Faculty Profiles - TANIGUCHI Hiroki

Phase Transition from Weak Ferroelectricity to Incipient Ferroelectricity in Li<inf>2</inf>Sr(Nb<inf>1- x</inf>Ta<inf>x</inf>)<inf>2</inf>O<inf>7</inf> Reviewed

Takayuki Nagai, Yasuhide Mochizuki, Hirokazu Shirakuni, Akitoshi Nakano, Fumiyasu Oba, Ichiro Terasaki, Hiroki Taniguchi

Chemistry of Materials Vol. 32 ( 2 ) page： 744 - 750 2020.1

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

Copyright © 2020 American Chemical Society. Pseudo Ruddlesden-Popper-type Li2SrNb2O7 undergoes a phase transition between paraelectric and weak ferroelectric phases at Tc (=217 K), whereas Ta-counterpart Li2SrTa2O7 stays paraelectric though its room temperature structure is identical to that of Li2SrNb2O7. In the present study, the ferroelectric phase transition of Li2Sr(Nb1-xTax)2O7 is investigated as a function of Ta-concentration x. As the Ta-concentration x increases, Tc is found to decrease monotonously, leading to the disappearance of the ferroelectricity at x = 0.4. This phase-transition suppression with increasing x in Li2Sr(Nb1-xTax)2O7 is supported by first-principles calculations, showing that the soft mode in Li2SrNb2O7 or the increase of x in Li2Sr(Nb1-xTax)2O7 enhances their bonding states. In the composition of x = 0.4, dielectric permittivity gradually increases on cooling to become independent of temperature around 0 K, indicating an incipient ferroelectricity of the system. Empirical analyses suggest existence of a quantum paraelectric state in a composition range of 0.3 < x < 0.4. The present study provides a novel playground to investigate a quantum para/ferroelectricity in layered-perovskite-type compounds.

DOI： 10.1021/acs.chemmater.9b04022

Novel Materials with an Optically Tunable Dielectric Response

TANIGUCHI Hiroki

The Journal of The Institute of Electrical Engineers of Japan Vol. 140 ( 1 ) page： 28-31 - 31 2020

Antiferroelectric to Antiferroelectric-Relaxor Phase Transition in Calcium Strontium Sulfoaluminate Reviewed

Toru Wakamatsu, Genta Kawamura, Tomohiro Abe, Yuki Nakahira, Shogo Kawaguchi, Chikako Moriyoshi, Yoshihiro Kuroiwa, Ichiro Terasaki, Hiroki Taniguchi

Inorganic Chemistry Vol. 58 ( 22 ) page： 15410 - 15416 2019.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

Copyright © 2019 American Chemical Society. Structural phase transitions of calcium strontium sulfoaluminate series, (Ca1-xSrx)8[AlO2]12(SO4)2 ((CS)AS-x) with x = 0.80-1.00, are systematically investigated by powder X-ray diffraction, dielectric measurements, and pyroelectric measurements, to clarify a phase diagram of (CS)AS-x (x = 0.80-1.00). A pure strontium sulfoaluminate, (CS)AS-1.00, is found to undergo three phase transitions, which take place successively on cooling from a prototypical cubic phase with the symmetry of Im3 m. Though the room-temperature phase of (CS)AS-1.00 was previously reported to be of polar Pcc2, the pyroelectric measurements clarified a nonpolar character of the crystal symmetry. The dielectric measurements suggest a possibility of an antiferroelectric ground state of (CS)AS-x in the Sr-rich compositions. As x decreases, the ground state changes to a short-range-ordered state, implying a unique phase transition from the antiferroelectric state to the antiferroelectric-relaxor state. The present study provides an intriguing playground for designing new ferro/antiferroelectric materials.

