Structural, Dielectric, and Magnetic Properties of Ba2Bi9-xFe5+xTi8O39 Tetragonal Tungsten Bronze Ceramics

The formulations of Ba2Bi9-xFe5+xTi8O39 (0 ≤ x ≤ 9) (BBFTO) tetragonal tungsten bronze structures were synthesized by the conventional solid-state method. The structural, dielectric, and magnetic properties of these ceramic compounds were investigated. Optimized composition (at x = 9) and optimized sintering temperature (Ts = 875 °C) of BBFTO were set where the highest magnetic behaviour achieved. X-ray diffraction (XRD) pattern of x = 9 reveals the crystal structure to be orthorhombic. The magnetic properties of x = 8 and x = 9 samples were analyzed with respect to the temperature (from −150 °C to 650 °C) during the applied magnetic field. The lower relative permittivity and lower ferroelectric properties were also reported. These type of TTB multiferroic ceramics can open new directions of application in the future.


Introduction
The tetragonal tungsten bronze (TTB), based ceramics are of great interest in today's research and technology. This class is regarded as the ferroelectric class of functional ceramics [1][2][3][4]. Tetragonal tungsten bronze (TTB) is a well-known family of non-stoichiometric compounds having general formulae, A 6 [3,5,6]. These crystal structures have three non-equivalent crystallographic sites for doping such as A-site, B-site, and C-Site is an additional interface (mostly vacant) [6,7]. In contrast to perovskites, these doping sites exercise the formation of a great variety of versatile compositions with profound physical properties and have the flexibility of multi-doping [8]. These sites can accommodate cations of various valences; the A-site interstices can accommodate divalent or trivalent cations while tetravalent or pentavalent cations are for the B-sites, while the C-sites are typically left empty for smaller cations [9]. Nonetheless, the C-site can also be filled; as some researchers have reported the Li + could be the participant of C-site [10].
L.G.Van Uitert et al [5] published a report on 'filled' tetragonal tungsten bronze-like structures but the selected composition was based on A 5 B 5 O 15 [5]. The work on TTB compounds is not much decoded; although this class of functional materials is still a hot topic of innovation . These compounds were first reported by V. A. Isupov et al [35]. They reported this class in the theme of ferroelectrics but did not report further structural and functional properties.
In this study authors decided to initiate an investigation to formulate new compounds of TTB-like class. A novel approach was adopted to optimize the best stoichiometric composition by increasing the Fe 3 + cations and decreasing the Bi 3 + volatile ions [36,37]. Further to this, the research also intends to determine how the selected compositions are affected in terms of properties by the different sintering temperatures. And finally, to analyze the reported compositions (Ba 2 Bi 9-x Fe 5+x Ti 8  Experimental procedure: TTB compounds having a formula Ba 2 Bi 9-x Fe 5+x Ti 8 O 39 (BBFTO), (where 0 x 9) were processed by the conventional solid-state method. This process is carried out using BaCO 3 (Riedel-de-haen 99%), Fe 2 O 3 (Sigma-Aldrich, 99%), TiO 2 (BDH, 99.5%), and Bi 2 O 3 (Jinangyin Advance Co., Ltd, 99.85%) as the raw materials. The mixed powders milled in ethanol, dried, and then calcined at 800°C. Calcined powder was remilled and pellets were pressed before sintered at 825°C to 1050°C. XRD patterns of calcined powders and sintered pellets was obtained using the Panalytical (X-Pert ProMPD) diffractometer (2θ range of 10°-80°) with a    scanning speed of 0.02°/min. and step size 0.06°, where the X-Ray source was CuKα radiation (λ = 1.5418 Å). The microstructural evaluation was analyzed through JEOL (15 kV, 10 mA). The Capacitance and tanδ values were measured using an LCR meter (Model Th2826, Tonghul company China) attached with a mini tube furnace (JC-H-220-2,220 V,); with respect to the temperature (from 25°C to 600°C) and frequency (100 kHz to 5 MHz) at 100 mV. All capacitance readings were converted to relative permittivity (ε r ) according to the following relationship.
The P-E loops were measured using a Ferroelectric testing machine (POLYK), at max. applied electric field (10kV cm −1 ), charges (<1 nC to >1 mC), and frequency (0.01-200 Hz). Magnetic saturation (M s ) and remanent magnetization (M r ) values of all reported samples were investigated. For getting magnetic properties, the ultemcup filled with calcined powder samples, and in the case of a solid sample, the small piece (less than 0.20 grams) of pellet was crushed. Nonetheless, the quartz transverse lolly-pop sample holder was used to hold the powderfilled ultem cup and solid piece samples; with the help of sticky wax (for room temperature) and ceramic paste for variable temperatures (i.e. −150°C to 650°C, only optimized samples of S8 and S9 compositions were tested in this range of temperature). For low-temperature measurements, the liquid N 2 with the flow of dry N 2 and for high temperature (up to 650°C) measurements, the Argon gas; was used during this test.

