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Synthesis and Characterization of Strontium Doped Barium Titanates Using Neutron Diffraction Technique

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Synthesis and Characterization of Strontium Doped Barium Titanates Using Neutron Diffraction Technique NUCLEAR SCIENCE AND APPLICATIONS Vol. 28. No. 1&2 2019 Synthesis and Characterization of Strontium Doped Barium Titanates using Neutron Diffraction Technique I. B. Elius1*, B. M. Asif2, J. Maudood1, T. K. Datta1, A. K. M. Zakaria3, S. Hossain1, M. S. Aktar1 and I. Kamal3 1Institute of Nuclear Science and Technology, Bangladesh Atomic Energy Commission, GPO Box No. 3787, Dhaka-1000, Bangladesh 2Department of Nuclear Engineering, University of Dhaka, Dhaka, Bangladesh 3Bangladesh Atomic Energy Commission, Agargaon, Sher-e-Bangla Nagar, Dhaka-1207 Abstract In the present study, structural changes due to gradual doping of Sr in BaTiO3 were investigated by both x-ray and neutron powder diffraction techniques. Ba1-xSrxTiO3 (x=0.0, 0.5 and 1.0) samples were synthesized by PVA evaporation method and purity was confirmed by x-ray diffraction experiment. After that, neutron diffraction experiments were carried out and diffraction data were analyzed by Rietveld least-square data refinement method using the computer program RIETAN-2000 and FullProf to determine various crystallographic structural parameters. The cation and anion position coordinates were also determined from the data refinement method which confirmed that Ba and Sr atoms possess tetrahedral A site and Ti atoms possess octahedral B site for all three samples BaTiO3, Ba0.5Sr0.5TiO3, and SrTiO3, respectively. Moreover, the lattice parameter values indicate, Ba1-xSrxTiO3 structure undergoes a phase transition from tetragonal to cubic somewhere before x=0.5. The concurrences between observed and calculated profiles were excellent and well consistent with the previously reported values. Furthermore, MEM based analysis was done for charge density measurement. Keywords: Neutron diffraction, Barium titanate, PVA evaporation method, X-ray diffraction, ferroelectric materials 1. Introduction and neutron diffraction studies were performed for Barium titanate is one of the most studied ferroelectric crystallographic studies via structural refinement. ceramic materials which exhibit photoelectric and piezoelectric properties [1–3]. It has been widely used as 2. Experimental electromechanical transducers, ceramic multilayer 2.1 Sample preparation capacitors (CMC), microphones, in non-linear optics etc. The role of polyvinyl alcohol (average molecular weight [4]. Nanoparticles of Barium titanate are used as nano- 125000) in the current method is to make a homogeneous carriers for drug delivery for its bio-compatibility, as distribution of cations in its polymeric network structure capacitor energy storages in electric vehicles, in electro- and further, restrict the precipitation or segregation from the optic modulation, in un-cooled sensors, in thermal nitrate-PVA solution. Upon heating, PVA is decomposed capacitors etc. [3, 5]. Depending on the temperature, and thus provides an oxidizing environment. NO2 leaves the BaTiO3 has four polymorphs (cubic, tetragonal, organic dark fluffy mixture during the evaporation process, orthorhombic and rhombohedric consecutively). But in which is the precursor. After the calcination process, room temperature, it is tetragonal in structure [6]. When porous-fine white powder is yielded as the organic parts strontium is doped, BaTiO3 goes through a phase transition evaporate in the form of CO , CO, water and other gas. from tetragonal to cubic [4, 7-8]. Because of its dielectric 2 nonlinearity, large tenability and good dielectric thermal The perovskites Ba1-xSrxTiO3 (x = 0.0, 0.5, and 1.0) were stability, strontium doped barium titanate materials have prepared by the PVA evaporation method at the material various uses as phase shifters, tunable RF-filters etc. [9-10]. synthesis laboratory in the Institute of Nuclear Science and Technology (INST) of Atomic Energy Research Barium titanates are represented with a stoichiometric Establishment (AERE), Savar, Dhaka. All samples were formula ABO3 which stoichiometrically belongs to prepared using High purity materials of Ba(NO ) (99% perovskite group of crystals. In this structure, A and B are 3 2 pure), Sr(NO3)2 (98% pure) and TiO2 (99% pure) powders cations and O is the anion (oxygen in this particular case). from Sigma Aldrich, India, in exact stoichiometric The ionic radii of A (typically Ba2+, Ca2+, Sr2+, La3+ etc.) is 3+ 3+ proportions. Then chemicals were weighted separately larger than B cations (Ti , Fe etc.), where A cations using a digital microbalance. At first, stoichiometric reside at the corners of the structural cage having a 12-fold proportions of Ba(NO3)2, Sr(NO3)2 and TiO2 were mixed cuboctahedral coordination (0,0,0) and the B-ions are with 200 ml water and the mixture was then heated in a surrounded by the anions having a six-fold coordination at magnetic heating stirrer to make it homogeneous. When the the very center (½, ½, ½) [3, 