Understanding of doping sites and versatile applications of heteroatom modified BaTiO3 ceramic

ABSTRACT Due to its multifunctional properties, BaTiO3(BT) is a crucial ceramic material in the semiconductor market. The development of doping techniques has received considerable attention in recent years as efforts continue to enhance the attractive qualities of BaTiO3 and broaden the range of modern technologies in which it can be used. The synthesis method and doping components must be carefully chosen in order to produce appropriate BaTiO3 particles with high purity and suitable grain size that are critical for the desired end applications of synthesized doped BaTiO3. Doping can be performed in either the A site or B site or in both sites proportionally. This brief review has been compiled to increase our understanding of the doping sites as well as the possible applications based on the previous literature.


Introduction
Perovskite, originally identified as CaTiO 3 by Gustave Rose in 1839, was later renamed after the Russian mineralogist Lev Perovski [1]. It comprises a broad group of crystalline compounds having the general chemical formula ABO 3 . In this formula, "A" stands for monovalent, divalent, and trivalent materials and "B" for tetravalent and pentavalent atoms. BaTiO 3 perovskite (BT) is a member of the perovskite family that has found applications as a commercial piezoelectric and ferroelectric material [2]. The excellent dielectric, ferroelectric, and piezoelectric properties of BT as a multifunctional perovskite ceramic material have led to research in a wide variety of fields, including thermostats, thermistors, dielectric in multilayer ceramic capacitors, sensors, energy storage devices, energy conversion technologies, catalysis, and biomedical applications [3]. This interest in BT among many scientists is owing to its unusual features. Different studies on BT-based materials have been conducted as a result. Perovskite has the potential to accommodate ions of any size due to its structural features. Thus, several studies have been conducted to investigate its properties, and researchers have been able to obtain varying outcomes by adjusting the sample preparation, grain size, sintering temperature, and phase transition, among other variables. These versatile characteristics of BT can be significantly impacted by doping elements [4].
However, BT's inherent qualities are less than ideal for most applications [5]. Maximizing the useful features of BT is therefore essential for achieving desired characteristics for industrial use. Researchers have resorted to doping pure BT ceramic with heteroatoms in order to achieve the desired characteristics. This approach to doping led to a significant decrease in electrical resistance, enhancement of piezoelectric capabilities, enhancement of ferroelectric properties, rise in dielectric permeability, and a capacity of previously non-magnetic BT to acquire magnetic moment. Doping BT also improves its thermal stability and makes it resistant to extremely strong electric fields [6]. The phase diagram for BT also shows a transition to a new phase at very low temperatures. Therefore, BT-doped materials offer an attractive research platform for investigating the underlying mechanisms of enhanced properties. The physical structure of BaTiO 3 crystal is depicted in Figure 1 [4]. Semiconductor materials based on BT are highly desirable for fundamental study since their functional qualities are sensitive to both microstructural and chemical alterations [5].
In this review, tables and figures have been plotted based on previous literature studies to give novice researchers an idea on dopant selection, and also scientists in the relevant chemical industries a brief overview of the ongoing research trends. Knowledge and proper understanding of the doping site are essential for determining the appropriate dopant [7]. Many novice researchers in the ceramics science field lack sufficient understanding of why dopants possess either an A site or a B site in the perovskite structure. Study of a good number of research articles on the doping of BaTiO 3 and its characteristics has enabled this review to summarize the selection of doping sites and the reported possible applications after the doping experiments. The economic potential and applicability of doped BT materials are evaluated in order to assess their technological relevance. As the attributes of BT have been used to establish its usefulness in various contexts, a data table detailing the potential contexts in which BT might be put to use is herein.
Thus, this article can serve as a roadmap for understanding the doping sites of BaTiO 3 . Some applications of the recently reported doped BaTiO 3 have also been presented to suggest new research directions for the near future.

