Journal of ELECTRONIC MATERIALS, Vol. 47, No. 7, 2018 https://doi.org/10.1007/s11664-018-6262-1 2018 The Author(s) Co-Precipitation Synthesis and Characterization of SrBi Ta O 2 2 9 Ceramic 1,2,3 2 2 MOHAMED AFQIR , AMINA TACHAFINE, DIDIER FASQUELLE, 1 2 MOHAMED ELAATMANI, JEAN-CLAUDE CARRU, 1 1 ABDELOUAHAD ZEGZOUTI, and MOHAMED DAOUD ´ ´ 1.—Laboratoire des Sciences des Materiaux Inorganiques et leurs Applications, Faculte des ´ ´ Sciences Semlalia, Universite Cadi Ayyad, Marrakech, Morocco. 2.—Unite de Dynamique et ´ ´ ´ ˆ Structure des Materiaux Moleculaires, Universite du Littoral- Cote d’Opale, Calais, France. 3.—e-mail: firstname.lastname@example.org Strontium bismuth tantalate (SrBi Ta O ) was synthesized by a co-precipi- 2 2 9 tation method. The sample was characterized by x-ray powder diffraction patterns (XRD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results of the dielectric properties are re- ported at room temperature. No secondary phases were found while heating the powder at 850C and the pure SrBi Ta O phase was formed, as revealed 2 2 9 by XRD. The characteristic bands for SrBi Ta O were observed by FTIR at 2 2 9 1 1 approximately 619 cm and 810 cm . SEM micrographs for the sample displayed thin plate-like grains. The grain size was less than 1 lm and the crystallite size of about 24 nm. Dielectric response at room temperature shows that the SrBi Ta O ceramic has low loss values, and the ﬂattening of the 2 2 9 dielectric constant at higher frequencies. The observed Curie temperature is comparable with those reported in the literature. Key words: SrBi Ta O , co-precipitation, dielectric 2 2 9 INTRODUCTION the synthesis of this compound. Other methods used for the synthesis of SrBi Ta O ceramic pow- 2 2 9 Aurivillius phases have the general formula 5 6 der are: the sol–gel method, chemical routes, 2+ 2 (Bi O ) (A B O ) , where A stands for a 2 2 n1 n 3n+1 7 solution combustion technique, and hydrothermal 2+ 2+ divalent cation (e.g., Ca ,Ba ), B for a tetravalent process. A study of many ceramics of the same 5+ 4+ or pentavalent (e.g., Nb ,Ti ), and are the per- chemical compound, but synthesized under differ- ovskite blocks A B O . Strontium bismuth n1 n 3n+1 ent conditions, might reveal trends in the crystallite tantalate (SBT), SrBi Ta O , belongs to the layered 2 2 9 size, which in turn can be related to the properties perovskite ferroelectrics where the crystal consists of the product. For random access memory applica- of the stacking of alternating layers of Bi O and 2 2 tions, it is necessary to obtain the SrBi Ta O 2 2 9 pseudo-perovskite SrTa O units with double TaO 2 7 6 materials with low dielectric loss, low conductivity octahedral layers along the c-axis. The layered and high fatigue properties. To prevent the perovskite-like ferroelectric of SBT is attractive for volatilization of Bi O during sintering above 2 3 the development of non-volatile random access 1000C, which could result in oxygen and bismuth memories because of its excellent fatigue character- vacancies, it is therefore desirable to dope SrBi 3 2 istic. SBT is usually obtained through the mixture 10,11 Ta O by rare earth elements. Meanwhile, an 2 9 of oxides, Bi O ,Ta O and SrCO , which gives 2 3 2 5 3 accurate amount of Bi O oxide must be added to 2 3 large particles and requires high temperatures for compensate experimentally for the loss characteris- tics of the Bi O oxide. To our knowledge, it does not 2 3 resolve many practical physical issues such as the microstructure–property relationship and the phys- (Received October 15, 2017; accepted March 23, 2018; ical nature leading to the change of the dielectric published online April 3, 2018) 3398 Co-Precipitation Synthesis and Characterization of SrBi Ta O Ceramic 3399 2 2 9 properties of SrBi Ta O ceramics. In this regard, vacuum. The dielectric