Ferroelectric Properties of Strontium Bismuth Titanate (SrBi4Ti4O15) Synthesized Using Solution Combustion Technique

The ferroelectric properties of layer-structured Strontium Bismuth Titanate (SBT) have been investigated in this study. SBT was prepared using solution combustion technique with glycine as a fuel. Single-phase formation of the layer-structured compound of SBT with orthorhombic structure was achieved after calcinations at 800 °C, and was confirmed by x-ray diffraction studies. Scanning electron micrograph shows that the grains exhibit a plate like morphology and possesses ne particle size. The as prepared sample exhibits ferroelectric properties with remnant polarization of 2Pr = 1.84 μC/cm2 at coercive field 2Ec= 2.61 kV/cm and displays low dielectric loss. Its ferroelectric transition temperature (Tc) is found to be 450 °C.


Introduction
Ferroelectric compounds, both in powder and thin film forms, have attracted considerable attention in recent years because of their potential application in non-volatile memories and as capacitors in dynamic random-access memories [1]. The most popular ferroelectric material used in memory devices is lead zirconate titanate (PZT). However, it (PZT) has drawback of being a possible environmental pollutor both during fabrication and at the stage of waste disposal, because of its toxic lead content. Thus, there is a need to develop lead free material [2] that could work as an alternative. Strontium-based layered perovskite is currently one of the most promising candidates for new generation non-volatile ferroelectric random access memory (NvFRAM) devices. Among several Bismuth layer structure ferroelectrics (BLSF) materials, SrBi 4 Ti 4 O 15 (SBT) has been extensively studied by many researchers for possible applications in piezoelectric devices. Recently, lot of attention has been paid to SBT due to its high Curie temperature, low coercive field, barrier type property, large retentivity and anisotropic physical properties [3]. Various techniques have been used for the preparation of SBT. Some of the well-known methods are: pulsed laser deposition [3], polymeric precursor method [2], solid state reaction [4],ball milling [5], high-energy milling [6,7], soft chemical method [8] and sol-gel [9] synthesis. The major problems associated with these methods are either the process duration or the difficulties in achieving the desired product phase composition. Also, all these techniques require special chemicals and equipments [10].
Among the various wet chemical routes, the solution combustion technique is regarded as one of the most effective and economic methods due to its convenient processing, simple experimental set up, significant timesaving and high purity products [11]. Also, combustion technique requires lower calcinations temperature due to which volatilisation of bismuth can be minimized [12]. This process involves a self-sustained reaction in homogeneous solution of different oxidizers (e.g., metal nitrates) and fuels (e.g., urea, glycine, hydrazides). Depending on the type of the precursors, as well as on conditions used for the process organization, the solution combustion synthesis (SCS) may occur as either volume or layer-by-layer propagating combustion mode. This process not only yields nanosize oxide materials but also allows uniform (homogeneous) grain formation in a single step [13].
Reports on the effect of solution combustion synthesis of SBT on its structure and ferroelectric properties are rare. Therefore, it is proposed to synthesize SBT powders via solution combustion technique using glycine as fuel and to investigate its ferroelectric properties. In this paper, we report a successful preparation of SBT powder through solution combustion with glycine as fuel and acetyl acetone as a chelating agent.

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Results and discussion XRD analysis. The X-ray diffraction pattern of SBT (calcined) sample recorded by using Cu K α radiation is shown in Fig. 1(c). The sharp peaks indicate the crystalline nature of the particles. It is seen that all the peaks match well with the standard JCPDS card No: 43-0973 as shown in Fig. 1(a).
No other secondary phases are seen. As seen from the Fig. 1(b), the combusted powder without calcination possesses amorphous nature, also some secondary phases are seen , which disappear and complete crystallinity is reached only after calcination at 800 ºC for 4 hour.  Fig.2a and 2b respectively. Fig.2(a) shows that the particles do not display any definite shape. A great number of agglomerates can be seen in the powder which is typical of combustion synthesis products [14]. The milling in agate mortar before compacting and sintering was adopted in order to break the agglomerates. Fig 2(b) shows that the grains of SBT pellets exhibit a plate like morphology. It was also found from the SEM micrograph that the grains of different sizes are homogeneously distributed. Similar grain morphology was observed in SBT prepared by other methods. It is known that plate-like grain formation is a typical characteristic of bismuth layer structured ferroelectrics (BLSFs) because they have highly anisotropic crystal structure [15]. The sintered surface shows dense structure. Dielectric Properties. Fig. 3 displays dielectric constant (ε) as a function of temperature of the as prepared sample measured in a frequency range 1 kHz to 1 MHz. As expected, a maximum of dielectric constant (ε) related to the ferroelectric-paraelectric transition is observed at the Curie Temperature T c = 450 ºC. The ferroelectric transition temperature, which determines the working temperature range of the as prepared sample , is well within the reported results that is around 500 °C [4,15]. Apart from this peak there is a slight hump seen at around 200 °C. This has been reported by earlier researchers as well and is attributed to phenomena such as structural distortions [16], space charge relaxation [17], and oxygen vacancy movement [18]. The temperature corresponding to this hump in the low temperature range is called as 'depolarization temperature' (T d ), which indicates the stability of ferroelectric domains [19]. The depolarization temperature also corresponds to the ferroelectric to antiferroelectric transition as the specimen is depolarized and loses its piezoelectric activity over this temperature [20]. Further, the transition temperature is found to be almost the same for different frequencies, which indicates that SBT belongs to the category of normal ferroelectric but not a relaxor ferroelectric [8].  Where e′ represents the electron and V o ″ represents the oxygen vacancy. So the conductivity of the samples can therefore be associated with the mobility of oxygen vacancies [12].

Conclusions
SBT powder can be successfully prepared by the solution combustion technique with glycine as fuel in a short duration. Complete phase formation and crystallinity was attained only after calcinations at 800 °C. Low remnant polarization of the sample was attributed to high conductivity due to oxygen vacancy movement. Variation of dielectric loss was also studied as a function of temperature.