Perspectives on doping
Due to the adaptability of BaTiO 3 earth metal perovskite, researchers frequently use dopants to modify and tune the material's characteristics for desired end applications [6]. BaTiO 3 is doped because its physical and chemical behavior is of interest and is under investigation [8]. Doping is a method whereby an impure atom in a compound's chemical structure is replaced by a pure one. In the world of semiconductors, this is not unusual [9]. Doping is essential in the development of semiconductor materials. As a result of its perovskite structure, BaTiO 3 can be doped with a large number of dopants, giving it varying properties of semiconducting material. The doping mechanism selected for BaTiO 3 is crucial to achieving the necessary properties and applications [10]. Given that BaTiO 3 is a high-quality material with unique characteristics, many experiments have been conducted to observe how the presence of various atoms doped at both sites affects the material's performance [10]. Experimental findings of Tao Shi et al. [11] following the formula Ba(Ti 1-x Sn x )O 3 , contributed to a better understanding of the physicochemical properties of a B site modified with pure BaTiO 3 . The first principles calculation results are displayed in Figure 2(a), which shows that the amount of overlapping between Sn and oxygen  atoms is noticeably lower compared to the overlapping of Ti and oxygen atoms, indicating that their orbital hybridization is weaker [11]. Hybridization between the B site doped ion and oxygen ions inside the ferroelectric crystal BaTiO 3 reduces slightly the repulsion, which causes off-center ion displacement, and the formation of dipoles in each unit thus occurs. Due to the strength of the ions' interactions with one another, a Coulomb field with a large spectrum can be generated. The domain configuration of the crystal can be seen to be directly influenced by this effect, as shown in Figure 2(b). As seen in Figure 2(c), if the concentration of dopant Sn atoms is large enough, the ongoing dipole-dipole interaction is broken. Due to this phenomenon, the matrix is unable to accumulate a long-range Coulomb potential and normal ferroelectricity is destroyed. Ferroelectric property changes are therefore clearly caused by doping Sn into BaTiO 3 ceramic. The Curie temperature (T c ) also varies depending on the distribution of Sn, which results from the previously mentioned effects. Ferroelectric relaxor-type Ba(Ti 1-x Sn x )O 3 material displays diffuse phase transition behavior due to the random distribution of the Sn dopant, which in turn causes the local Curie temperatures to vary.
Dielectric characteristics, for instance, can be enhanced by doping to reduce the temperature coefficient of capacitance (TCC), lower the dielectric loss (tan δ), and maintain the relative permittivity (ε r ) at a relatively high value [12]. Sometimes doping increases the atomic density and grain size and hence the dielectric properties therefore increase [13]. Doping at the A site tends to make the grains of BaTiO 3 smaller. This is due to a reduction in the lattice parameter of the crystal that occurs when Ba 2+ ions are replaced by smaller cations such as Sr 2+ or Ca 2+ . The end result is an increase in the lattice strain, which in turn encourages the development of smaller grains during the sintering process [14]. However, the grain size increases with B-site doping. Substituting Ti 4+ ions with larger cations such as Zr 4+ or Sn 4+ raises the lattice parameter. Hence, the lattice strain diminishes and sintering produces larger grains. Most of the time, doping of metal atoms with larger atomic radii increases the dielectric nature [15]. Changes in dialectic properties are largely a matter of atomic polarization [16]. The above discussion shows how polarization can be affected by incorporating dopant atoms. The goal sometimes is to form a core-shell structure with pure BT grains at the center, surrounded by a shell of dopant atoms. Let us once more consider BT's resistivity characteristics. At room temperature, the unaltered BT exhibits insulator-like behavior with a band gap between 3 and 3.4 eV [17]. However, doping may improve the conductivity due to electron repositioning. Doping can also cause a phase transformation, which can affect a material's properties; for instance, in the case of Sr doping, the Curie temperature of a perovskite compound falls, causing conversion of the material from ferroelectric to paraelectric [18]. As stated by Yu Wang et al. [19], this transformation can be employed and regulated to obtain qualities of BT that are of interest. The doping methods for perovskite compounds have already been addressed at length. Atoms can be doped into both the "A" and "B" sites, as well as the O 3 domain. Later sections discuss the doping sites in detail.

