Programme

P.8 -- Posters, VCTP-50

Date: Tuesday, 5 August 2025

Time: 08:30 - 10:00

Theoretical elucidation of local atomic structures and colossal permittivity properties of Nb-doped TiO$_2$

Ngoc Dung Dinh (1,2), Van An Dinh (1), Quang Minh Ngo (2,3), Yoshitada Morikawa (1)

(1) Department of Precision Engineering, Graduate School of Engineering, The University of Osaka, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan; (2) Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam; (3) University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam

Recently, materials with colossal permittivity (CP) have attracted significant interest due to their potential applications in device miniaturization and high-energy-density storage. To date, CP-based capacitors have primarily relied on materials such as BaTiO$_3$, CaCu$_3$Ti$_4$O$_12$ (CCTO), La$_{2–x}$Sr$_x$NiO$_4$ (LSNO). However, BaTiO$_3$ exhibits temperature-sensitive permittivity, while CCTO and LSNO suffer from high dielectric loss [1-3]. Achieving both CP and low dielectric loss in a single material remain a significant challenge. TiO$_2$ doped with pentavalent element (V, Nb, Ta) material has emerged as a promising alternative [4]. Experimental studies have shown that at high temperatures, its CP is primarily attributed to the Maxwell-Wagner-Sillars effect, where electrons accumulate at grain boundaries, leading to a large dielectric dipole. However, at low temperatures, even though the Maxwell-Wagner-Sillars effect is suppressed, the permittivity of doped TiO$_2$ remains very high (about four-fold compared to the undoped case) [5]. Therefore, the underlying mechanism responsible for the high permittivity at this regime remains unclear. In this study, we investigate the mechanisms underlying the high permittivity of Nb-doped TiO$_2$ at low temperatures using Density Functional Theory (DFT). Pentavalent ions such as Nb have an additional electron in their outer shell compared to Ti atoms. When Ti is substituted by Nb in TiO$_2$, this excess electron becomes localized, inducing the local lattice distortion and forming a quasi-particle called polaron. Our DFT results show that the excess electron localizes on a Ti atom neighboring the Nb-dopant, forming a molecular polaron configuration. The energy barrier for polaron hopping between adjacent Ti sites of Nb atom is relatively small (about 20 meV), which allows for flip-flop motion of the molecular polaron even at low temperatures. This hopping process contributes to the formation of large dielectric dipoles, leading to the manifestation of colossal permittivity. Furthermore, when compressive (tensile) pressure is applied to Nb-doped TiO$_2$, our DFT calculations reveal a decrease (increase) in the polaron hopping barrier. This observation highlights the role of external pressure in modulating the local structure and the permittivity of Nb-doped TiO$_2$, providing insight into their pressure-dependent behavior. References (1) S. Krohns, et al., Appl. Phys. Lett. 94, 122903 (2009) (2) S. Krohns, et al., Nat. Mater. 10, 899 (2011) (3) P. Lunkenheimer, et al., Eur. Phys. J. Special Topics 180, 61 (2010) (4) W. Hu, et al., Nat. Mater. 12, 821–826 (2013). (5) M. Kawarasaki, et al., Sci. Rep. 7, 5351 (2017)

Presenter: Dinh Ngoc Dung