Shunta Harada1,2,Naoki Kosaka1,Shunya Sugimoto1,Takashi Yagi3,4,Miho Tagawa1,Toru Ujihara1,3
Nagoya University1,Japan Science and Technology Agency2,National Institute of Advanced Industrial Science and Technology3,Jaoan Science and Technology Agency4
Shunta Harada1,2,Naoki Kosaka1,Shunya Sugimoto1,Takashi Yagi3,4,Miho Tagawa1,Toru Ujihara1,3
Nagoya University1,Japan Science and Technology Agency2,National Institute of Advanced Industrial Science and Technology3,Jaoan Science and Technology Agency4
Control of heat conduction through the manipulation of phonons as coherent waves have been attracted great interest for the advanced thermal management [1]. Although smooth interfaces were reported to be obtained in artificial superlattices, preparation of coherent interfaces for terahertz phonons having nanoscale periodicity with atomic-scale perfection are still challenging since MOCVD and MBE were nonequilibrium growth process. Therefore, we focus on natural superlattice titanium oxides, in which periodic structures are obtained as thermodynamically stable phases. A homologous series of the titanium oxides is so-called crystallographic shear (CS) structure, in which planar faults are periodically introduced in the mother rutile structure with their spacing depending on the oxygen. Recently we have revealed that the titanium oxide with CS structure possesses the pristine interface, which is expected to behave as coherent interface for almost all phonons [2]. Furthermore, we have successfully demonstrated the tuning of the interspacing in the CS planes in titanium oxides by the addition of chromium oxides to form oxygen deficiency [3]. In the present study, we have investigated thermal conduction in bulk titanium oxide natural superlattices with CS structures prepared by reductive annealing of rutile TiO<sub>2</sub> and crystal growth by the floating zone method. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) revealed that (132)<sub>rutile</sub> and (121)<sub>rutile</sub> CS planes with interspacings of 2.7 and 1.0 nm were introduced in the mother rutile structure. Time-domain thermoreflectance (TDTR) revealed that the thermal conductivity decreased by the introduction of CS planes, but that the decrease is not monotonic with increasing density of CS planes. Calculation of the thermal conductivity and the mean free path for phonons revealed that a crossover from incoherent to coherent thermal conduction took place, and coherent interfaces with nanoscale periodicity were formed as thermodynamically stable phases in bulk titanium oxide natural superlattices.<br/>[1] M. Maldovan, Nature 503 (2013) 209–217.<br/>[2] S. Harada <i>et al.</i>, J. Phys. Chem. C 125 (2021) 11175–11181.<br/>[3] S. Harada, S. Sugimoto <i>et al.</i>, J. Phys. Chem. C 125 (2021) 15730–15736.