`International Journal of Inorganic ChemistryVolume 2012 (2012), Article ID 751676, 8 pageshttp://dx.doi.org/10.1155/2012/751676`
Research Article

1Materials Science Unit, Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
2Materials Science Unit, Chemistry Department, Faculty of Science, Taif University, 888-Alhawyah-Taif, Saudi Arabia
3Ceramic Unit, Chemistry Department, National Research Center, Tahrir Street, Dokki, Egypt

Received 3 November 2011; Revised 18 January 2012; Accepted 24 January 2012

Copyright © 2012 Khaled M. Elsabawy et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The aurivillius phase of Mn-substituted samples with general formula where and 0.6 mole were prepared by solid-state reaction technique and ceramics procedures. The X-ray structural measurement analysis confirmed the formation of single-phase-layered hexagonal structure which is observed in all samples. The thermal stability and phase change of the green powders were studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). SEM revealed that the average grain size increases with increasing Mn content. The infrared absorption spectra recorded a series of vibrational modes within the range of 400–1600 cm−1 were investigated. The present work also studied the effect of Mn-doping concentration interactions on both DC-electrical conductivity and ESR spectra.

#### 1. Introduction

Ferroelectic aurivillius ceramic has attracted the attention of many investigators due to its potential applications in electronics devices (DRAMs) [13]. Recently, there is interest in the study of bismuth-layered-structured ferroelectric materials for memory applications; one of the bismuth-layered-structured compounds is a promising candidate for ferroelectric random access memories (FRAM) as it has very little fatigue under polarization switching [4]. The study of microstructure of the SBN thin film is very important [5, 6]. The antiphase boundaries [APBs] are important factors of Bi-layered perovskite properties [79]. The layered crystal structure compounds have an anisotropic lamellar morphology, in which the major faces of the lamellar are perpendicular to the c-axis of the structure.

In order to use moderate sintering temperatures so as to prevent compositional changes and exaggerated grain growth, and to attain low porosity, the ceramics of these compositions must be prepared by hot pressing technique [1013].

Several bismuth-layered perovskites such as strontium bismuth niobate [SBN] [12] and strontium bismuth tantalate [SBT] [14] have been shown to exhibit much elongated fatigue durability and are capable of withstanding 1012 erase and rewrite operations. It is found that the substitution of niobium with vanadium in SrBi2Nb2O9 leads to enhancement of ferroelectric properties together with a lowered processing temperature [15, 16].

It has recently been reported that there occurs (BiFeO3) doped Sr Bi Nb [17]. Most of the work of the layered perovskite Sr Bi oxides reported on the improvement of the dielectric and ferroelectric properties is based on A-site substitution [18, 19]. For example, the replacement of Sr2+ ions by a smaller cations Ca2+ results in an increase in its dielectric content and Curies temperature [20].

Coondoo et al. reported the effect of tungsten substitution for tantalum on the structural, dielectric and impedance properties, of SrBi2Ta2O9 ferroelectric ceramics [21]. It was difficult to accommodate the Mn3+ ion with large ionic radius to the lattice because the ionic radius of the La3+ ion was smaller than that of the Sr2+ ions [22, 23].

It is well known that the addition of 3d transition metals for example, Mn, Fe, Cr, and Cu improves the dielectric properties of bismuth strontium titanate and that Mn is the most effective among them; Mn ions are believed to substitute Ti and act as acceptors [2426]. Recently, it was found by Liu and Fan that Ca and Mn codoping affects the structure and dielectric properties of sol-gel derived BST ceramics [27].

Recently many researchers [2831] have investigated conductivity, dielectric, structure, substitutions effects, and magnetic properties of different molecular formulas of aurivilius phases such as Bi2.5Gd1.5Ti3O12, PbBi4Ti4O15, BaBi3.8M0.2(Ti3.8Nb0.2)O15 where (M = Ba, Sr, Mg and Ba, Mn) and most of them confirmed that substitutions on the different sites of aurivillius phases affect sharply and remarkably on both of structural and physical properties.

The essential goal of the present paper is to investigate wide range of Mn-dopings on vanadium sites of 212 Bi-Sr-V-O regime on;(a)Structural and microstructural properties,(b)Thermal and processing temperature,(c)Spectroscopic properties (IR, ESR),(d)Conduction behaviour of 212-Bi-Sr-V-O system.