Results and discussions:
Sintering colour and bulk density: The sintered sample of S0, has a mustard colour, this colour gradually converts to brown with increasing Fe 3 + content, when sintered at 875°C. All samples were turned black except S9 which remained dark brown chocolate in colour when sintered at 1050°C. Here the optimized sample is S9 as per the highest magnetic properties. With the help of crystal maker software 10.8, theoretical densities were determined using ICCD card number 04-022-1782. The theoretical and bulk densities are represented in figure 1 and are also reported in table 1. Figure 2 shows XRD patterns of all BBFTO ceramics pellets sintered at 875°C. All XRD patterns have a common highest peak at 44.42°to 44.72°. The XRD patterns (figure 2(a)) shows all BBFTO samples sintered at 875°C, (figure 2(b)) sample S9 (Ba 2 Fe 14 Ti 8 O 39 ) sintered at different sintering temperature from 825°C to 1050°C, and (figure 2(c)) all BBFTO samples sintered at 1050°C.

Structural and Microstructural characterization:
The XRD pattern of S9 exhibits mixed symmetry of cubic and hexagonal at 800°C due to incomplete reaction at low calcination temperature. The presence of cubic-phase is about 46.6% and another phase is 53.6% [38]. However, there is a single phase existence in S9 samples sintered at 875°C and 1050°C, as shown in table 2. The orthorhombic phase is strongly indexed with the diffraction peaks of 875°C, and subsequent hightemperature sintering converts the orthorhombic phase into a monoclinic structure. This is due to the active diffusion mechanism that results in a single phase at high-temperature sintering; which has a significant impact on the transition of crystal structure.  The hexagonal and orthorhombic phase has limited temperature stability and is metastable between monoclinic and tetragonal symmetries. The metastable condition causes the transition to orthorhombic, then to monoclinic [39][40][41]. The presence of orthorhombic symmetry has good agreement with vibrating sample magnetometer (VSM) data of S9 (sintered at 875°C) due to its high magnetic properties obtained [42].
The samples of optimized composition (S8 & S9) were analyzed by scanning electron microscope, as shown in figure 3, to observe the morphology of samples sintered at 875°C and 1050 o C. The samples sintered at 875 o C,  reveal the solid-state sintering mechanism, and low inter-diffusion kinetics may be encouraged by sintering conditions. Irregularly sized grains appear to be distributed uniformly throughout the samples and contain porosity, while high-temperature sintering samples favour the liquid-phase-sintering mechanism and may be curtailed the porosity due to earlier softening of low melting phase, thus gets a better-sintered surface.