Doping sites
The term "doping site" refers to a particular region inside a crystal lattice that is available occupation by an atom or molecule as a dopant. In this case, there is a limitation on doping BaTiO 3 that presents the addition of more than 3% of any one atom to ensure that the ceramic's core qualities are preserved [17,18]. Perovskite materials can undergo either "A" site doping or "B" site doping, both of which involve the introduction of a dopant. Several criteria must be met when selecting the dopant to be used at a particular doping site [20]. Doping without following the proper rules usually results in atomic instability. Doping policies vary from one synthesis method to another method. That is, we can dope just the "A" site, just the "B" site, or just the O 3 domain, not only both the "A" and "B" sites, etc., according to ABO 3 structure of the perovskite. Perovskites often have a primitive cube-shaped crystal structure, with larger "A"-type monovalent, divalent, or trivalent cations in the corners, smaller "B"-type pentavalent, tetravalent cations in the center, and O 3 (typically the oxygen employed as an anion) ions in the center of the face edges. It is therefore apparently stated that any atoms may be doped into either the "A" or "B" site depending on the atomic size and valency. Table 1 presents suitable atoms for A-site, B-site, and both-site doping as they appear in the periodic table. The choice of whether an atom is doped in the "A" site or the "B" site is determined by several factors. The ionic radius of an atom, for example, plays a major role in determining the site of substitution. Since ionic radii are of modest dimensions, cations such as La 3+ and Ce 3+ can be used as substitutes in the A-site. However, the BaTiO 3 chemical structure allows for substitution of ions with larger ionic radii, such as Lu 3+ and Yb 3+ , in the B site [21]. Additionally, the cation lattice positions in BaTiO 3 can be occupied by ions that have amphoteric behavior such as Y 3+ , Dy 3+ , Ho 3+ , and Er 2+ .
The A/B ratio is another formula that can be used to select the site will for doping. The process and dopant atom may vary depending on the intended use or desired qualities. Any researcher can obtain an idea of the atoms that can be employed for the "A" site, "B" site, both "A" and "B" sites, and O 3 domain doping using Table 1 in Figure 3.
Drawing on the many studies that have been undertaken on doped BaTiO 3 , Table 1 provides a useful overview for novice researchers. Some of the atoms in Table 1 apply to both site doping and the generic formula for doping, A 1-x M x BO 3 , which is displayed on the right side of the table. Here, x represents the concentration and M represents the dopant atom. Similarly, the general doping formula is depicted next to atoms doped in the "B" site in the lower section. A rectangle in the case of both-site doping encloses the atom. Doping can occasionally involve substitution at the O 3 site (an anion site), and this is indicated by presenting the dopant atom in the bottom row of the periodic table.

Sample preparation methods
In the initial stage of the process of material characterization, sample preparation is crucial when investigating the many different properties of dielectric ceramics and other materials [14]. Sample preparation techniques can vary depending on the type of research conducted on the characterization of doped BaTiO 3 . The crystal structure, grain size, and other parameters are all controlled by the sample preparation processes, and hence play a significant role in achieving the desired qualities [15]. The different methods with the dopant atoms and volume of content are listed in Table 2 based on previous literature. A number of steps are taken to prepare samples for BaTiO 3 doping. The sample preparation methods can influence the BaTiO 3 's final characteristics [17][18][19]. Many articles and studies have detailed the many methods used to synthesize BaTiO 3 ceramics. Different processes have been used for different purposes.

Solid-state reaction method
The solid-state reaction method is a sample synthesis technique that is applied in the absence of liquids, whether by grinding at the outset of the reaction or applying heat treatment. After the chemicals have been measured out and weighed to the appropriate levels, they are combined [42]. A mortar and pestle agate is typically used when hand mixing of lowvolume amounts is required. The conditions of the reaction and the anticipated characteristics of the result both play a role in the choice of reactant chemicals. It is essential to select a container material that is suitable for the subsequent reaction involving hightemperature heat treatment and that does not induce any chemical activity in the reaction when heated under the relevant conditions. The process of heat treatment requires a reliable furnace. It is recommended that researchers pelletize samples before heating them since this improves the contact surfaces among the grains. A polycrystalline solid sample is produced by performing a solid-state reaction on the solid substances. Solid-state processes are favored in order to maximize efficiency and minimize chemical wastage [43].

Sol-gel process
The sol-gel process involves solidifying a substance that includes components that produce a significant level of chemical activity by means of a solution, a gel, or sol, followed by the application of heat treatment to an oxide or another substance in order to prepare the sample under moderate conditions [44]. The basic materials are dispersed in a solvent, after which they undergo a hydrolysis reaction to generate an active monomer. This is the molecular process behind the sol-gel technique. Polymerization of a monomer leads to the formation of a sol, and eventually to a gel with a defined three-dimensional shape. The preparation will be successful following drying and thermal treatment [45].