#### 2. Experimental

The pure Bi2SrV2O9 and doped samples with the general formula Bi2SrV2−x MnxO9, where , 0.1, 0.2, 0.3, 0.6 mole and were prepared by conventional solid state reaction route and sintering procedure using (physical method) the appropriate amounts of Bi2(CO3)3, SrCO3, (NH4)2VO3 and MnO (each purity >99%). The particles size of MnO used as dopant ranged between 60–90 nm. The mixture was ground in an agate mortar for one hour. Then the finely ground powder was subjected to firing at 800°C for 10 hours, reground and finally pressed into pellets (8 Ton/cm2) with thickness 0.2 cm, diameter 1.2 cm, and Sintered at 850°C for 10 hours. Then the furnace is cooled slowly down by rate −5°/min, to room temperature. Finally, the materials are kept in vacuum desiccator over silica gel dryer.

##### 2.1. Structural Measurements
###### 2.1.1. X-Ray Diffraction (XRD)

The X-ray diffraction measurements (XRD) were carried out at room temperature on the fine ground Bi2SrV2O9 and Bi2SrV2−x MnxO9 systems in the range (2 = 10–70°) using Cu-Kα radiation source and a computerized [Bruker Axs-D8 advance] X-ray diffractometer with two-theta scan technique.

###### 2.1.2. Scanning Electron Microscopy

Scanning electron microscope (SEM) measurements were carried out using small pieces of prepared samples on different sectors to be the actual molar ratios by using “TXA-840, JEOL-Japan” attached to XL30 apparatus with EDX unit, accelerant voltage 30 kv, magnification 10x up to 500.000x and resolution 3 nm. The samples were coated with gold.

##### 2.2. Conductivity Measurements

The DC-electrical conductivity of the samples was measured using the two terminals DC method. The pellets were inserted between spring loaded copper electrodes, and A KEITHLEY 175 multimeter (ASA) was employed from room temperature up to 500 K. The temperature was measured by a calibrated chromel-alumel thermocouple placed firmly at the sample. Energy gab () and the number of (e) in conduction band increase as the ratio of Mn doping increases from to mole.The calculations of and were estimated according to (1) and (2) as shown below, where in (1) and are conductance and specific conductance, respectively. is Boltezmann constant and is absolute temperature in Kelvin. Measurements were conducted in such a way that at each temperature, sufficient time was allowed to attain thermal equilibration.

##### 2.3. Thermal Analyses Measurements

The thermogravimetric analysis (TGA) and the differential thermal analysis (DTA) measurements were carried out on the green mixtures (starting powders) of the prepared samples using a computerized Shimadzu c Japan TGA/DTA analyzer and Al2O3 as a reference for DTA measurements.

##### 2.4. Solid Infrared Absorption Spectral Measurements

The IR absorption spectra of the prepared samples were recorded using “Nexus 670 FT IR spectrometer in the range 500–2500 cm−1 using pure KBr matrix.”

##### 2.5. Electron Paramagnetic Resonance Measurements

The electron spin resonance spectra (ESR) were recorded at room temperature for the prepared samples at -band frequencies on a “Bruker- ELEXSYS E 500 Germany” spectrometer at the National Research Center, Egypt.

#### 3. Results and Discussion

##### 3.1. Phase Identification
###### 3.1.1. X-Ray Diffraction

Figure 1(a)–(f) displays the X-ray diffraction patterns of pure and Mn-doped samples having the general formula Bi2SrV2−x MnxO9, where , 0.1, 0.2, 0.3, 0.6 mole.

Figure 1: XRD patterns recorded for (a) pure Bi2SrV2O9 and Mn-doped samples, (b) Bi2SrV1.95Mn0.05O9, (c) Bi2SrV1.9Mn0.1O9, (d) Bi2SrV1.8Mn0.2O9, (e) Bi2SrV1.7Mn0.3O9, (f) Bi2SrV1.4Mn0.6O9.

Analysis of the corresponding 2 values and the interplanar spacings d (Å) by using computerized program proved that the compound mainly belongs to distorted aurivillius structure type with hexagonal crystal form, that expressed by assigned peaks. The unit cell dimensions were calculated using parameter of the most intense X-ray reflection peaks and is found to be a = b = 5.7804 Å and c = 7.104 Å for the pure 212 Bi-Sr-V-O regime.

The refinement of X-ray diffraction pattern indicated that the fraction volume of pure phase is reaching to ~93% while ~88% for Mn-substituted aurivillius ceramic.

The substitution of Mn for V5+ would induce B-site cation vacancies in the aurivillius layer structure which leads to an increasing of internal stress for the shrinkage of unit cell volume. It is observed that the single phase layered auivillius structure is obtained in the range since the intensity of the peaks increases as the Mn doping increases. The lattice parameter c shows an increasing as the -values increase, due to the stress inside the lattice which leads to increase the shrinkage of lattice (Figure 2).

Figure 2: The variation of c-axis versus Mn-content.