DOI： 10.1021/acs.inorgchem.9b02495

Exciton Transport in the Electron–Hole System Ta₂NiSe₅

Nakano Akitoshi, Nagai Takayuki, Katayama Naoyuki, Sawa Hiroshi, Taniguchi Hiroki, Terasaki Ichiro

Journal of the Physical Society of Japan Vol. 88 ( 11 ) page： 113706 - 1-4 2019.11

Language：English Publishing type：Research paper (scientific journal) Publisher：日本物理学会

We have measured the dielectric constant and thermopower of single crystal samples of the excitonic insulator candidate Ta2NiSe5. The dielectric constant below 50 K shows relaxor-like relaxation, implying the existence of randomly distributed electric dipoles. We have evaluated the concentration of the dipoles to be 10^19 cm^−3 at 30 K, which gives a rough estimate of the exciton concentration in this compound. A large thermopower of 600 µV/K at 100 K suddenly drops toward zero down to 50 K, and this zero thermopower evidences the neutral particles flow against the applied temperature gradient. We have semi-quantitatively explained the above two anomalies in terms of spontaneously generated excitons at low temperatures where thermally excited holes are quenched.ファイル公開：2020/11/15

Weak Ferroelectricity in n = 2 Pseudo Ruddlesden-Popper-Type Niobate Li<inf>2</inf>SrNb<inf>2</inf>O<inf>7</inf> Reviewed

Takayuki Nagai, Hirokazu Shirakuni, Akitoshi Nakano, Hiroshi Sawa, Hiroki Moriwake, Ichiro Terasaki, Hiroki Taniguchi

Chemistry of Materials Vol. 31 ( 16 ) page： 6257 - 6261 2019.8

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

© 2019 American Chemical Society. Li2SrNb2O7 (LSNO) crystallizes in a structure closely related to n = 2 Ruddlesden-Popper-type compounds, which are generally formed by intergrowth of two-dimensional perovskite-type blocks and rocksalt-type layers. The present study demonstrates the coexistence of spontaneous polarization and an anti-ferroelectric-like nonlinear response in LSNO at 80 K, suggesting weak ferroelectricity below the phase transition temperature of 217 K. A combination of first-principles calculations and single-crystal X-ray diffractions clarifies a polar P21cn structure for the ground state of LSNO, where an in-plane antiferroelectric displacement and an out-of-plane polar shift simultaneously take place. The present study offers a new perspective to design ferroelectric and antiferroelectric materials with Ruddlesden-Popper-type compounds.

DOI： 10.1021/acs.chemmater.9b02373

Efficacy of Taping Treatment for Umbilical Hernia Compared With Monitoring Without Treatment in Infants: Analysis of a Questionnaire Survey Presented at the 46th Annual Meeting of the Kyushu Pediatric Surgery Study Group Open Access

Taniguchi Naoyuki, Kai Hiroki, Matsuo Susumu, Matsuura Toshiharu, Taguchi Tomoaki

Journal of the Japanese Society of Pediatric Surgeons Vol. 55 ( 5 ) page： 920 - 926 2019.8

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Japanese Society of Pediatric Surgeons

Purpose: To examine the effectiveness of taping treatment for infantile umbilical hernia, a retrospective questionnaire survey was carried out in the pediatric surgery institutes in the Kyushu Pediatric Surgery Study Group.Methods: Infants with umbilical hernia registered from January 2013 to December 2015 were included in this study. Their therapeutic results were assessed at the end of June 2016.Results: Replies were obtained from 24 institutes. Cure was judged on the basis of the closure of the hernia orifice. The number of infants who underwent the taping treatment was 1,320. Cure was achieved in 908 infants, whereas no cure was found in 158 infants; 48 infants dropped out owing to skin disorders and results were not known in 206. Among the 333 infants in whom monitoring without treatment was performed, spontaneous cure was seen in 146 infants and no cure in 111 infants; results were not known in 76. The cure rate of taping treatment was significantly higher than that of monitoring without treatment (85.2% vs. 56.8%, p<0.05, χ2 test). Furthermore, the duration until cure was less than 4 months in about 80% of the infants with taping treatment, whereas the duration in many infants monitored without treatment was more than 1 year.Conclusions: The taping treatment for infantile umbilical hernia shortened the duration until cure, but the cure rate of this treatment did not exceed the 90% spontaneous cure rate of the untreated patients at 2 years of age as described in conventional textbooks. However, the spontaneous cure rate found by our questionnaire survey was unexpectedly low. Therefore, the spontaneous cure rate of infantile umbilical hernia should be further investigated by prospective studies.