properties were investigated 2 2 9 we report a co-precipitation method to obtain at a range from room temperature to 400C using a SrBi Ta O with sub-micron grain size. X-ray pow- LCR meter HP 4284A. 2 2 9 der diffraction patterns (XRD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron RESULTS AND DISCUSSION microscopy (SEM) are used to characterize the Figure 2a shows the XRD pattern of SrBi Ta O 2 2 9 synthesized material. Dielectric measurements ceramic powder. All Bragg peaks of this sample were performed. were found to correspond to the layered perovskite SrBi Ta O crystal structure (JCPDS no 01-070- 2 2 9 EXPERIMENTAL 4062). No pyrochlore phase was found while heating A ﬂowchart of the co-precipitation procedure used the powder at 800C. The reﬁned lattice parameters ˚ ˚ ˚ is shown in Fig. 1. A stoichiometric amount of are a = 5.51752 A ± 0.00085 A, b = 5.52500 A ± ˚ ˚ ˚ SrCl Æ6H O (Normatom) was dissolved in distilled 0.00023 A and c = 25.08384 A ± 0.00203 A. These 2 2 water, Bi(NO) Æ5H O (Fluka Chemika, 99.0%) was parameters are comparable with those reported 3 2 ˚ ˚ ˚ dissolved in a minimum amount of dilute HNO (a = 5.5146 A, b = 5.5246 A and c = 24.9017 A)in (Panreac, 65%) to avoid precipitation of Bi ions, and the literature. Ta O (Fluka Chemika, 99.9%) was dissolved in a Crystallite size and micro-strain were measured 2 5 13,14 minimum amount of HF (Riedel-de Hae ¨ n, 40%). The according to the method of Williamson and Hall three limpid solutions were mixed together, fol- (Fig. 2b). lowed by the addition of KOH (Acros Organics) 0:89 solution (10 M) until pH = 12 to ensure complete b cos h ¼ k þ 4e sin h ð1Þ precipitation. The precipitate was ﬁltrated by wash- ing several times with distilled water. After drying where b is the integral breadth, D is the crystallite in the oven, the precursor was heated in a furnace size, (e) the micro-strain and (h) the diffracted angle. at 400C for 12 h, and then at a rate of 5C/min to The reﬂections with a low intensity of the peaks 800C and kept at this temperature for 24 h. The were excluded from the analysis. The slope corre- pellet was sintered at 850C for 12 h. Both sides of sponds to the strain and the intercept corresponds the sintered sample were painted with silver paste to the crystallite size. The average crystallite size and ﬁred to form the electrodes. was 24 nm and micro-strain was about X’Pert HighScore Plus software (PANalytical) 7.26 9 10 . The crystallite size/strain deduced was used to estimate the degree of crystallization. from the bcosh versus sinh described the formation The dried sample was ground with KBr to form a of nanoparticles. mixture containing 1% mass of the sample, and a Figure 3 shows the FTIR spectrum of the SrBi small pellet of the mixture was pressed and scanned Ta O ceramic powder. The bands at 3452 cm and 2 9 by FTIR (KBr-pellet; Bruker Vertex 70 DTGS). The 1630 cm are due to water molecules. The bands microstructure of the sintered pellet was analyzed located at 2387 cm can be attributed to CO . 1 1 1 by SEM with 10 kV of accelerating voltage in a high Bands at 1386 cm , 2853 cm , and 2929 cm Fig. 1. Flowchart for the preparation of SrBi Ta O powder by co-precipitation. 2 2 9 3400 Afqir, Tachaﬁne, Fasquelle, Elaatmani, Carru, Zegzouti, and Daoud Fig. 3. FTIR spectrum of SrBi Ta O ceramic powder. 2 2 9 Fig. 2. (a) XRD pattern and (b) corresponding Williamson–Hall plot for SrBi Ta O ceramic powder. 