From Figure 1(a)–(f) one can observe that the substitution of Mn are successful in all ranges up to 0.6 mole and there is no evidence for impurities in the diffractogram so, the Mn-dopant can substitute in the V-sites successfully in all ranges [18, 32]. Only remarkable observation is decreasing the intensities of maximum intense assigned by black circles as Mn-doping increases that reflects the decreasing of major phase as Mn-dopings ratio increase.

###### 3.1.2. Microstructural Properties (SEM)

Figures 3(a), 3(c) and 3(e) displays the SEM-micrographs captured for the synthesized materials. The grain size of pure 212BiSrV is found to be 1.5 μm. The presence of bismuth leads to attraction between the grains with each other and porous structure appeared between the grains due to bismuth evaporation (Figure 3(a), 3(c) and 3(e)).

Figure 3: SE-micrograph images recorded for pure and some selected Mn-doped 212-Bi-Sr-V-O system with two different magnification factors 2000x μm & 5000x μm where (a) Bi2SrV2O9, (c) Bi2SrV1.9Mn0.1O9 and (e) Bi2SrV1.7Mn0.3O9.

The grain size increased drastically with increase of Mn addition from 0.1 to 0.3 moles and found to be in between 2.49–2.6 μm, respectively. The ionic radius of Mn2+ is 67 pm which is close to the ionic radius of V5+ 58 pm, Mn will replace V at the B-site of the perovskite ABO3 structure and bring the distorted perovskite unit cell, which promotes the grain growth as observed in Figures 3(a), 3(c) and 3(e). The doping of Mn ions have the tendency to rearrange and aggregate within limited space, leading to an increase in the size of particles and distortion of crystal [28].

###### 3.1.3. (TGA and DTA) Thermal Analyses Measurements

The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements were carried out on the green mixture of pure 212BiSrV and some selected Mn-doped samples with general formula Bi2SrV2−x MnxO9, where and 0.3 mole.

Owing to the two curves TGA and DTA Figures 4(a) and 4(b) for samples a, c, and e, respectively, the TGA analysis was constituent with four stages, a gradual mass loss from room temperature to 250°C can be assigned to the evaporation and elimination of the bonded water and decomposition of ammonium vanadate to NH3 and vanadium oxide. The mass loss in the temperature range from 250°C to 450°C was mainly caused by the decomposition of Bi2(CO3)3 into Bi2O3 and CO2. The third loss region from 450°C–700°C is due to partial decomposition of SrCO3 incorporated with the initial phase formation reaction. The further mass loss beyond 700°C was owing to the formation of solid state oxide and the release of CO2 results from the final decomposition of SrCO3. Moreover, it could be found in the (DTA) curves that there existed different endothermic and exothermic peaks which clearly exhibited the formation of solid state oxides with an increase in an annealing temperature [33].

Figure 4: Thermogravimetric (TGA) and differential thermal analyses (DTA) curves recorded for green mixture of pure and some selected Mn-doped 212-Bi-Sr-V-O system where (a): Bi2SrV2O9, (c): Bi2SrV1.9Mn0.1O9 and (e): Bi2SrV1.7Mn0.3O9.
##### 3.2. Electron Paramagnetic Resonance Measurements

Figure 5(a), (c) and (e) shows the ESR spectra for pure 212BiSrVO and different Mn concentration with the general formula Bi2SrV2−x MnxO9, where and 0.3 mole. It is clearly seen that the effective -values () exhibit an increase from mole to mole due to the strong coupling between Mn2+ and V5+ ions successfully at low dopant concentration (Figure 6). The anisotropy occurred as a result of dopant cation and/or lattice defects could be the reason why effect varies as a function of dopant cation [34, 35]. Furthermore it is well known that Mn+m under thermal treatment program is possible to be oxidized into multi oxidation states such as Mn+2, Mn+3, Mn+4,…, Mn+7 so the values of and are many due to different oxidation state of Mn in the samples, and generally averaged values are increased as Mn-dopant concentration increased from to mole.

Figure 5: ESR spectra at room temperature for pure and some selected Mn-doped 212-Bi-Sr-V-O system where (a) Bi2SrV2O9, (c) Bi2SrV1.9Mn0.1O9 and (e) Bi2SrV1.7Mn0.3O9.
Figure 6: The relationship between and -values of Mn-content.
##### 3.3. DC-Electrical Conductivity Measurements

Figures 7(a)7(f) shows the relation between DC-electrical conductivity and reciprocal of absolute temperature (1000/T) K−1 as a function of Mn doping with the general formula Bi2SrV2−x MnxO9, where , 0.1, 0.2, 0.3, 0.6 mole.

