Reaction mechanism on the formation of (Sr, Ba)TiO3 and Ba(Zr, Ti)O3 at near room temperature by using A(OH)2•8H2O (A = Sr, Ba) and BO2•nH2O gel (B = Zr, Ti, Zr0.45Ti0.55)

ABSTRACT Recently, our group synthesized perovskite-type oxide ABO3 (A = Sr, Ba; B = Ti, Zr) with high crystallinity simply by leaving a powder mixture of A(OH)2•nH2O and BO2 hydrous gel at room temperature to 323 K for 3–10 days. In this study, perovskite solid solution (Ba, Sr)TiO3 and Ba(Zr, Ti)O3 were synthesized to investigate the reaction mechanism in this synthesis method in detail. The perovskite solid solution (Ba, Sr)TiO3 was obtained by using barium hydroxide, strontium hydroxide and TiO2 hydrous gel as the staring materials. On the other hand, in the case of using barium hydroxide and the mixture of TiO2 hydrous gel and ZrO2 hydrous gel as the starting materials, a mixed phase of BaTiO3 and BaZrO3 was obtained. B-site-substituted perovskite solid solution Ba(Zr0.45Ti0.55)O3 was obtained by using barium hydroxide and (Zr0.45Ti0.55)O2 hydrous gel as the starting materials. From the above, it was clarified that in this synthetic method, the Sr and Ba ions diffuse into the water part of hydrous gel and then slowly react with Ti(OH)4 to form a perovskite lattice.


Introduction
In the perovskite-type oxide ABO 3 , a relatively large A cation occupies the cage formed by vertices shared BO 6 octahedra framework [1,2]. The perovskite structure can accommodate various elements, and compounds containing multiple types of ions at each site can also be synthesized. Perovskite-type Pb(Zr, Ti)O 3 [3] are industrially important materials due to the structural-related properties such as ferroelectricity and piezoelectricity [4].
Recently, our group developed a low-temperature synthesis method for perovskite-type ABO 3 (A = Sr, Ba; B = Ti, Zr, Hf) with high crystallinity. The process was simply by leaving a powder mixture of A(OH) 2 •8H 2 O and BO 2 hydrous gel (hereafter described as BO 2 •nH 2 O) at room temperature to 353 K [5][6][7][8]. The reaction temperature was related to the tolerance factor t, given by t = (r A + r O )/2(r B + r O ), where r A , r B , and r O was ionic radii of the respective ions. SrTiO 3 , BaZrO 3 and BaHfO 3 , which all have t ~ 1 and cubic structure, were obtained at room temperature [8]. In particular, SrTiO 3 exhibited very sharp X-ray diffraction (XRD) peaks due to its high crystallinity, which was comparable to that synthesized by the solid-state reaction at 1373 K [6]. The crystallinity was affected by the reaction temperature. When the reaction proceeded rapidly at a relatively high temperature of 383 K, a sample with low crystallinity was obtained [5]. Dehydration occurs with the progress of the reaction, which was clearly observed as the sample in the reaction vessel getting wet [5]. Unlike other low-temperature synthesis method such as hydrothermal synthesis, this method does not need to add solvents or additional reagents, and the by-product is only water. In addition, high crystallinity was obtained even at low temperatures.
In the hydrothermal synthesis method which is a widely used as low-temperature process, the dissolution-precipitation has been widely proposed as the reaction mechanism [9]. On the other hand, it is considered that the synthesis method we developed have a reaction mechanism different from the dissolutionprecipitation in following points [5]. In the powder mixture of starting materials, Sr(OH) 2 dissolves in a small amount of water to make a strong alkali solution. However, TiO 2 •nH 2 O did not dissolve even in the strong alkali solution of pH14 [5]. Therefore, so-called dissolution-precipitation seems to be not suitable for this process. In addition, the reaction temperature to produce ATiO 3 was dependent on the kind of A ion. If Ti component in the gel dissolves into a water, the reaction temperature should be similar regardless of the kind of A ion. On the synthesis of SrTiO 3 , the reaction did not proceed when n value in TiO 2 •nH 2 O was 0.97 or less [5]. TiO 2 •nH 2 O contains OH groups and water molecules in the structure, and is more accurately described as TiO 2-x (OH) 2x •(n-x)H 2 O [10,11].
The internal water in hydrous titania gel TiO 2 •nH 2 O was decreased by the addition of Sr(OH) 2 saturated solution, preventing the reaction [5]. Hence, the existence of the internal water in the gel is important for the reaction. It is expected from those results that A ions diffuse into the water part of hydrous titania gel and then slowly react with partially polymerized Ti(OH) 4 (body of the gel) to form a perovskite lattice.
As shown in Equation (1), driving force of this reaction is neutralization, in which A(OH) 2 acts as base and In this study, the reaction mechanism proposed above has been verified by the syntheses of (Ba, Sr)TiO 3 4 and ZrCl 4 in a volume ratio of 1:9 was used. The perovskite-type ABO 3 was synthesized by the following procedure. The staring materials were mixed with stoichiometric ratio in an alumina mortar. The mixture was placed in a vial and sealed with a cap. These operations were performed in an Ar-filled glove box to prevent the formation of carbonate. The vial was kept at 296 K -323 K in a constant temperature bath and then left for 1-5 days. The reacted powder was washed with 1.0 M acetic acid and pure water. Table 1 shows the target composition, starting materials, and reaction conditions. Powder XRD measurement was performed using a RIGAKU MiniFlex600 X-ray diffractometer (CuKα radiation). The program VESTA was used for drawing crystal structures [12]. To check the n value in BO 2 •nH 2 O, thermogravimetric (TG) analysis was performed (heating rate of 5 K/min in air, and α-Al 2 O 3 as standard sample). The particle shape of the powder samples was observed with a field emission scanning electron microscope (FE-SEM) JEOL JSM-7600. The composition analysis was performed by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a SHIMADSU ICPE-9000. The solution for composition analysis was prepared by the following procedure. A few milligrams of powder sample and 5 mL of concentrated HCl were placed in a vial vessel and then heated to near 373 K on a hot plate. Once the sample was dissolved, it was diluted to 100 mL with ultrapure water.