Dielectric and Ferroelectric properties:
As per scrutiny of figure 4, the tangent losses seem to be constant and start to raise after 400°C for S8 but are random for S9. The values of tanδ and ε r , at room temperature are 0.1521 and 16.59406 for S8, 0.14631 and 10.74202 for S9, and at 600°C, the values are found to be 2.82905 and 97.33882 for S8, 1.17236 and 57.30875 for S9, respectively at 1 MHz. The variation in all dielectric curves is possibly due to the polarization mechanisms; i.e. ionic, orientation, interfacial, and electronic polarization are dielectric polarization which all occurs at different frequencies. Space charge polarization is slowest and occurs around 10 4 Hz. The contribution of space charge polarization decreases the ε r (with respect to the frequency). In addition to this, the ε r decreases especially for high-sintered samples due to the presence of defects, oxygen vacancies, and impurities [43]. The molecules must cross the energy barrier to polarize the materials or to switch polarization if molecules possess less energy than barrier energy, then this time lag will freeze out the orientation modes and it cannot contribute to overall polarization; hence relative permittivity will drop out [44]. At high temperatures, the dielectric losses and constant both increase with increasing temperature due to the dipole polarization mechanism [45]. The S8 and S9 samples have T c across 650°C, and this result is well versed with figures 5 and 7; where the M s (emu/g) drops dramatically at 650°C, due to the formation of long-range frustrated ferromagnetic domains.
The P-E hysteresis loops of BBFTO compounds reflect different behaviours as highlighted in figure 5. Some BBFTO formulations like S1, S4, and S8 sintered at 875°C and S0, S1, S2, and S3 sintered at 1050°C, show antiferroelectric type loops. And all other PE curves (including S9) are leaky and ramify conduction phenomena due to porosities, low densities, and sintering flaws; resulting in elliptical and asymmetrical hysteresis loops [46][47][48]. The B-site cation fosters the ferroelectric distortion and hybridization between the Ti 4+ 3d°states and the oxygen 2p states in Ti-O octahedral, are required for ferroelectricity [49]. BFTO possesses Ti 4+ as a favourable condition for ferroelectric, the fact B-site Fe 3+ creates oxygen vacancies causing oxygen vacancy defect dipoles [49]. For spontaneous polarization, the dipoles align themselves along a preferred direction, and/or diffuse to high-stressed locations of domain walls or grain, resulting in low hysteresis loops and conduction, which readily breaks the samples when electric field is increased [50]. S9 shows conductivities arise due to absence of Bi 3+ ions, which play a critical role to rise ferroelectricity [51]. The displacement of Bi 3+ ions from a central position is conducive to polarization [52].
If Ba 2+ cause the hindrance in the displacement of Bi 3+ , it would be difficult to observe hysteresis loops [53]. The simultaneous occurrence of ferroelectricity and ferromagnetism from the same central atom is challenging. The partially filled d°orbitals are essential for magnetism and on the other hand, empty d°orbitals are requisite for ferocity [54][55][56][57]. In improper ferroelectric class broken spatial symmetries will cause the multiferrocity, and magnetism-driven ferroelectricity, can be obtained by magnetic order that will break the inversion symmetry [58][59][60][61][62]. The empty d shell at B-site transition ion like Ti 4+ does not necessarily provide ferocity [63]. The displacive type multiferroics do not favour the origin of multiferrocity via a single central atom simultaneously [64]. The formation of defect-dipole can be resolved through doping, which may improve ferroelectric. The reduced ferocity for S9 is due to the absence of active 6s lone pair of Bi +3 ion [37].  Figure 6; shows the magnetization-field (M-H) curves for all BBFT0 ceramics samples calcined at 800°C, and sintered at 875°C, and 1050°C were analyzed at 20 o C. The appearance of hysteresis loops at 800°C and paramagnetic curve at 1050°C gives a hint that optimal temperature lies between these ranges. To optimize the magnetic properties via sintering temperature, and to obtain a conducive structure as a function of sintering  temperature, S8 and S9 samples were sintered at different sintering temperatures in the range of 800°C to 1050°C

