Site Preference of Carriers and Percolation Conductivity in BaZrO3 System

S. Yamaguchi1), S. Miyoshi1), A. Ebara1), H. Takahashi2), and I. Yashima2)

1) Department of Materials Engineering, The University of Tokyo

2) Engineered Materials Sector R&D Center, Mitsui Mining & Smelting Co., Ltd. We often encounter non-linear conductivity against the acceptor dopant

concentration in heavily and even lightly doped oxides. Although various mechanisms have been proposed, such as carrier trapping model, variable range hopping model, and so on, most of those models rely basically on the impurity semiconductor theory, which is valid in dilute dopant concentration range, where isolated dopant-carrier clusters are isolated. As carriers are trapped at deep trap centers, usually by the dopant ions, and are necessary to be thermally exited from local trapping site to matrix domain for carrier migration in matrix. On the other hand, if the dopant concentration is high enough for such dopant-carrier cluster shells to overlap each other, new percolation pathway that does not require thermal excitation to matrix is formed. Recently, new experimental results on the possible presence of such percolation conductivity of electron holes are reported in doped BaZrO3 systems and theoretical approach will be reviewed in this study, for the discussion on the characteristics and conduction properties governed by the site-preference and percolation along the 1st nearest neighbor (1NN) and the 2nd nearest neighbor (2NN) shell on the oxide ion sublattice. It is also a mater of our inter-ests whether or not similar percolation conductivity is working behind on proton con-ductivity. Our present estimation favors the presence of contribution of percolation conductivity for protons as well, and the hole and proton conductivity in perovskite ox-ide systems will be summarized. The nonlinear conductance with cubic dependency against the carrier concentration is a typical behavior of percolation conductance origi-nated from the site preference of both holes and protons due to a local distortion intro-duced by a mismatch of the size between dopant and matrix B-site cation. From the DFT calculation using CASTEP, both ionic and electronic defects prefer to occupy the 2NN site upon doping of large dopant ions, while the carriers localize at the 1NN site upon doping of smaller cations. By allowing carrier hopping between 1NN to 2NN O site in the neighboring dopant octahedrons with the probability from 0 to 1 relative to the hopping probability between 1NN O sites, the percolation threshold varies from 0.31 to 1.13. The 2NN site preference leads to an extremely low percolation threshold of 0.03 to 0.05 under the similar condition. On the other hand, a blocking effect by the 1NN shell, when heavily doped, will take over and complicated behavior with a non-linear variation of conductivity against dopant conductivity appears. The isothermal conductivity variation of proton, showing excellent agreement with Kreuer's data[1], suggests the 2NN percolation conductance.

[REFERENCE] [1] K.D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333.

Dr. Shu Yamaguchi is Professor at Dept. of Materials Engineering, School of Engineering, The University of Tokyo, Japan. He was graduated in 1978 from Tokyo Institute of Technology, earning Dr. Engg., in Metallurgy on Thermodynamic activity measurements in sodium silicate and phosphate melts using Na beta-alumina as a solid electrolyte in 1983 under the guidance of Professor K. S. Goto. He then worked as postdoctoral fellow with Professor Wayne L. Worrell at Department of Materials Sci-ence and Engineering University of Pennsylvania, and returned to Japan as Assistant Professor at Nagoya Inst. of Tech. from 1985. He moved to The University of Tokyo as Professor in 2002.

His expertise in solid state chemistry has initiated from the research on tracer diffusivity measurements of 18O in Fe1-xO and Co1-xO using SIMS in 1980, engaged in the interdisciplinary area between chemical thermodynamics and solid state chemistry, such as the electronic structure in oxide protonics materials, atomic switch, surface pro-tonics, and so on.

He has been serving as President of international (ISSI: 2013-2015) and local community (SSIJ: 2010-present) of Solid State Ionics. He is currently serving as Edi-tor-in-Chief of Science and Technology of Advanced Materials (STAM) published by Taylor and Francis and continuing challenge to develop high visibility journal in mate-rials science area from Asia.