2 2 9 indicate the presence of residual organics, and 1 1 619 cm and 810 cm are expected for octahedral stretching TO . These two infrared absorption bands are related to the crystallization of the phase 12,14 SrBi Ta O . Also, note there is no presence of 2 2 9 residual precursors or unwanted secondary phases at a temperature of 800C. Figure 4 shows the cross-sectional microstructure investigated by SEM of SrBi Ta O ceramic. The 2 2 9 sintered pellet exhibits less porosity, relatively dense structure, small rod-like and plate-like shaped grains. The plate-lake morphology has been consid- ered to be the characteristic grain growth of Auriv- 15–17 illius microstructures. The obtained samples are micro- to nanograined and therefore contain well developed grain boundaries and free surfaces. It has recently been demonstrated that the physical prop- erties of pure and doped ﬁne-grained oxides strongly depend on the presence of defects like interphase 7,14 Fig. 4. (a, b) SEM images of SrBi Ta O ceramic. 2 2 9 boundaries and grain boundaries. Co-Precipitation Synthesis and Characterization of SrBi Ta O Ceramic 3401 2 2 9 Figure 5 shows the temperature-dependent frequencies and the ﬂattening of dielectric constant, dielectric constant (e¢) and dielectric loss (tand) for which are very desirable in nonvolatile ferroelectric SrBi Ta O ceramic at room temperature. Both the random access memory applications. The plot of the 2 2 9 e¢ and tand values decrease with increasing fre- temperature-dependent dielectric constant (e¢) and quency. The dielectric constant decreases signiﬁ- dielectric loss (tand) at different frequencies for cantly with frequency and ranges from 287 to 94 in SrBi Ta O ceramic is shown in Fig. 6. A Curie 2 2 9 the frequency range of 100 Hz–10 kHz. In general, temperature is observed at 330C, which is in good for all materials, the dielectric constant continu- agreement with the reported value 17,21 ously decreases with frequency, due to the space (310C ± 20C). It may also be noted that the charge polarization and interface effect. Another values of dielectric loss do not exceed the value of explanation suggests that these observations may 0.5 over the temperature range of 50–400C. be due to the fact that the dipoles cannot follow the As noted in the ‘‘Introduction’’, the SrBi Ta O 2 2 9 15,18 rapid variation of the applied ﬁeld. For the ceramics suffer from high dielectric loss due to the analysis of frequencies below 10 kHz down to volatilization of bismuth during the sintering pro- 1 MHz, e¢ remains constant around a value of cess at temperatures greater than 1000C. The 86. The dielectric loss is found to be less than losses are attributed to increased conduction aris- 0.005 when measured at 100 kHz, which is lower ing from lattice defects induced through volatiliza- 22, than those reported from SrBi Ta O ceramics tion of the bismuth oxide. With the synthesis 2 2 9 19,20 prepared by a solid-state method. The frequency conditions described above, the grain sizes are dependence of the dielectric properties at room found to be extremely small, and may give rise to temperature shows low loss values at higher resistive and capacitive grain boundaries. As Fig. 5. (a) Frequency dependence of dielectric constant and (b) Fig. 6. (a) Temperature-dependent dielectric constant and (b) dielectric loss at room temperature for SrBi Ta O ceramic. dielectric loss at different frequencies for SrBi Ta O ceramic. 2 2 9 2 2 9 3402 Afqir, Tachaﬁne, Fasquelle, Elaatmani, Carru, Zegzouti, and Daoud 23,24 improved elsewhere, the dielectric loss appears REFERENCES to be a strong function of grain size. Thus, the 1. E.P. Kharitonova and V.I. Voronkova, Inorg. Mater. 43, decrease in loss can be understood from the fact 1340 (2007). that the lower conductivity of the ﬁne-grained 2. V.A. Isupov, Inorg. Mater. 42, 1094 (2006). 3. Q.-H. Li, M. Takahashi, T. Horiuchi, S. Wang, and S. Sa- ceramics is due to the higher density of resistive kai, Semicond. Sci. Technol. 23, 45011 (2008). grain boundaries. It can be concluded that our 4. B. Li, L. Li, and X. Wang, Ceram. Int. 29, 351 (2003). ceramic processing conditions may reduce lattice 5. W. Wang, H. Ke, J. Rao, M. Feng, and Y. Zhou, J. 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