta2O9 based glass-ceramic nanocomposites: Crystallization kinetics, optical and dielectric properties
Introduction
Mixed bismuth oxide layer compounds (known as Aurivillius compounds) were reported first by Bengt Aurivillius [1] in 1949, which was followed by investigation of ferroelectricity in these compounds by Smolensky et al. [2]. A member of the Aurivillius family, bismuth layered-structure ferroelectrics (BLSFs) has attracted great attention in recent years due to their unique properties, such as large dielectric breakdown strength (BDS), high Curie temperature (TC) and excellent fatigue free polarization [3], [4]. These properties make them a promising candidate for application in ultrasound transducers, high temperature piezoelectric devices, thin film capacitors, pyroelectric sensors, optical memories, displays and non-volatile ferroelectric random access memories (FE-RAM).
The BLSF crystals are represented by the general formula (Bi2O2)2+(Ax−1BxO3x+1)2−, where (Ax−1BxO3x+1)2- are the pseudo-perovskite unit cells interspaced between bismuth oxide ((Bi2O2)2+) layers along the c-axis. In the pseudo-perovskite block ‘A′ can be a monovalent (Na+, K+), bivalent (Ca2+, Ba2+, Sr2+, Pb2+) or trivalent cation (Bi3+), ‘B′ represents a tetravalent, pentavalent, or hexavalent ion (Ti4+, Nb5+, Ta5+, W6+, and V6+) and x can be any integer or ½ integer. The CaBi2Ta2O9 (CBT) crystal belongs to the BLSF family which consists of double TaO6 octahedral units within the (CaTa2O7)2− perovskite blocks stacked between the (Bi2O2)2+ layers [5]. It is a polar orthorhombic crystal phase with A21am space group [6]. The presence of small sized Ca2+ ion (ionic radius 1.00 Å) in the A-site of (ATa2O7)2− layers leads to a significant lattice mismatch between the CaO and TaO2 planes of the layers which caused structural distortion of crystals. Due to this structural distortion, the CBT crystals become non-centrosymmetric which contributes to larger spontaneous ferroelectric polarization [7]. CBT also exhibits higher Curie temperature (> 600 °C). Das et al. [8] synthesized CBT thin films and reported some of its ferroelectric and dielectric properties. A maximum polarization of 13.4 μC/cm2, remanent polarization of 3.44 μC/cm2, coercive field strength of 112 kV/cm, dielectric constant of 116 at 100 kHz and low dielectric loss of 0.01 were reported. Photoluminescence properties of rare earth doped CBT ferroelectrics synthesized through conventional solid state reactions technique have been reported in recent times. Ruirui et al. observed outstanding tunable emissions and generation of warm white color in Eu3+/Tb3+ co-doped CBT powders [9]. Strong red emission peaks at 622 nm for Pr3+ and 615 nm for Eu3+ were observed for CBT powders doped with 0.02 mol% of Pr3+ and 0.15 mol% of Eu3+ respectively [10]. They also reported significant enhancement of luminescence intensity of Eu3+ ion in CBT by co-doping with La3+ and the highest emission intensity was reported for europium ion concentration of 0.15 mol% [11]. Such rare earth doped non-centrosymmetric CBT crystals along with their inherent ferroelectric and non-linear optical (NLO) properties could be useful for designing of future generation opto-electronic devices [12].
In comparison to ferroelectric ceramics, fabrication of ferroelectric glass-ceramics (FGCs) through melt-quenching technique offers several advantages like zero/low porosity, higher mechanical and dielectric breakdown strengths and the possibility of altering the properties by varying the volume fraction of the ferroelectric phase and size of the crystals dispersed in the glass matrix. Transparency in FGCs can be achieved through homogenous nucleation and controlling growth of crystallites in nanometer range that are too small for Rayleigh scattering. However, for application of transparent FGCs in integrated optical devices, the size of the crystallites must be large enough to generate a reasonably good ferroelectric response. A tradeoff between these two attributes remains a big challenge for the researchers [13]. Very recently we have reported the optical and dielectric properties of Eu3+ doped BaBi2Ta2O9 (BBTE) glass and glass-ceramics and observed favorable properties, like, a five-fold increase in photoluminescence intensity in the BBTE glass-ceramics and higher dielectric constant (εr) (> 100) along with low dielectric losses and dissipation factors [14]. To the best of our knowledge, there is no report available in the literature on rare earth doped transparent nanostructured glass-ceramics containing ferroelectric CBT (CaBi2Ta2O9) crystal phase, in spite of its favorable properties making it suitable for various advanced applications.
