| Development of High-Pressure Treatment Technique for Epitaxial Thin Films and Fabrication of α-PbO2-type TiO2 Epitaxial Thin Films Yuki Sasahara1; Koki Kanatani2; Hiroaki Asoma3; Cédric Tassel1; Kazunori Nishio3; Hiroshi Kageyama1; Ryota Shimizu3; Masaki Azuma2; Norimasa Nishiyama2; Taro Hitosugi4; 1KYOTO UNIVERSITY, Kyoto, Japan; 2TOKYO INSTITUTE OF TECHNOLOGY, Yokohama, Japan; 3TOKYO INSTITUTE OF TECHNOLOGY, Tokyo, Japan; 4THE UNIVERSITY OF TOKYO, Tokyo, Japan; PAPER: 441/SolidStateChemistry/Regular (Oral) SCHEDULED: 16:20/Mon. 28 Nov. 2022/Andaman 1 ABSTRACT: Materials under ultrahigh pressure (HP) exhibit a variety of interesting properties.[1] However, some of the HP-phase materials that are thermodynamically stable under HP (> 10 GPa) transforms into amorphous or different crystalline phase during decompression. If this back-transformation can be suppressed, we can obtain some functional HP-phase materials to be utilized in the future. To obtain such HP-phase materials under ambient pressure, we focused on the epitaxial stabilization of metastable-phase materials and came up with the idea of applying HP to thin-film samples.[2] We first investigated an HP-phase, <i>α</i>-PbO<sub>2</sub>-type TiO<sub>2</sub> (orthorhombic, <i>a</i> = 0.454 nm, <i>b</i> = 0.549 nm, <i>c</i> = 0.491 nm). This phase is obtainable under ambient pressure. Unfortunately, most of the reported data was about the product in the form of powder, and only a few reports about the fabrication of single crystals are currently available. In particular, single-phase epitaxial thin films have not been reported. In this study, we developed a technique for applying HP (8 GPa) to thin-film samples. Using a rutile TiO<sub>2</sub>(100) epitaxial thin film as a precursor, we fabricated epitaxial thin films of single-phase <i>α</i>-PbO<sub>2</sub>-type TiO<sub>2</sub>(100) by inducing a structural phase transition at ultrahigh pressure. Thin films of epitaxial rutile TiO<sub>2</sub>(100) (thickness: ~100 nm) were deposited as precursors on Al<sub>2</sub>O<sub>3</sub>(001) (5 mm in diameter and 0.5 mm in height) using pulsed laser deposition. HP treatment for thin films was performed using a Kawai-type multi-anvil HP apparatus. The precursor thin film was heated up to 1000 °C under HP of 8 GPa, and then kept for 0.5 h. After the heating step, the film was quenched to room temperature (RT), followed by decompression. The results of X-ray diffraction and Raman spectroscopy indicate that a single-phase <i>α</i>-PbO<sub>2</sub>-type TiO<sub>2</sub>(100) epitaxial thin film has been obtained. It should be stressed here that rocking-curve full width at half maximum of the 200 peak showed a quite small value of 0.11°, indicating very high crystallinity. Our present study indicates that HP treatment to thin-film samples allows us to fabricate high-quality HP-phase epitaxial thin films. References: [1] Snider <i>et al.</i>, <i>Nature</i>, <b>586</b>, 373 (2020). [2] Sasahara <i>et al.</i>, <i>AIP Adv.</i>, <b>10</b>, 025125 (2020). |