DOI： 10.11164/jjsps.55.5_920

Open Access

Interplay between quantum paraelectricity and thermoelectricity in the photo-Seebeck effect in a SrTiO<inf>3</inf> single crystal Reviewed Open Access

Yuuka Shiraishi, Kenji Tanabe, Hiroki Taniguchi, Ryuji Okazaki, Ichiro Terasaki

Journal of Applied Physics Vol. 126 ( 4 ) 2019.7

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

© 2019 Author(s). We report the electrical conductivity and the Seebeck coefficient of a SrTiO 3 single crystal under 405-nm laser illumination from 10 to 30 K. We find that the photoconductivity exponentially increases with decreasing temperature, suggesting a gradual metal-insulator transition. Assuming the carrier mobility reported in the preceding studies, we have evaluated the carrier concentration to be 8 × 10 9 cm - 3 at maximum, which corresponds to 10 - 7 ppm impurities in the case of chemical doping. Such ultralow doping is realized only when the energy of the incident light is slightly lower than the bandgap. In this situation, the incident light penetrates the whole sample with a small probability of electron-hole creation. We find that the observed photo-Seebeck coefficient seriously disagrees with the calculated values from the carrier concentration. In order to remedy this discrepancy, we have proposed a phenomenological model in which the quantum paraelectric behavior of SrTiO3 screens the thermoelectric voltage.

DOI： 10.1063/1.5106384

Open Access

Plastic/Ferroelectric Crystals with Easily Switchable Polarization: Low-Voltage Operation, Unprecedentedly High Pyroelectric Performance, and Large Piezoelectric Effect in Polycrystalline Forms Reviewed

Jun Harada, Yuto Kawamura, Yukihiro Takahashi, Yohei Uemura, Tatsuo Hasegawa, Hiroki Taniguchi, Koji Maruyama

Journal of the American Chemical Society Vol. 141 ( 23 ) page： 9349 - 9357 2019.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

© 2019 American Chemical Society. Molecular ferroelectric crystals have attracted growing interest as potential alternatives to conventional lead-based ceramic ferroelectrics. We have recently discovered that a class of compounds known as plastic crystals can show multiaxial ferroelectricity, which allows ferroelectric performance even in polycrystalline forms. Here, we report new plastic/ferroelectric ionic molecular crystals that exhibit remarkably small coercive electric fields at room temperature. The easily switchable ferroelectric polarization enables low-voltage switching operations and high-frequency performance. Such ferroelectric crystals can be readily processed into bulk polycrystalline forms with desired shapes that are characterized by unprecedentedly high pyroelectric figures of merit and large piezoelectricity. These multifunctional molecular crystals represent highly attractive prospects for device elements with a diverse range of applications, which will significantly boost the development of molecular ferroelectric crystals.

DOI： 10.1021/jacs.9b03369

Structural Phase Transitions and Possibility of the Relaxor-like State in Improper Ferroelectric Strontium-Substituted Calcium Sulfoaluminates

Wakamatsu Toru, Kawamura Genta, Abe Tomohiro, Kawaguchi Shogo, Moriyoshi Chikako, Kuroiwa Yoshihiro, Itoh Mitsuru, Terasaki Ichiro, Taniguchi Hiroki

Journal of the Physical Society of Japan Vol. 88 ( 3 ) page： 34718 - 34718 2019.3

High-pressure compressibility and electronic properties of bismuth silicate Bi2SiO5 from synchrotron experiments and first-principles calculations Reviewed Open Access

A. Girard, M. Stekiel, W. Morgenroth, H. Taniguchi, V. Milman, A. Bosak, B. Winkler

Physical Review B Vol. 99 ( 6 ) 2019.2

Language：Japanese Publishing type：Research paper (scientific journal)

© 2019 American Physical Society. The high-pressure structural properties of bismuth oxide Bi2SiO5 have been investigated up to 29 GPa using in situ powder synchrotron x-ray diffraction (XRD) and up to 50 GPa with density functional theory (DFT) calculations. We found a compressibility anomaly at about 20 GPa that is observed both in our XRD and DFT results. The rotation of the corner-connected SiO4 tetrahedra produces a straightening of the silicate chains, yielding a lower compressibility above 20 GPa for the structure with straight chains. We analyzed the stereochemical activity of the Bi3+ lone electron pair, which is found to decrease with increasing pressure, but it can still be identified in the calculated electron density difference maps at 50 GPa.

