Preparation of nanocrystalline ferroelectric BaBi2Nb2O9 by citrate gel method

Preparation of nanocrystalline ferroelectric BaBi2Nb2O9 by citrate gel method

Materials Letters 58 (2004) 3729 – 3731 Preparation of nanocrystalline ferroelectric BaBi2Nb2O9 by citrate gel method ...

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Materials Letters 58 (2004) 3729 – 3731

Preparation of nanocrystalline ferroelectric BaBi2Nb2O9 by citrate gel method S.P. Gaikwada, Violet Samuelb, Renu Pasrichac, V. Ravia,* a

Physical and Materials Chemistry Division, National Chemical Laboratory, Pashan Road, Pune 411008, India b Catalysis Division, National Chemical Laboratory, Pune 411008, India c Center for Materials Characterization, National Chemical Laboratory, Pune 411008, India Received 30 May 2004; received in revised form 20 July 2004; accepted 1 August 2004 Available online 17 August 2004

Abstract A gel was formed when a aqueous solution of BaCl2, BiCl3, NbF5 and citric acid in stoichiometric ratio is heated on a water bath. This gel on decomposition at 600 8C produced the nanocrystallites of ternary oxide, BaBi2Nb2O9 (BBN). The phase contents and lattice parameters were studied by the powder X-ray diffraction (XRD). Particle size and morphology was studied by transmission electron spectroscopy (TEM). The room temperature dielectric constant at 1 kHz is 90 8C. The ferroelectric hysteresis loop parameters of these samples were also studied by a home built Sawyer–Tower circuit. D 2004 Elsevier B.V. All rights reserved. Keywords: Powders chemical preparation ; Electron microscopy ; Dielectric properties ; Ferroelectric properties

1. Introduction Recently, bismuth-based compounds with Aurivillustype structure have attracted much attention because of their low operating voltage, fast switching speed, negligible fatigue up to 1012 switching cycles, excellent retention characteristics and low leakage current density on Pt electrodes for integrated device applications in nonvolatile ferroelectric random access memories (FRAM) [1]. Large remnant polarization, low coercive field and high Curie temperature are required for better performance and reliable operation of the FRAM devices. The Aurivillus family of compounds [2–5] is represented by a general formula (Bi2O2)2+ (An 1Bn O3n+1)2 , where A=Sr, Ca, Ba, Pb, etc., is in 12-fold coordination, B=Ta, Nb, etc., is in sixfold coordination, and n is any integer or 1/2 integer. The lattice structure of this kind of compounds compose of n number of

* Corresponding author. Tel.: +91 20 58934002273; fax: +91 20 5893044. E-mail address: [email protected] (V. Ravi). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.08.004

(An 1Bn O3n+1)2 unit cells sandwiched between (Bi2O2)2+ slabs along pseudo tetragonal c-axis, while A-site cation occupies the cubo-octahedral sites in perovskite layers. SrBi2Ta2O9 (SBT) and SrBi2Nb2O9 (SBN) are most intensively studied members of this in comparison to BaBi2Ta2O9 (BBT), BaBi2Nb2O9 (BBN), CaBi2Ta2O9 (CBT), CaBi2Nb2O9 (CBN), etc. [2–5]. In this communication, we present a simple citrate gel method to produce single-phase BBN powders. However, bulk BBN ceramic powder synthesis has very limited reports using nonconventional techniques other than conventional solid state method [6]. The bulk BBN powders can be used as a target for DC sputtering and laser ablation methods for the preparation of corresponding thin films. The properties of ceramics are greatly affected by the characteristics of the powder, such as particle size, morphology, purity and chemical composition. Using chemical methods, e.g., co-precipitation, sol–gel, hydrothermal and colloid emulsion technique, have been confirmed to efficiently control the morphology and chemical composition of prepared powder. Among the reports of these wet chemical techniques sol–gel using alkoxides, hydrothermal and colloid


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Fig. 1. XRD of BBN precursor powder calcined at 600 8C.

emulsions are time-consuming and involve highly unstable alkoxides and difficult to maintain reaction conditions. The purpose of this study was to prepare ultrafine BaBi2Nb2O9 powder using citrate gel technique from simple inorganic salts. This process can avoid complex steps such as refluxing of alkoxides, resulting in less time consumption compared to other techniques. The citrate gel process offers a number of advantages for the preparation of fine powders of many complex oxides as quoted in the literature [7,8].