Hydrothermal method
The hydrothermal technique involves heating and pressurizing an aqueous solution in a confined reaction vessel to produce a high-temperature and highpressure reaction atmosphere [46]. Sometimes vapor pressure is produced internally by the substance within the reaction vessel. One basic advantage of this method is that it enables insoluble or weakly soluble substances to be dissolved and recrystallized. The first stage of this process involves dissolving the Table 2. Sample preparation method with the doping formula applied and the doping content range of each experiment. The grain size range is shown beside the formation tempertaure (°C) and the doping site is also shown.  Energy storage applications High-frequency stability [10] reactants in a hydrothermal medium, at which point they join the solution as chemical groups or ions. Second, the temperature gradient between the top and bottom of the pot causes the ions or molecules to split. Adsorption, decomposition, and desorption of ions or molecule groups then occur at the junctions between the growing components. The adsorbate migrates upon contact, and the dispersed substance eventually crystallizes [47]. Which strategy for material synthesis is most appropriate is determined by several factors, including the intended purpose, budget, time available, number of steps, complexity of the procedure, kind of laboratory equipment, and availability of the starting material [48]. If precision is required, this must also be taken into account. However, reliable data and high-quality raw materials are prerequisites for accurate property measurements in the lab. For characterization purposes, it is crucial to have a high-quality powder from the synthesis process [49]. The properties are optimized, and more accurate data is obtainable when the powder is fine and homogeneous. Although many different synthesis methods are utilized to dope BaTiO 3 , only a few wet chemical and solid-state synthesis procedures, such as the sol-gel, solid-state reaction, sol hydrothermal, mixed oxide route, and so on, are commonly used for sample preparation. The solidstate approach often invalues investment of less processing time, less money, and fewer resources.
Several of the papers we studied employed solidstate reaction approaches since these are simpler than other methods [49]. Even while many trials used the sol-gel process, only a small fraction of those using different techniques were successful. Wet chemical synthesis processes, which offer high purity and uniform ultrafine powder at low temperatures, are one way to get beyond the restrictions seen in other approaches. Wet chemical procedures reduce the calcination temperature by roughly 300°C because they shorten the diffusion distance and provide a wellproportioned mixing ratio [4]. Doping experiment sample preparation factors such as sintering and calcination time, temperature, and media are introduced and compared in detail in Table 1. Many of these techniques have been used in an air atmosphere, even though some of these environments also contain oxygen, nitrogen, and argon.

Raman spectroscopy
Raman spectroscopy is an experimental method that can be utilized to differentiate between various site occupancies and compensatory mechanisms [50]. Analysis of the changes that occur in the Raman spectra of a material after it has been doped is performed with Raman spectroscopy, and the results can then be used to locate the doping site in a material. The vibrational modes of a material that are detected by Raman spectroscopy can be altered by the introduction of a dopant. For this purpose, the Raman shifts and intensities of individual vibrational modes can be monitored and used to pinpoint the doping location [50]. When a dopant is introduced into a solid substance, it can substitute an atom in the crystal lattice, altering the associated vibrational modes. The spectra shown in Figure 4 are representative of those that have been measured on undoped BaTiO 3 ceramics at temperatures ranging from room temperature to 300°C [50]. Additional data on the B-species site's composition can be gleaned from the presence and strength of the A 1 g octahedral breathing mode in the Raman spectra. This can lead to modifications to the Raman spectra with respect to undoped samples, such as repositioning or weakening of the peaks. It has been confirmed that V Ti //// (vacancy of titanium in BaTiO 3 ) is the main ionic compensation mechanism for samples that are made in flowing oxygen when the donor A site is substituted [50]. This is evidenced by the A 1 g octahedral breathing mode becoming Raman active. Raman spectra for BC100×T (A site doped with Ca into BaTiO 3 ) at ambient temperature are displayed in Figure 4(b). The presence of Ca in the A site is verified by the absence of any unusual modes in the spectroscopy near the 800 cm −1 region [50]. Figure 4(c) shows the spectra for BTC100× (B site with Ca doped into BaTiO 3 ) revealing a wide A 1 g breathing mode at 800 cm −1 , thus demonstrating that the B site is occupied by Ca. The precise way in which doping affects the Raman spectra is dependent on the material and dopant employed. However, the doping site can be determined and the effect of the dopant on a material's properties can be gleaned from a comparison of the doped and undoped Raman spectra [50].

X-ray diffraction (XRD) method
Crystal structure analysis using X-ray diffraction (XRD) is a highly effective method for locating doping sites [51]. In some circumstances, it may be more informative than Raman spectroscopy, which is more frequently used for this purpose [50]. Rietveld refinement of XRD data is commonly used to establish the doping ion site occupancy. The crystal structure of a substance can be determined by beaming a stream of X-rays at it and analyzing the resulting diffraction pattern [51]. A material's XRD pattern can be altered by the introduction of a dopant because the material's altered crystal structure is altered. Depending on the ionic radius of the dopant ion, the lattice parameters determined using XRD could either diminish or increase. This demonstrates that the dopant has made its way to the displacement location. If we continue to increase the concentration of the dopant, however, the lattice parameter will begin decreasing.
This occurs if the parameter is increasing at a lower concentration. Depending on the nature of the dopant and the crystalline structure of the material, different modifications will be seen in the XRD spectrum [52]. In some instances, the doping site can be determined by examining the modifications to the XRD pattern. Changes in the area and strength of the diffraction peaks in the XRD pattern can result from a dopant's replacement of an atom in the crystal structure. The XRD patterns of the doped and undoped materials can be compared to identify the precise site of doping [52].