Results and discussion
The perovskite-type SrTiO 3 , BaTiO 3 , and BaZrO 3 were synthesized by conditions shown in Table 1. Similar to the previous report [5,7], all were obtained as cubic perovskite phases. The lattice constants of the samples 1-5 days Figure 6 synthesized in 5 days of reaction were 0.39292(11) nm for SrTiO 3 , 0.40447(3) nm for BaTiO 3 , and 0.42176(3) nm for BaZrO 3 . Figure 1 shows SEM images of TiO 2 •nH 2 O and SrTiO 3 . If a perovskite phase was formed by diffusion of Sr 2+ and Ba 2+ ions into the hydrous gel, the grain shape of perovskite phase should be similar to initial hydrous gel. However, it has not been confirmed because no clear particle size was observed from SEM in both the hydrous gel and perovskite due to agglomeration of particles. Therefore, we verified that the reaction process of this synthesis method was due to diffusion of Sr 2+ and Ba 2+ ions in hydrous gel by synthesizing perovskite solid solution using various starting materials. Synthesis of (Ba, Sr)TiO 3 solid solution was performed by using Ba(OH) 2 •8H 2 O, Sr(OH) 2 •8H 2 O and TiO 2 •nH 2 O as starting materials. Figure 2 shows the XRD patterns of the products synthesized by leaving at 323 K for 1-5 days. Single phase of cubic perovskite phase (Space group Pm � 3m) was obtained on the first day, and the peak intensity became stronger day by day. The peak position was between SrTiO 3 and BaTiO 3 , suggesting the formation of perovskite solid solution. The solid solution obtained was found to be (Ba 0.60 Sr 0.40 )TiO 3 by ICP-AES. Ba and Sr were considered to have disordered arrangement because superlattice peaks due to the ordering of Ba and Sr in perovskite A-site were not observed.
Next, the synthesis of (Ba 1-x Sr x )TiO 3 series were performed by leaving at 323 K for 5 days. Table 2 shows composition of Ba and Sr of the starting materials and products measured by ICP-AES. The ratio of Ba and Sr of the starting materials and products were in substantial agreement. Figure 3 shows the XRD patterns of the obtained samples. Cubic perovskite phase (Space group Pm � 3m) was obtained in all compositions. Figure 4 shows the relationship between the lattice parameter and composition. The lattice parameter increased with increasing Ba content. This relationship appears to follow the Vegard's law, based on the 12coordinated ionic radii [13] of Ba 2+ (0.161 nm) and Sr 2+ (0.144 nm). From the above results, it was found that A-site-substituted perovskite solid solution (Ba, Sr)TiO 3 was synthesized by starting materials of Ba(OH) 2 Figure 5 shows the XRD patterns of the products synthesized by using 2 Ba(OH) 2 ·8H 2 O + ZrO 2 •nH 2 O + TiO 2 •nH 2 O at 323 K for 3-5 days. Diffraction peaks showing a two-phase mixture of perovskite phases were observed. The lattice parameters calculated for each phase were 0.40555(3) nm and 0.42091(3) nm, which were close to those of end members of BaTiO 3 (0.40447 nm) and BaZrO 3 (0.42176 nm), respectively. This result indicated that BaTiO 3 and BaZrO 3 were individually generated when the mixture of ZrO 2 •nH 2 O and TiO 2 •nH 2 O were used as starting materials. However, the XRD peak intensity of BaTiO 3 was much smaller than that of BaZrO 3 . This may be due to the difference in atomic    Table 2.
