Characterization of (Bi1/2Na1/2)TiO3 using electric modulus spectroscopy

Abstract

(Bi1=2Na1=2)TiO3 (BNT) is a perovskite material which undergoes several complex ferroic phase transitions between room temperature and ,540 8C [1–4]. Single crystal studies have demonstrated [5] that non-stoichiometry can occur in this system, which leads to differences in the degree of rhombohedral lattice distortion at room temperature. Although the space groups for the rhombehedral and cubic polymorphs of BNT at room and high temperatures are known, R3c and Fm3m, respectively, there is some confusion regarding the space group of an intermediate temperature tetragonal polymorph. In an attempt to assign the correct space group, Zvirgzds et al. [3] employed fixed frequency permittivity measurements to determine whether the tetragonal polymorph is polar or non-polar. High temperature, fixed-frequency dielectric measurements reported on BNT ‘‘revealed’’ a peak maximum in permittivity data at ,450–540 8C. The authors suggested that this dielectric anomaly was related to a tetragonal to cubic phase transition known to occur in BNT from X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements at 540 8C. This led Zvirgzds et al. [3] to postulate, on the basis of fixed-frequency electrical data, that the tetragonal polymorph is polar (point group 4mm). In this letter we report a.c. impedance spectroscopy (IS) results, which have been used to characterize the high temperature electrical properties of polycrystalline BNT. IS is a convenient method for measuring the a.c. response of electromaterials having resistance values between ,102 and 108 U over a wide frequency range, typically 10y2 – 107 Hz. In favourable circumstances, IS can offer several advantages over traditional fixed-frequency measurements, e.g. tan δ and e9, and is particularly useful when attempting to separate intra-granular (bulk), inter-granular and sample=electrode effects [6]. It has been widely used to quantify bulk characteristics of solid electrolytes but has not yet been used extensively in the study of ferroelectric materials. This is primarily due to the high resistivity (. 108 Ucm) associated with dielectric materials at moderate temperatures, i.e. , 400 8C. As a result, fixed-frequency measurements at 1 kHz remain the most convenient and widely used method for electrical characterization of dielectric materials at moderate temperatures. The IS results presented in this letter demonstrate that the high temperature permittivity anomaly observed in fixed-frequency measurements [3] is not a bulk property, and that, in fact, the tetragonal polymorph is non-polar, as has been previously suggested [7, 8]. We also show how spectroscopic plots of the imaginary component of the electric modulus, M 0, provide a convenient and reliable method for calculating accurate intra-granular permittivity and conductivity values. The reagents, Bi2O3 (99.5%), Na2CO3 (99.9%) and TiO2 (99.99%) were dried at 300, 300 and 700 8C, respectively and stored in a vacuum desiccator prior to use. A reaction mixture of (Bi1=2Na1=2)TiO3 totalling 3–4 g was weighed, mixed into a paste with acetone using an agate mortar and pestle, dried and fired in a Pt boat at 750 8C for 24 h. The powder was reground, analysed by XRD using a Hagg Guinier camera (CuKα1 radiation) and reheated several times between 750 and 1000 8C until phase purity was achieved. A Stoe Stadi psd-based diffractometer, using CuKα1 radiation was used for cell parameter determination. Pellets for electrical property measurements were cold-pressed, sintered at 1160 8C for 1 h and Au paste electrodes fired on at 200–600 8C. The a.c. impedance measurements were made over 25– 800 8C in air using combined Solartron 1250=1287 and Hewlett-Packard 4192 instrumentation to sweep the frequency range 10y2 to ,107 Hz with an applied voltage of 100 mV. Impedance data were corrected for the parallel capacitance of the ‘‘blank’’ conductivity jig. Samples were allowed to equilibrate at constant temperature for 45 min prior to each set of impedance measurements. After the completion of electrical measurements, the pellets were polished, carbon coated and analysed by electron probe microanalysis (EPMA) using a Cameca electron probe, model SX51. Bi2CuO4, NaAlSi2O6 and SrTiO3 were used as calibration standards for Bi, Na and Ti, respectively. Spot analysis was conducted on ten different grains under beam conditions of 15 kV and 20 nA. The polycrystalline samples prepared were phase pure by XRD and EPMA. The XRD pattern was fully indexed on a rhombohedral unit cell given in JCPDS file 36-340 with refined lattice parameters of a ˆ 0.5477(2) and c ˆ 0.6766(4) nm. EPMA established that the pellets used for conductivity measurements were stoichiometric within experimental errors, i.e. Bi0:48(3)Na0:51(3)Ti1:01(3)O3. The variation in permittivity for BNT at 1 kHz as a function of temperature is shown in Fig. 1. The permittivity maxima at ,150 and ,320 8C are in