# Ferroelectric Transition in Sr- and W-Doped BaTiO3 Solid Solutions
## 1 Introduction  
In the field of condensed-matter physics, ferroelectricity has been studied for decades because spontaneous electrical polarization can be obtained without an electrostatic potential, and the direction of polarization can be tuned by using an external electric field [1; 2; 3]. Ferroelectric materials are widely applicable in piezoelectric devices, condensers, IR detectors, nonvolatile memory devices, and ferroelectric random-access memory [4; 5; 6]. Perovskite BaTiO3 (BTO), which is the best-known ferroelectric material, undergoes a distinct transition between the ferroelectric and paraelectric phases with high latent heat (-0.3 J/g) as the transition temperature, which is called the Curie temperature (T\({}_{\text{C}}\)) [7; 8]. The ferro-paraelectric transition in BTO originates from the spontaneous transition between tetragonal and cubic structures around the T\({}_{\text{C}}\) of 120 \({}^{\circ}\)C [9; 10]. When the room-temperature ferroelectric phase, tetragonal BTO, is heated above T\({}_{\text{C}}\), the lattice constants of a and c become identical, resulting in a cubic structure, which exhibits no spontaneous polarization. BTO has the formula of perovskite (ABO3), and the cations Ba2+ and Ti+ occupy the A and B sites, respectively. The dopant atoms usually have preferred substitutional sites (A or B in the perovskite ABO\({}_{3}\) system) depending on their atomic radius, the valence states, or their coordination numbers [11; 12; 13]. For example, Ce3+ atoms are more likely to occupy the A sites, while Ce4+ atoms are more likely to occur at the B sites [14]. Many doped BTO systems have been reported so far, and it is known that proper doping at an A or B site can effectively influence the crystal structure of BTO, as well as its ferroelectricity [15; 16; 17]. For example, A-site doping with divalent atoms, such as Sr2+ and Ca2+, or B site-doping with tetravalent atoms, such as Zr4+, Sn4+, or Hf4+ atoms, were expected to have an effectiveinfluence on ferroelectric properties, such as T\({}_{\rm C}\) and latent heat [18; 19; 20]. However, the effects of multivalent atoms, such as transition metal atoms, are hardly predictable for exact substitutional sites. Moreover, the ferroelectric properties can also be diverse depending on the synthetic conditions, such as temperature, ambient gas, or pressure [7]. There have been many studies of BTO systems that were doped with Sr\({}^{2+}\) and that had high solubility [21]. The T\({}_{\rm C}\) values of Sr-doped BTO decreased with an increase in the content of Sr atoms [15]. In the case of transition metal dopants, such as tungsten (W), W dopants have possible valence states of 4+, 5+, or 6+ when tungsten atoms substitute a Ti\({}^{4+}\) atom into a BTO system. [PERSON] et al. reported that substitutional tungsten atoms in the hexavalent state (W\({}^{6+}\)) in BTO could effectively improve the ferroelectric properties by creating trivalent titanium (Ti\({}^{3+}\)) atoms due to the charge neutrality of the system [22]. Because dopants with hetero-valence states create vacancies or interstitial states, the ferroelectric properties and the phase stability of W-doped BTO are still under debate. In this study, we showed the change in structural phase transition in polycrystalline BaTiO\({}_{3}\) through the doping of the A or B sites with Sr or W atoms. The Ba atoms were replaced with Sr atoms at the A sites, whereas the Ti atoms were replaced with W atoms at the B sites. X-ray diffraction revealed that the Sr- and W-doped BTO has a tetragonal structure for all samples in this study. The ratio of the lattice constants in the tetragonal structure (c/a) decreased from 1.008 to 1.005 as the content of Sr and W dopants increased. The transition between the ferroelectric tetragonal (room-temperature phase) and paraelectric cubic (high-temperature phase) structures was checked by using differential scanning calorimetry (DSC) measurements.