DOI： 10.1103/PhysRevB.99.064116

Local structure analysis of PbTiO<inf>3</inf> in high-temperature cubic phase Reviewed

Y. Yoneda, H. Taniguchi, Y. Kitanaka, Y. Noguchi

Ferroelectrics Vol. 538 ( 1 ) page： 57 - 62 2019.1

Language：Japanese Publishing type：Research paper (scientific journal)

© 2019, © 2019 Taylor & Francis Group, LLC. High-energy X-ray diffraction study was performed on lead titanate (PbTiO3). Short-range order structure was revealed using atomic pair-distribution function (PDF) method. In the high-temperature cubic phase, there is a large deviation between local and average structure. Especially, Pb atoms deviated from the cubic lattice framework, but establish Pb-O-Pb random network.

DOI： 10.1080/00150193.2019.1569986

Structural Phase Transitions and Possibility of the Relaxor-like State in Improper Ferroelectric Strontium-Substituted Calcium Sulfoaluminates Reviewed

Toru Wakamatsu, Genta Kawamura, Tomohiro Abe, Shogo Kawaguchi, Chikako Moriyoshi, Yoshihiro Kuroiwa, Mitsuru Itoh, Ichiro Terasaki, Hiroki Taniguchi

Journal of the Physical Society of Japan Vol. 88 ( 3 ) page： 034718 2019

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

©2019 The Physical Society of Japan Structural phase transitions of strontium-substituted calcium sulfoaluminates, (Ca1−xSrx)8[AlO2]12(SO4)2 [(CS)AS-x], are systematically investigated by powder X-ray diffraction (PXRD) and dielectric measurements. In a pure calcium sulfoaluminate (CS)AS-0.0, an unreported phase transition and dielectric anomaly are found in the low-temperature region below the ferroelectric phase-transition temperature. The XRD pattern recorded as a function of Sr substitution and temperature clearly resolves Tc suppression with Sr content x from a structural change perspective. Although no macroscopic change in the crystal structure is observed for (CS)AS-0.4, the broad dielectric peak with frequency dispersion remains for this composition, indicating the fragmentation of polar ordering. Furthermore, (CS)AS-0.4 shows a specific heat anomaly along with the broad dielectric peak, suggesting a relaxor-like short-range ordered state of the system.

DOI： 10.7566/JPSJ.88.034718

Indium-Free Amorphous Ca-Al-O Thin Film as a Transparent Conducting Oxide Reviewed

So Hee Sim, Kyeong Tae Kang, Sangyun Lee, Miso Lee, Hiroki Taniguchi, Suyoung Kim, Seulki Roh, Jun Hyeob Oh, Sang A. Lee, Jong Seong Bae, Jae Hyuck Jang, Jungseek Hwang, Seungwu Han, Tuson Park, Woo Seok Choi

Chemistry of Materials Vol. 31 ( 19 ) page： 8019 - 8025 2019

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER CHEMICAL SOC

© 2019 American Chemical Society. Transparent conducting oxide (TCO) is a promising material system for transparent electrodes, which is one of the most essential elements in current electronic and energy devices. In particular, In-based TCOs such as Sn:In2O3 (ITO) and In-Ga-Zn-O (IGZO) have shown large optical band gap and high electrical conductivity, sufficient for the applications. However, In is an expensive element, which hampers its facile industrial application. Moreover, In-based TCOs show an abrupt decrease in conductivity when their thickness decreases below ∼100 nm, possibly due to inhomogeneity within the thin films. Here, we introduce an amorphous Ca-Al-O (CAO) thin film as a promising candidate for the In-free TCOs. The amorphous CAO thin film has very low resistivity (∼10-5 ω cm) at room temperature, as well as high transparency in the visible region of the light spectrum (>80%). The isotropic Ca s-orbital in the conduction band is found to be responsible for the superior performance of CAO as a TCO. Owing to the exceptional structural homogeneity of the CAO thin film, thickness-independent transport characteristics are observed, maintaining its TCO performance down to 10 nm of thickness.

DOI： 10.1021/acs.chemmater.9b02399

Zeolite-type compounds with improper ferroelectricity

Exciton transport in the electron–hole system Ta<inf>2</inf>NiSe<inf>5</inf> Reviewed Open Access

Akitoshi Nakano, Takayuki Nagai, Naoyuki Katayama, Hiroshi Sawa, Hiroki Taniguchi, Ichiro Terasaki

Journal of the Physical Society of Japan Vol. 88 ( 11 ) page： 113706 2019

Language：English Publishing type：Research paper (scientific journal) Publisher：PHYSICAL SOC JAPAN

©2019 The Physical Society of Japan We have measured the dielectric constant and thermopower of single crystal samples of the excitonic insulator candidate Ta2NiSe5. The dielectric constant below 50 K shows relaxor-like relaxation, implying the existence of randomly distributed electric dipoles. We have evaluated the concentration of the dipoles to be 1019 cm−3 at 30 K, which gives a rough estimate of the exciton concentration in this compound. A large thermopower of 600 µV=K at 100 K suddenly drops toward zero down to 50 K, and this zero thermopower evidences the neutral particles flow against the applied temperature gradient. We have semi-quantitatively explained the above two anomalies in terms of spontaneously generated excitons at low temperatures where thermally excited holes are quenched.