2. Experimental For preparing BaBi2Nb2O9, niobium (V) oxide, bismuth (III) nitrate, calcium chloride were and citric acid used as starting materials, which were of AR grade (LOBA cheme). A stoichiometric amount of BaCl2d 6H2O (2.44g) was dissolved in distilled water (100 ml), Bi(NO3)3d 5H2O (4.64g) was dissolved in minimum amount of dilute HNO3 to avoid precipitation of Bi ions, and Nb2O5 (2.65g) was dissolved in minimum amount of HF after heating at hot water bath for 20 h. An excess quantity of concentrated HCl is added to the above solution to dissolve the barium fluoride formed by the mixing of NbF5 and BaCl2d 6H2O. The ratio of the total metal cations to citric acid is one. Since there was no precipitation during mixing of citric acid and these metal salts, the pH of the solution was not varied. On heating on a water bath at 100 8C a light yellowish gel was formed after evaporation of water. Subsequently, the gel is decomposed at various temperatures ranging from 400 to 800 8C. The gel initially started to swell and filled the beaker producing a foamy precursor. This foam consists of very light and homogeneous flakes of very small particle size. For comparison, BBN samples are also prepared by ceramic method. The corresponding oxides or carbonates are taken in stoichiometric ratio and mixed, ground several times and heated at 1000 8C for 72 h. The calcined powders were mixed

with few drops of 1 wt.% solution of polyvinyl alcohol and pelletized at 1–2 tons. The green pellets were sintered at 1050 8C for 2 h. The surfaces of the sintered pellet were polished and electroded with low-temperature curing silver paint. The ferroelectric hysteresis loop parameters were measured with aid of a home-built Sawyer–Tower circuit. A LCR meter was used to measure the room temperature dielectric constant of the samples at 1 kHz. Various techniques, such as X-ray diffraction (XRD, Philips PW 1710 diffractometer) and transmission electron spectroscopy (TEM), were employed to characterize these powders. The powder X-ray pattern were recorded for all the samples sintered at various temperatures by using Philips PW-1710 model X-ray diffractometer using Cu–K~. For lattice parameter and interplanar distance (d) calculation, the samples were scanned in the 2h range of 10–808 for a period of 5 s in the step-scan mode. Silicon was used as an internal standard. Least squares method was employed to determine the lattice parameters. The TEM picture was recorded with JEOL model 1200 EX instrument at the accelerating voltage of 100 kV. The fine powders were dispersed in amyl acetate on a carbon coated TEM copper grid.

3. Result and discussion The citric acid added acts as a complexing agent. The mixture of citric acid and aqueous metal salts forms a gel on heating on a water bath which decomposes at higher temperaturesN300 8C. During calcinations process, a black fluffy mass (foamlike) is formed which occupies large volumes of the furnace. As the temperature increases, the black mass turns to white in colour with the removal of carbon. Samples calcined at 500 8C for 40 min shows less than 1% of carbon. At higher temperatures of calcinations, no carbon was found to present. Fig. 1 shows the XRD pattern of BBN powder calcined at 600 8C, indicating formation of phase pure BBN. The crystal structure of BBN

Fig. 2. TEM of BBN precursor powder calcined at 600 8C.

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70 kV/cm without occurring an electric breakdown of the sample. It is well known that the ferroelectric properties obtained depends on sinter density and defects present in the sample. The samples prepared by the ceramic technique have remnant polarization, P r=0.3 AC/cm2 and coercive field, E C~10 kV/cm at an applied field of 50 kV/cm without occurring an electric breakdown. The room temperature dielectric constant measured at 1 kHz is 90 8C for the citrate process-derived BBN samples.

Fig. 3. P–E loop for the sintered BBN pellet derived from citrate process.

is tetragonal and all the d-lines pattern match with reported values [6]. The calculated lattice parameters by least square fit are a=3.929 2 and c=25.598 2. Conventional solid state method also forms BBN phase at 1000 8C [6] after prolonged heating (72 h) with comparatively larger particle size of ~1 Am. The particle size and morphology of the calcined powders were examined by transmission electron microscopy. Particle morphology of calcined powder (600 8C for 6 h) prepared by citrate process was irregular in shape, with an average primary particle size around 100nm (Fig. 2). The particle size calculated from Scherrer’s formula (t=Kk/Bcosh B), where t is the average size of the particles, assuming particles are spherical, K=0.9, k is the wavelength of X-ray radiation, B is the full width at half maximum of the diffracted peak, and h B is the angle of diffraction), is 200 nm. The average particle size of BBN powders prepared by conventional ceramic method was in the range of 1–2 Am (not shown). The density of all the sintered samples (both ceramic and citrate process derived) are above 93% of the single crystal values. Fig. 3 shows the curve between polarization ( P) and applied electric field (E). The ferroelectric hysteresis loop parameters measurements of the pellet sintered at 1050 8C showed the values of remnant polarization P r=1.6 AC/cm2 and coercive field, E C=9.5 kV/cm at an applied voltage of

4. Conclusions A simple citrate gel method was used to prepare ultrafine particles of BBN. The BBN phase was found to be formed at 600 8C with average particle size of 100 nm. The dielectric and ferroelectric properties of these samples were also reported.

Acknowledgements One of the authors (V.R.) acknowledges DST, Govt. of India (grant no. SP/S1/H-19/2000) for financial assistance.

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