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Colossal dielectric behavior of Ga+Nb co-doped rutile TiO2 Wen Dong, Wanbiao Hu, Adam Berlie, Kenny Lau, Hua Chen, Ray L Withers, and Yun Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07467 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015
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ACS Applied Materials & Interfaces
Colossal Dielectric Behavior of Ga+Nb Co-doped Rutile TiO2 Wen Dong1, Wanbiao Hu 1*, Adam Berlie1,2, Kenny Lau1, Hua Chen3, Ray L. Withers1 and Yun Liu1* 1
Research School of Chemistry, The Australian National University, ACT 2601, Australia
2
The Bragg Institute, Australian Nuclear Science and Technology Organisation, New Illawarra
Road, Lucas Heights, Sydney 2234, Australia. 3
Center for Advanced Microscopy, The Australian National University, ACT 2601, Australia
*To
whom
the
correspondence
should
be
addressed:
[email protected],
[email protected] Abstract Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO2, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO2 i.e. (Ga0.5Nb0.5)xTi1-xO2, where Ga3+ is from the same group as In3+ but with a much smaller ionic radius. Colossal permittivity up to 104 ~ 105 with an acceptably low dielectric loss (tan δ = 0.05 ~ 0.1) over broad frequency/temperature ranges is obtained at x = 0.5% after systematic synthesis optimizations. Systematic structural, defect and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (~ 10 – 40 K), polaron-like electron hopping/transport at higher temperatures and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent colossal permittivity observed. We believe this work provides a comprehensive guidance for the design of new CP materials. Keywords: Colossal permittivity, dielectric properties, rutile TiO2, defect dipole, ceramic 1. Introduction Colossal permittivity (CP) (> 103) materials are drawing increased attention due to their promising potential for applications in the device miniaturization and energy storage areas.1 High performance CP materials, however, are rare. While a very high permittivity, up to 104~105, can be achieved in BaTiO3-based perovskite materials,2-3 for example, they typically suffer from a 1 ACS Paragon Plus Environment
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strong temperature dependence of this permittivity, especially in the vicinity of their ferroelectric-to-paraelectric phase transition. Other known CP materials e.g. CaCu3Ti4O12 (CCTO) and NiO-based systems have relatively stable CP properties over a broad temperature range but poor dielectric losses, typically higher than 0.1.4-7 As a result, the main issue for the development of high-performance CP materials is the simultaneous lowering of the dielectric loss and improvement of the temperature/frequency stability of the dielectric properties. To this end, a new electron-pinned defect-dipole mechanism, by means of which electrons can be localized within appropriately constructed defect clusters, was proposed8 and In+Nb co-doped rutile TiO2 cited as an example of this mechanism.8-9 This material exhibits both a large temperature/frequency-independent colossal permittivity (>104) as well as a low dielectric loss (< 0.05) over a broad temperature range, from 80 to 450 K. It is suggested that ‘triangular’ 3+ 3+ 4+ In23+VO••Ti 3+ and ‘diamond’ shaped Nb25+ Ti 3+ ATi (A=Ti /In /Ti ) defect complexes are strongly
correlated, leading to large defect-dipole clusters containing highly localized electrons that are together responsible for the high performance CP properties of this materials system.8 Donor doping of Nb5+ on its own into rutile TiO2 can easily lead to colossal permittivity but also leads to a high dielectric loss unless an In3+ acceptor is simultaneously co-doped into the system. The co-doped In3+ ions thus play a vital role in controlling dielectric loss in In+Nb co-doped rutile TiO2. This kind of acceptor-donor co-doped rutile TiO2 material has achieved the required balance between a temperature-independent CP and a sufficiently low dielectric loss. There is an ongoing need for further research into which kinds of high performance, CP materials systems can be achieved via appropriate acceptor-donor co-doping, given that TiO2 is an archetypical unitary oxide. Cheng et al
10
investigated the colossal permittivity of TiO2
ceramics co-doped with niobium and other trivalent cation acceptors, but there is still a lack of understanding of the type/s of dielectric mechanisms operating in these systems. While it is known that Al3+ is too small to build defect dipole clusters,11 the general relationship between the ionic radii of the acceptor ion and the presence or absence of a defect dipole effect remains largely unknown. It is therefore desirable to understand what happens when a different cation with a significantly different ionic radius such as e.g. Ga3+ ( r 3+ Ga group as In3+ ( r 3+ In
= 76 pm ) ,
= 94 pm ) but with an ionic radius closer to that a Ti
2 ACS Paragon Plus Environment
4+
from the same
ion, is used.
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ACS Applied Materials & Interfaces
For In+Nb co-doped rutile TiO2, bond valence sum (BVS) calculations12-13 indicate that replacing a Ti4+ ion by an In3+ ion in the rutile structure makes In3+ highly overbonded with an apparent valence (AV) of +5.2389. By contrast, using Ga3+ instead of In3+ to replace a Ti4+ only slightly increases the calculated Ga AV to +3.2192 due to its smaller ionic radius (r
Ti 4 +
( 74.5 pm )