11-12]. solutions started boiling, PVA of a proportional amount The current study, Ba1-xSrxTiO3 (x = 0.0, 0.5, and 1.0) was mixed with the solutions. Different amounts of PVA samples were synthesized by PVA (polyvinyl alcohol, was used for each sample. [CH2CH(OH)]n) evaporation method which is an Then the new PVA solutions were heated until the water environment-friendly, feasible, low cost and a low- was completely vaporized and the mixture turned into the temperature method [11, 13]. Finaly, both x-ray diffraction ashes. After that, the ashes were heated in a muffle furnace in air environment at 300°C for 2 hours and then 500°C for *Corresponding author: [email protected] 2 hours each (the temperature was raised and cooled down 57 NUCLEAR SCIENCE AND APPLICATIONS Vol. 28. No. 1&2 2019 at a rate of 5°C/min). Then the mixtures of all the samples resolution of 0.05°, covering an angular range of 110° [18]. were ground in an agate mortar until the powder mixture The machine has The powdered samples were poured in a became slippery. After grinding, all the samples were cylindrical vanadium can which was placed on a rotating calcined in a muffle furnace at 700°C in air for 6.5 hours, in table. The neutron beam emerging from the TRIGA Mark- order to homogenize the end product. The raising and the II research reactor gets converted to monochrome using a cooling temperature was 5°C/min, respectively. Then the Si-single crystal monochromator. Only the neutron beam mixture of each sample was cooled down to room having a wavelength of 1.5656Å can pass through the temperature in the furnace having air atmosphere at a rate filtration system which is then collimated to impinge on the of 5°C/min. Appropriate amount of polyvinyl alcohol sample can. The scattering pattern recorded by Position (PVA) was added to each dried mixture as a binder and Sensitive Detectors (PSD) is then processed and stored by mixed intimately by agate mortar. It was then pressed into the data acquisition system. small pellets of cylindrical form with dimensions of 13mm The recorded neutron diffraction data were analyzed by diameter and width of 2-3 mm in a hydraulic press. The Rietveld least-square refinement method [19] using the pellets did not require pre sintering as the ingredients do not computer program RIETAN-2000 [8] and FullProf [20] in have carbonates and higher oxides to be decomposed. The order to find crystal structure. The Pseudo-Voigt function pellets were then sintered in air at 1250°C for 9 hours and was used to model the peak profiles. The background was subsequently cooled in the furnace at a slow rate. Finally, fitted using a six-factor polynomial. Parameters like scale the pellets of each sample were pulverized into powder factor, zero position parameter, isotropic profile parameters, form again by grinding with the agate mortar. Time taken lattice parameters, atomic positional parameters etc. were for each sample to convert from solid to powder was also refined. The refinements were done for the whole approximately 3 hours. angular range, leaving no unmatched or unaccounted peaks. 2.2 Data collection and refinement For barium titanate sample, split Pseudo-Voigt function was The samples were subjected to x-ray diffraction studies to used to model the peaks; other refinements were carried out confirm their phase purity. These studies were carried out in a similar manner. using a Phillips PW 3040 X’Pert Pro x-ray diffractometer with Cu(K) radiation having an average wavelength of 1.54178Å, located at Atomic Energy Centre (AECD) of BAEC, Dhaka [14]. The XRD patterns were recorded within an angular range of 2θ = 20°~70° having a step size of 0.02°. The comparison between the positions of the peaks in the XRD patterns of the samples and the standard samples confirmed the formation of desired phases [12, 15]. Moreover, the peak positions were indexed with Chekcell peak matching software. Absence of any other extra peaks indicated that no impurity or other chemicals were found in the samples. Space groups were identified using Chekcell software. The XRD patterns of the samples are presented in Fig. 1. Bragg’s peaks at (100), (101), (111), (200), (210), (211), (220) and the splitting of (200) peak to afterwards (approaching to higher angle) suggested that the samples belong to tetragonal (P4mm) symmetry [16], which are shown in Fig. 1(a) and Fig. 1(b)). On the other hand, no splitting in any of the peaks in the SrTiO3’s pattern indicated the formation of cubic symmetry (Pm m) [12, 17], it is shon in Fig. 1(c) . Upon confirmation of the formation of a single-phase Fig. 1: X-ray diffraction pattern of (a) BaTiO , (b) Ba Sr TiO structure from x-ray diffraction data, long exposure neutron 3 0.5 0.5 3 and (c) SrTiO3, respectively diffraction measurements were performed on the samples. X-ray photons interact with the outer shell electrons, 3. Results and Discussion whereas, neutrons interact with the nuclei of atoms and thus more specific and reliable when structural position data, It is well evident from previous studies that BaTiO3 is bond lengths etc.
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