Applications
Almost all studies aim to shed light on the unknown and put what has been learnt into practice. Despite BaTiO 3 being initially introduced as a dielectric material, it has been revealed via extensive study that BaTiO 3 is also well suited for many other uses as a result of its combination of the beneficial and practical features discussed thus far. BaTiO 3 is widely employed as a dielectric in the capacitor sector, especially in the production of multilayer ceramic capacitors (MLCC) [34]. The rising need for shrinking necessitates the usage of a multilayer capacitor structure, which permits the maximum capacitance achievable from a thin dielectric to be packed into the smallest space possible without compromising mechanical integrity. Modifying a BT ceramic to fix associated problems opens up new uses for the material. Doping trivalent dopants into BT ceramic to replace Ba 2+ causes a positive temperature coefficient of resistance (PTCR) effect, which can be employed in PTCR thermistors [37].
Barium titanate, in polycrystalline form, is a semiconductor with PTCR characteristics. Thermistors and other self-regulating electric heating systems are also manufactured using the PTCR effect. Doping can also convert BT ceramic from a dielectric to a semiconductive substance. Numerous articles detail the various ways in which the alterations in properties caused by doping could be put to use. Several potential uses are listed in Table 3; these were derived from our analysis of the aforementioned literature. PTC materials created via BT ceramic doping have several potential applications due to their electrical and thermal properties [34]. Because of their piezoelectric qualities, they are frequently employed in microphones and other types of transducers. Also, piezoelectric qualities are also put to use in electromechanical devices [31]. The importance of BT ceramic-based materials in the development of piezoelectric materials cannot be overstated. The toxicity problem is mitigated when they are used in place of lead-based piezoelectric materials. High beam coupling gain and the ability to work at visible and near-infrared wavelengths are two further benefits of BT ceramic's application in non-linear optics [28]. Due to its higher dielectric constant, pure BT ceramic perovskite can be utilized as an insulator and in capacitor modules. Mics and other transducers can benefit from BTP's potential piezoelectric capabilities, which can be investigated by introducing small changes. Small amounts of other metals, such as yttrium, scandium, and samarium, can be doped into barium titanate to turn it into a semiconducting material [37]. Barium titanate crystals are useful in nonlinear optics because they have a high beam-coupling gain and can operate at wavelengths close to infrared and visible light. This review features a conceptual table and a focus on the ways in which prior research might be applied in the real world. Since doping barium produces noticeable alterations, investigations have demonstrated potential applications based on the elements' properties. Possible applications suggested by a review of the relevant literature are listed in Table 3. Some of the atoms used in doping are particularly effective in altering attributes in certain contexts. Doping with iron in BTP increases its sensitivity to ferroelectric characteristics since Fe has ferromagnetic properties. Fe doping is useful in a wide variety of electronic applications because of its sensitivity and low noise [27]. Since barium titanate shows a wide range of doping-induced improvements in properties, it is surprising that research into this material has increased dramatically. Due to its abundance, barium is an ideal candidate for doping, allowing us to create useful new qualities for the aforementioned purpose easily and cheaply. It is hoped that the information presented in this study will be useful in guiding readers toward the optimal doping sample for their needs.

Summary and conclusions
We have conducted a comprehensive review aimed at understanding of the doping site of BT-based doped materials, including a comparative analysis of various synthesis techniques and attributes based on past literature, as well as a brief discussion of possible applications after doping. Ample experimental research on doping in BT perovskite is available, making this ceramic material popular, and for this reason, we recognized a need to summarize the reasons why different site occupancies occurred depending on the dopant in BT ceramics. We chose this topic to review in light of the current trends in research on ferroelectric perovskites such as BaTiO 3 .
In the previous literature, doping has been shown to alter four fundamental properties: dielectric, ferroelectric, electrical, and piezoelectric properties. After reviewing the related literature, a summary of the findings has been displayed above in the form of tables and figures. Since several studies have been conducted on BaTiO 3 by doping of the A site or B site, their findings have been compiled and compared with those of other experiments. The grain size, formation temperature, doping range, dopant, and other characterization parameters for doped BT are all tabulated, and data from the relevant studies and reports are cited. While doping can improve some characteristics, it can also produce undesirable side effects. With the goal of giving industrial and material scientists a quick idea of the viability of various doped BT-based materials in various applications, we have reviewed potential applications based on the results for all their properties along with dependency of dopant sites occupancy.

Disclosure statement
No potential conflict of interest was reported by the author.