Magnetic properties
(as shown in figures 6(d) and (e)). S9 sintered at 875°C to 950°C, has the highest magnetic saturation (M s ), which reduces from 975°C to 1050°C. It can be observed that the sharp curve area converts to a diagonal straight line, M s and M r values are reduced due to phase transformations.
The hysteresis loop obtained for all BBFTO compositions sintered at 1050°C shows a paramagnetic behaviour except S9; has a minor loop area due to the co-existence of ferromagnetic and antiferromagnetic bonds. Ferromagnetic interactions are affected during sintering due to small distortions arises due to the difference in ionic radius size and valence state of Fe 2+ and Fe 3+ . The distortion in the structure leads to the misalignment of an antiparallel magnetic moment which fosters weak ferromagnetic loops (figures 5 and 6) [51,65].
At 800°C weak ferromagnetic properties are due to canting of antiferromagnetic interactions, and at 875°C magnetic moment arises due to B-site Fe 3+ and orbital moment with electrons spin contribution atomic magnetic moment. The paramagnetic behaviours arise due to non-interactions of dipoles or weak diploe reinforcement with an applied field at 1050°C. The sintering mechanism and presence of oxygen vacancies may cause hindrance and drive the centrosymmetric symmetries [66][67][68].
The classical magnetic hysteresis loops were obtained for all BBFTO compositions sintered at 875°C (figure 6(c)). A-site and B-site occupations have great significance on magnetic properties. The reduction of Bi 3+ from A-site and the rise of Fe 3+ on B-site have influenced propitious magnetic structure formation. According to the findings, the presence of ferromagnetism in BBFTO ceramics was triggered not only by Fe 3+ , but also by the formation of a preferable structure at 875°C, orthorhombic phase permits high magnetization and ferroelectricity [69,70].
The figure 7, represents the M s and M r parameters for all BBFTO composition and S8 and S9 sintered at different sintering temperatures. All samples have high M s and M r values at 875°C. S8 has 1.824 emu g −1 , and S9 has 4.621 emu g −1 at 875°C. The sudden increase in M s and M r values for S9 confirms that Bi 3+ ion is volatile, creates a vacant position, and does not stabilize the structure thus complete removal of Bi 3+ is beneficial. This drastic increase for S9 is also due to the minimization of Bi-O diffusive phase transition, thus domains favour increased ferromagnetic behavior (figures 6(e) and 7(b)) [70]. These values are reduced at 1050°C (S8 has 0.391688 emu g −1 , S9 has 0.502138 emu g −1 ).
To understand the subzero temperature behaviours, perceive figure 9, which demonstrates that M s values are higher at cariogenic temperature and gradually decrease with higher temperature and drop at 650°C, thus we can conclude that T c lies at 650°C or above 650°C.
The temperature dependence M-H hysteresis loops were measured from negative field cooling (−150 o C) to zero-field cooling then the temperature was increased from ambient to 650 o C as shown in figures 8 and 9. Sample S9 exhibits higher M s and M r at −150°C, 6.11 emu g −1 and 2.733 emu g −1 , respectively and reveals a more suitable structure at −150°C, further obtained M-H curves trend indicates the possibility of the presence higher M s and M r , values below −150°C to certain temperature limit. Thermal energy enhances domain mobility, making it easier to align them, but it also prevents them from keeping aligned when the field is withdrawn. As a result, at high temperatures, saturation magnetization, remainent, and the coercive field, all are lowered. Ferromagnetic behaviour is no longer exhibited when the temperature exceeds the curie temperature (T c ) instead, the material takes on the characteristics of a paramagnetic substance [67,71] as shown in figures 8 and 9. S9 has improved emu/g , i.e. M s and M r , 4.6 emu g −1 , and 1.935 emu g −1 respectively at 875°C, which higher than Ba 4 Sm 2 Fe 2 Nb 8 O 30 [72], doped BiFeO 3 (Sm, Co doped x = 0.12) has M s = 0.04 emu g −1 and M r = 0.02 emu g −1 [73]. In addition, 0.75BaTiO 3 −0.25Ni 0.6 Zn 0.4 Fe 2 O 4 also exhibited the maximum saturation (M s = 1.732 emu g −1 ) and remnant magnetization (M r = 0.025 emu g −1 ) [74]. Figure 10, summarizes the possible reason, how the reduction of Bi 3+ from the A-site and the rise of Fe 3+ on the B-site have affected the functional properties of S9. Ferromagnetic switching is easier, especially at −150°C. Ferroelectric switching also occurs but internal defects hindered the polarization during the applied electric field. Furthermore, it seems that the domain favours the magnetic dipoles. P-E curves are better for certain BBFTO compositions at 1050°C due to improved density and M-H loops are excellent at 875°C for all compositions

Conclusion
The Ba 2 Bi 9−x Fe 5+x Ti 8 O 39 (0 × 9) compositions were successfully synthesized by the conventional solidstate method. Their structural, dielectric and magnetic properties have been systematically studied. The obtained XRD data was investigated and structural parameters obtained. The theoretical densities were calculated and compared with bulk-densities (obtained after optimized sintering). All samples showed antiferroelectric and leaky behaviours; this could be the reason of intrinsic defects in samples. S9 sample (T s = 875°C ) of BBFTO was found to be orthorhombic crystal system; this composition permits max. M s of 4.021 emu g −1 at room temperature and 6.11 emu g −1 at −150°C and lowest 0.172 emu g −1 at 650°C. BBFTO possess ferromagnetic as the dominant property and ferroelectricity as subservient property due to hindered growth of ferroelectric domains. The reduction of Bi 3+ from the A-site and the rise of Fe 3+ on B-site have played the role of magnetic properties. S8 and S9 can open new applications in electroceramics. To drive further multiferroic properties, doping strategies should be considered to achieve polarization in both electrical and magnetic fields.

Data availability statement
As per university project, the data that support the findings of this study are available upon reasonable request from the authors.