Figure 7: (a) The variation of DC-electrical conductivity as a function of temperature for pure 212-Bi-Sr-V-O system. (b) The variation of DC-electrical conductivity as a function of temperature for (b) Bi2SrV1.95Mn0.05O9. (c) The variation of DC-electrical conductivity as a function of temperature for (c) Bi2SrV1.9Mn0.1O9. (d) The variation of DC-electrical conductivity as a function of temperature for (d) Bi2SrV1.8Mn0.2O9. (e) The variation of DC-electrical conductivity as a function of temperature for (e) Bi2SrV1.7Mn0.3O9. (f) The variation of DC-electrical conductivity as a function of temperature for (f) Bi2SrV1.6Mn0.4O9.

Figure 7(a) displays two behaviors, the first is conducting (metallic behavior) since the conductivity decrease as the temperature rise and the second is semiconductor behavior in which the conductivity increases as the temperature rise. The samples with different Mn doping have the same behaviors as shown in Figures 7(b)7(f). Also we found that the energy gab () and the number of (e) in conduction band increases as the ratio of Mn doping increase from to mole (Figures 8 and 9) [36], (See (1) and (2)).

Figure 8: The variation of versus Mn-content (-values).
Figure 9: The variation of and Mn-content (-values).

In all cases Figures 7(a)7(f) the results display semiconduction mechanism in the low temperature region and metallic conduction mechanism in the high-temperature region.

Figure 8 displays the variation of energy gap as a function of Mn-dopant ratios. It is clear that the sample with mole is recording the minimum energy gap while the maximum energy gap was recorded for sample with mole. These observations are enhanced by data recorded in Figure 9 as clear maximum number of electrons in the conduction band was for sample with mole.This means optimum ratio of Mn-dopings is equal to mole which is considered the boundary separates semiconduction and metallic behavior as shown in Figures 7(a)7(f).

Sharma et al. [36] reported the synthesis of aurivillius-type phases incorporating magnetic M4+ cations (M = Mn, Ru, Ir), based on the substitution of M4+ for Ti4+ in Bi2Sr2(Nb,Ta)2TiO12. They confirmed that the key to incorporating these magnetic transition metal cations appears to be the partial substitution of Sr2+ for Bi3+ in the α-PbO-type layer of the aurivillius phase, leading to a concomitant decrease in the M4+ content; that is, the composition of the prepared compounds was .

These compounds only exist over a narrow range of , between an apparent minimum ( mole) Sr2+ content in the α-PbO-type [Bi2O2] layer required for aurivillius phase to form with magnetic M4+ cations and an apparent maximum ( mole) Sr2+ substitution in this [Bi2O2] layer.

Our results are partially consistent with Sharma et al. [36] such that the optimum Mn-ratio was for mole in which maximum number of electrons was in conduction band.

##### 3.4. Solid Infrared Absorption Spectral Measurements

The IR absorption spectra of pure 212BiSrV and Mn doped samples were carried out at room temperature in the IR range of 400–600 cm−1 as shown in Figure 10(a)–(f). The system 212BiSr is mainly belongs to deficient perovskite structure and extra oxygen atom . Oxygen nine makes the structure to be distorted perovskite, so the vibrational modes of IR spectra of perovskite are closely appear.

Figure 10: The room temperature solid infrared absorption spectra recorded for (a) pure Bi2SrV2O9, (b) Bi2SrV1.95Mn0.05O9, (c) Bi2SrV1.9Mn0.1O9, (d) Bi2SrV1.8Mn0.2O9, (e) Bi2SrV1.7Mn0.3O9, (f) Bi2SrV1.6Mn0.4O9.

In the system under investigation, the V site in the ABO3 crystal structure is being modified. Further, it is also well accepted that the displacement of V ions from its center caused ferroelectricity in these materials. Therefore an investigation of the infrared absorption is expected to reveal valuable information about the modification caused in the interatomic forces between V and O ions with the substitutions.

The IR absorption bands in the range of 400–600 cm−1 could be assigned to the stretching and bending modes of vibration of Bi-O, Sr-O, V-O/Mn-O and Bi-O-V, Bi-O-Sr, respectively [37]. The band around 800 cm−1 is reported to be dominated by the motion of oxygen sub-lattice [38].

#### 4. Conclusions

The conclusive remarks inside this paper can summarized in the following points:(1)Mn substitutions succeeded on the Bi2SrV2−x MnxO9 aurivillius structure;(2)optimum concentration was found to be mole;(3)Mn dopings interacted sharply with both of IR- and ESR-spectra;(4)two kind of conduction mechanism were observed for Mn-aurivillus compounds;(5)numbers of electrons evaluated at conduction band are enhanced by Mn dopings.

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