scattering factors of X-rays between Zr and Ti, as well as the difference in reactivity between Ba(OH) 2 •8H 2 O and BO 2 •nH 2 O (B = Ti, Zr). Yamaguchi and coworkers reported the minimum temperature required for synthesizing each perovskite compound by this method as 303 K for BaZrO 3 and 313 K for BaTiO 3 [8]. BaZrO 3 can be formed at the lower temperature than BaTiO 3 . It is considered that crystallization of BaZrO 3 proceeded faster than that of BaTiO 3 and as a result, BaZrO 3 exhibited higher XRD peak intensity. Next, we attempted to synthesize single-phase Ba (Zr, Ti)O 3 by using (Zr 0.45 Ti 0.55 )O 2 •nH 2 O as a starting material. Figure 6 shows the XRD patterns of products. Unlike the case using ZrO 2 •nH 2 O + TiO 2 •nH 2 O, single phase of cubic perovskite (Space group Pm � 3m) was obtained. The lattice parameter, 0.41274(10) nm, was almost in the middle of the end members of BaTiO 3 (0.40447 nm) and BaZrO 3 (0.42176 nm). ICP-AES measurement revealed that the ratio of Ti and Zr in product did not change from starting material of (Zr 0.45 Ti 0.55 )O 2 •nH 2 O. In addition, superlattice peak due to the ordered arrangement of Zr and Ti ions in perovskite B-site was not observed, indicating that Zr and Ti ions were disorderly arranged in perovskite B-site. It was found that the use of (Zr, Ti)O 2 •nH 2 O hydrous gel was important for the formation of Ba(Zr, Ti)O 3 solid solution in this method.
From the above results, the reaction mechanism in this synthesis method is considered. Ba(OH) 2 and Sr(OH) 2 dissolve in water to show strong alkaline, respectively. As described above, B component in BO 2 •nH 2 O gel is insoluble in strong alkaline water. It is  deduced that the reaction proceeds in the gel, as shown in Figure 7. On the other hand, in the hydrothermal synthesis method, which is a similar lowtemperature process, so-called dissolutionprecipitation mechanism is proposed. In this mechanism, B component must dissolve in water to react with A 2+ ion, precipitating a perovskite [9]. If B component dissolved slightly, there would be a possibility to form (Ba, Sr)TiO 3 solid solution perovskite via dissolutionprecipitation. Ba(Zr, Ti)O 3 also could be obtained from (Zr, Ti)O 2 •nH 2 O gel via dissolution-precipitation. However, in this study, when two kinds of gel (ZrO 2 •nH 2 O and TiO 2 •nH 2 O) were used, BaZrO 3 and BaTiO 3 were obtained as clearly separated two phases. These two phases never be formed via dissolutionprecipitation, as shown in Figure 7(c). Therefore, it is verified as the reaction mechanism in this method that A ions of Ba 2+ and Sr 2+ diffuse into the hydrous gel to react slowly with BO 2 •nH 2 O to form a perovskite lattice. More precisely, A ions diffuse all over the inner water channel in the gel, because the reaction does not proceed without inner water. This is also supported by the fact that in synthesis of Ba(Zr 0.45 Ti 0.55 )O 3 , the ratio of B cation was unchanged between starting hydrous gel and perovskite product.

Conclusion
The perovskite solid solutions (Ba, Sr)TiO 3    of this synthesis process was verified by investigating the relationship between the starting materials and the obtained perovskite phase. A-site substituted perovskite solid solution (Ba, Sr)TiO 3 were formed by using Sr(OH) 2  The reaction process in this study is not based on the dissolutionprecipitation, but on the long-range diffusion of Ba 2+ and Sr 2+ ions in the inner water channel of hydrous gel.

Disclosure statement
There is no potential competing interest in this work.