DOI： 10.7566/JPSJ.88.113706

TANIGUCHI Hiroki

Oyo Buturi Vol. 88 ( 10 ) page： 668-672 - 672 2019

Language：English Publishing type：Research paper (scientific journal) Publisher：The Japan Society of Applied Physics

Our group has recently found improper ferroelectric materials in aluminate-sodalite-type compounds that belong to a zeolite-family. The aluminate-sodalite-type compounds are mainly composed of eco-friendly elements, which are ubiquitous in the earth's crust. A conventional strategy for the development of ferroelectric materials is based on perovskite-type compounds, leading to many practical applications for device elements such as capacitors and frequency filters. Aluminate-sodalite-type ferroelectrics, on the other hand, show excellent performance for pyroelectric energy harvesting, which exceeds that of lead-based perovskite-type compounds, indicating the potential of non-perovskite-type ferroelectrics for functional materials. Here we give an overview of the improper ferroelectricity in aluminate-sodalite-type compounds and discuss their performance for pyroelectric energy harvesting in connection with their improper ferroelectricity.

DOI： 10.11470/oubutsu.88.10_668

Systematic tuning of the photo-dielectric effect in Ba(Al<inf>1-</inf><inf>x</inf>Zn<inf>x</inf>)<inf>2</inf>O<inf>4-δ</inf> Reviewed

Dielectric properties in Ruddlesden-Popper-type oxide Li2SrNb2O7 Open Access

Nagai Takayuki, Nakano Akitoshi, Moriwake Hiroki, Sawa Hiroshi, Terasaki Ichiro, Taniguchi Hiroki

Meeting Abstracts of the Physical Society of Japan Vol. 74.1 ( 0 ) page： 2549 - 2549 2019

Structural analysis in Ruddlesden-Popper-type oxide Li2SrNb2O7 using synchrotron X-ray diffraction experiments Open Access

Nakano Akitoshi, Nagai Takayuki, Sawa Hiroshi, Terasaki Ichiro, Taniguchi Hiroki

Meeting Abstracts of the Physical Society of Japan Vol. 74.1 ( 0 ) page： 2550 - 2550 2019

A re-entrant structural phase transition in the aluminate-sodalite-type oxide (Ca1-xSrx)8[AlO2]12(MoO4)2 (0.5≦x≦1.0) Open Access

Aoki Nobumasa, Nakano Akitoshi, Nakahira Yuki, Moriyoshi Chikako, Kuroiwa Yoshihiro, Terasaki Ichiro, Taniguchi Hiroki

Meeting Abstracts of the Physical Society of Japan Vol. 74.1 ( 0 ) page： 2551 - 2551 2019

Takayuki Nagai, Kenji Tanabe, Ichiro Terasaki, Hiroki Taniguchi

Applied Physics Letters Vol. 113 ( 21 ) page： 212902 2018.11

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

© 2018 Author(s). Systematic tuning of photo-induced enhancement of the dielectric permittivity (i.e., the photo-dielectric effect) in Ba(Al1-xZnx)2O4-δ (BAZ-x) has been demonstrated by changing Zn concentration x and intensity of the incident light. The relative change of the dielectric permittivity (ϵ′), defined as Δϵ′photo/ϵ′dark, increases linearly with an increase in the Zn concentration (x), finally reaching approximately 30% at 1 MHz. The dependence of Δϵ′photo/ϵ′dark on the intensity of the incident light shows a two-step growth as the irradiation light intensity increases, suggesting the existence of multiple photo-induced processes influencing the photo-dielectric effect of BAZ-x. The results of the present study not only give a clue to the mechanism of the photo-dielectric effect but also aid in the development of innovative photo-tunable functional devices.

DOI： 10.1063/1.5036982