Programme

I.2 -- Invited, VCTP-50

Date: Monday, 4 August 2025

Time: 10:30 - 11:10

Origin for the Colossal Permittivity in Nb-doped TiO2

†Van An Dinh(1), †Yujiro Hashimoto(2), Koji Kimura(3,4,5), Taro Kuwano(2), Dung Ngoc Dinh(1,6), Ryoji Asahi(2), Koichi Hayashi(3,4), *Hiroki Taniguchi(2), and *Yoshitada Morikawa(1) †V.A.D. and Y.H. contributed equally to this work.

(1)The University of Osaka (2)Nagoya University (3)Nagoya Institute of Technology (4)Japan Synchrotron Radiation Research Institute (JASRI), SPring-8 (5)National Institute for Materials Science (6)Graduate University of Science and Technology, Vietnam Academy of Science and Technology,

We investigated the origin of colossal permittivity observed in Nb-doped rutile TiO2 using experimental and theoretical methods. Initially, we observed a significant enhancement of permittivity in Nb-doped TiO2, reaching up to 106. However, most of this enhancement diminishes below 20 K, indicating that it primarily arises from carrier redistribution at grain boundaries, so-called the Maxwell-Wagner effect. Even below 20 K, the permittivity remains around 103, four times higher than pristine TiO2. Additionally, the permittivity shows strong anisotropy, measuring approximately 1000 along the [001] direction and about 300 along the [110] direction. To clarify the physical mechanism behind the permittivity enhancement due to Nb singly doping in TiO2, we performed density functional theory (DFT) calculations using high-quality hybrid energy functionals. Our findings reveal that Nb dopant ionizes to form Nb5+ ion at a Ti site, donating electron to the TiO2 lattice, thereby forming polaron Ti3+. Doped Nb5+ ion and the polaron Ti3+ attract each other, and at low temperatures they localize at the first nearest neighbor sites along the [001] direction. The binding energy between Nb5+ ion and polaron Ti3+ is approximately 50 meV, indicating the formation of free polarons at tempetures above 20 K, contributing to colossal permittivity via the Maxwell-Wagner effect. Below 20 K, however, polarons are trapped at the first nearest neighbor sites of Nb5+ ions. Our DFT calculations using hybrid functionals suggest that the charge of the polaron distributes between the Nb5+ ion and the neighboring Ti ions, forming a molecular polaron. Despite being trapped near Nb dopant, the activation energy for polaron flip-flop motion between nearest neighbor Ti sites through Nb is only 15 meV, indicating unfrozen motion even at liquid helium temperatures. Based on these observations, we conclude that the significant permittivity enhancement observed at liquid helium temperatures originates from the flip-flop motion of molecular polarons facilitated by Nb dopants under an applied electric field. Moreover, we found strong mutual attraction between Nb ions that may cause the formation of Nb-Nb dimers within the TiO2 lattice. Consequently, increasing Nb dopant concentration may hinder the flip-flop motion at low temperatures. This is the origin for the saturation of permittivity observed experimentally at Nb concentrations around 1% or higher. Finally, to examine the local atomic arrangements around doped Nb, we conducted X-ray fluorescent holography experiments at 100 K. At low Nb concentrations (approximately 0.1%), Nb atoms showed negligible displacement from bulk Ti sites, whereas at higher concentrations (1%), significant Nb displacement was observed, supporting our hypothesis of single-doped Nb with free polarons at lower concentrations and Nb-Nb dimer formation at higher concentrations.

Presenter: Morikawa Yoshitada