In the present work, we report for the first time synthesis of Eu3+ doped ferroelectric CaBi2Ta2O9 (CBTE) glass-ceramics using conventional melt quenching and ceramming technique. A comparative study of crystallization kinetics of the CBTE glass has been made through linear (Kissinger, Augis-Bennett, Ozawa, Matusita) and multivariate nonlinear (nth dimension Avrami-Erofeev) model-fitting approaches to ascertain the nature of nucleation and mechanism of crystal growth. A novel approach of isothermal prediction has been adopted through the nonlinear kinetics study in order to optimize the heat-treatment protocol for controlled crystallization. The theoretically optimized heat-treatment protocol was successfully exploited experimentally to synthesize transparent CBTE GCs by controlled crystallization. The microstructure, thermal, optical and dielectric properties of the CBTE GCs has been correlated to their processing conditions.
Section snippets
Experimental procedure
Precursor CBTE glass was prepared using a molar composition of 17.5K2O-51.3SiO2-10BaO-10Bi2O3-10Ta2O5 doped with 0.5 mol% of Eu2O3 and 0.7 mol% of CeO2. The glass was synthesized through melt quenching technique by preparing glass batch of 120 g and melting in a platinum crucible at 1475 °C for 2 h employing an electric furnace. High purity raw materials are used for synthesis of glass, such as silica, SiO2 (99.8%; Sipur A1 Bremtheler Quartzitwerk, Usingen, Germany), anhydrous potassium
Physical properties
The CBTE glass is a heavy metal oxide (HMO) glass containing 10 mol% of Bi2O3, prepared by melting at 1475 °C in a platinum crucible. When melted at above 1000 °C color of the glass changes to black from pale yellow [15]. The coloration was intensified and become black with the increase in melting temperature. This is due to the auto thermal reduction of Bi2O3 in air atmosphere, which could be contained by addition of some oxidizing agents like Sb2O3, As2O3 and CeO2. In the present glass
Conclusions
Eu3+ doped SiO2-K2O-CaO-Bi2O3-Ta2O5 (CBTE) glasses have been synthesized and reported for the first time. Detailed crystallization kinetics analysis of the glass was done and a homogenous nucleation with a diffusion controlled three dimensional crystal growth has been determined. The prediction of optimum crystallization temperature and time has been possible from the kinetics study, which was experimentally exploited to fabricate transparent CBTE glass-ceramics. The results of XRD, TEM, and
Acknowledgements
Anirban thanks UGC (212510213), India for his Junior Research Fellowship (JRF) grant. The authors are thankful to Dr. Ranjan Sen, Head, Glass Division and Dr. K. Muraleedharan, Director CSIR-CGCRI for their constant support and encouragement.
References (55)
- et al.
Study of pulsed laser ablated CaBi2Ta2O9 thin films
Solid State Commun.
(2001) - et al.
Blue excited photoluminescence of Pr doped CaBi2Ta2O9 based ferroelectrics
J. Alloy. Compd.
(2012) - et al.
Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption
J. Non-Cryst. Solids
(2006) - et al.
How many non-crystalline solids can be made from all the elements of the periodic table?
J. Non-Cryst. Solids
(2004) - et al.
Fabrication, properties and applications of bulk glassy alloys in late transition metal-based systems
Mater. Sci. Eng. A
(2006) - et al.
ICTAC kinetics committee recommendations for performing kinetic computations on thermal analysis data
Thermochim. Acta
(2011) - et al.
Model-free and model-fitting approaches to kinetic analysis of isothermal and non-isothermal data
Thermochim. Acta
(1999) - et al.
Kinetic study of crystallization of glass by differential thermal analysis - criterion on application of Kissinger plot
J. Non-Cryst. Solids
(1980) - et al.
Crystallization kinetics analysis of BaF2 and BaGdF5 nanocrystals precipitated from oxyfluoride glass systems: a comparative study
Thermochim. Acta
(2015) - et al.
FT–Raman spectroscopic study of calcium-rich and magnesium-rich carbonate minerals
Spectrochim. Acta A
(2005)