Preparation of {100}-Oriented Epitaxial (1-x)(Bi,K)TiO₃-xCaTiO₃ Solid Solution Films by Hydrothermal Method and Their Ferroelectric Property

(Bi,K)TiO₃ is a well-known traditional tetragonal ferroelectric material discovered in the 1950s by Smolenskii <i>et</i>.<i> al</i>. [1] However, the preparation of high-quality (Bi,K)TiO₃ is still challenging due to the preparation difficulty. The fundamental ferroelectric properties have not been well understood so far. In 2022, our group succeeded to grow the polar-axis-oriented epitaxial (Bi,K)TiO₃ films at the low deposition temperature of 240 ^oC by hydrothermal method.[2] Their tetragonality (<i>c</i>/<i>a</i> ratio) and remnant polarization (<i>P</i>_r) values are 1.046, and 84 μC/cm², respectively. These values are larger than previously reported ones for (Bi,K)TiO₃ceramics. [3] The enhanced tetragonality of the (Bi,K)TiO₃films is possibly attributed to the unique displacement of the Bi and K in A-site cations ascertained by TEM observation. These (Bi,K)TiO₃films showed high Curie temperature, <i>T</i>_c, above 800 ^oC. Based on this research, solid solution epitaxial films of (Bi,K)TiO₃ with ferroelectric rhombohedral (Bi,Na)TiO₃ were successfully obtained by hydrothermal method.[4] X-ray diffraction analysis showed a continuous decrease of the <i>c</i>/<i>a</i> ratio with increasing K/(K+Na) ratio and the <i>c</i>/<i>a</i> ratio become unity near the K/(K+Na) ratio = 0.15. This suggests the existence of tetragonal-rhombohedral morphotropic phase boundary near <i>c</i>/<i>a </i>= 1. This research demonstrated the preparation of solid solution film by hydrothermal method. As the next challenge, we should decrease the extremely high <i>T</i>_c above 800 ^oC of (Bi,K)TiO₃films for the application of pyroelectric devices.<br/>In this study, we tried to hydrothermally grow epitaxial solid solution films of (Bi,K)TiO₃with paraelectric CaTiO₃, (1-<i>x</i>)(Bi,K)TiO₃-<i>x</i>CaTiO₃ films to increase the pyroelectric coefficient by decreasing <i>T</i>_c. The film composition region can be divided into three from the viewpoint of the crystal structure. Films in the range of <i>x </i>= 0 - 0.12 (Region 1) and 0.60 - 1.0 (Region 3) consisted of single phases and their lattice parameters continuously changed with the <i>x</i> value. The phases are tetragonal and pseudo-cubic symmetry in Region 1 and Region 3, respectively. This suggests a formation of a solid solution in these two regions. On the other hand, these two phases (tetragonal and pseudo-cubic symmetry) coexisted in the intermediated range of <i>x</i> = 0.18 - 0.43 (Region 2). In this coexistence composition region, the volume fraction of these two phases changed with the <i>x</i> value, while their lattice parameters were almost independent of the <i>x</i> value. This suggests the solubility limit of this system. In Region 1 and Region 2, <i>P</i>-<i>E</i> hysteresis loops originating from the ferroelectricity were clearly observed. The <i>P</i>_r values continuously decreased with increasing <i>x</i> in Region 1, mainly due to the decrease in the <i>c</i>/<i>a</i> ratio of the ferroelectric phase. This decrease of <i>P</i>_r with <i>x</i> was also observed in Region 2 mainly due to the decrease in the volume fraction of the ferroelectric phase. In the next step, these films were annealed at 950 ^oC by face-to-face annealing to decrease <i>T</i>_c. After annealing the film with <i>x </i>= 0.12, the film showed lowered <i>T</i>_cof 450 ^oC and a higher pyroelectric coefficient than that of the film with <i>x </i>= 0 ((Bi,K)TiO₃). These data show that the properties of (Bi,K)TiO₃ films can be controlled by the synthesis of a solid solution.<br/><br/>[1] G. A. Smolenskii, Sov. Phys. Solid State. <b>1, </b>1562 (1959).<br/>[2] Y. Ito <i>et.al.</i> Appl. Phys. Lett. <b>120</b>, 022903 (2022).<br/>[3] Y. Hiruma <i>et.al.</i> Jpn. J. Appl. Phys. <b>44</b>, 5040 (2005).<br/>[4] Y. Huang <i>et.al.</i> Jpn. J. Appl. Phys. <b>59</b>, SPPB10-1 (2020).