Effects of Three-Dimensional Strain on Electric Conductivity in Au-Dispersed Pr 1.90 Ni 0.71 Cu 0.24 Ga 0.05 O 4+δ Junji Hyodo, † Ken Tominaga, † Jong-Eun Hong, † Shintaro Ida, †,‡ and Tatsumi Ishihara* ,†,‡ † Department of Applied Chemistry, Faculty of Engineering and ‡ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan * S Supporting Information ABSTRACT: The effects of tensile strain on the electronic properties of Cu- and Ga-doped Pr 1.9 NiO 4 (PNCG) were investigated. The difference in the thermal expansion coefficient between PNCG (α = 13.5-13.9 × 10 -6 K -1 ) and Au (α = 14.2 × 10 -6 K -1 ) can induce tensile strain in PNCG, resulting in changes in electrical conductivity. Hall-effect measurements indicated that the tensile strain stabilized the oxidized state of PNCG, and the electrical conductivity increased because of the increased hole concentration. This suggests that the tensile strain affected the valence numbers of cations in PNCG, increasing the hole concentration and raising the conductivity. Furthermore, the BO 6 octahedral distance in the K 2 NiF 4 structure was increased by the induced strain, decreasing the hole mobility. 1. INTRODUCTION Recently, considerable attention has been paid to the “strain effect” in which crystal distortion changes electronic charge carrier and/or mass transport properties. 1-6 The electronic effects of induced strain in a crystal lattice are highly interesting, because these effects may enable performance improvements in electrochemical solid state devices such as solid oxide fuel cells (SOFCs). There have been several reports on the positive effects of strain on electrical conductivity and oxygen diffusivity. 7-20 Most approaches employ epitaxial thin film growth, and strain is induced by a crystal lattice mismatch between the substrate and the film. However, for thin films the quality of the film such as partial amorphous part, may strongly influence the electrical conductivity and/or mass transport behavior. Therefore, the effects of strain on the electronic behavior of thin films can be quite complex. It is well-known that the thermal expansion coefficients of metal and metal oxides are very different. Therefore, when we connect a metal to a metal oxide at an elevated temperature and then cool it down, a large strain will be introduced at the interface between the metal and the metal oxide. According to the difference in thermal expansion coefficients between the metal and the metal oxide, we can expect three-dimensional (3D) tensile or compressive strain introduced, as schematically shown in Figure 1. The quality of a bulk sintered sample should be independent of the metal dispersion in the grains if the sintering temperature is high enough. Therefore, the effects of strain on electrical conductivity can be more directly observed, independ- ent of the quality or crystallinity of the film sample. For demonstration purposes, Cu- and Ga-doped Pr 1.9 NiO 4 (PNCG) and Au particles were chosen, because the thermal expansion coefficient of PNCG (α = 13.5-13.9 × 10 -6 K -1 ) is slightly lower than that of Au (α Au = 14.2 × 10 -6 K -1 ). 21,22 PNCG is a promising new candidate cathode material in SOFCs with a K 2 NiF 4 structure and has an extremely high oxygen permeability, as was previously reported. 23-28 In this study, the effect of 3D tensile strain on the detailed charge transfer behavior in metallic- gold-dispersed PNCG was investigated using in situ Hall-effect measurements. 2. EXPERIENTAL SECTION In our previous report, the amount of Cu and Ga dopant was optimized for oxygen permeability. The optimized amount of Cu was 24 mol % and that of Ga was 5 mol % in Ni-site with A-site deficient. 25-28 Pellets of 1 mol % Au-added Pr 1.90 Ni 0.71 Cu 0.24 Ga 0.05 O 4+δ were prepared by solid state reaction, using Pr(NO 3 ) 3 ·6H 2 O (99.9%, Mitsuwa Chemicals Co., Ltd.), Ni(CH 3 COO) 2 ·4H 2 O (98%, Wako Pure Chemical Industries, Ltd.), Cu(NO 3 ) 2 ·3H 2 O (99%, Wako Pure Chemical Industries, Ltd.), Ga(NO 3 ) 3 ·nH 2 O (99.99%, Mitsuwa Chemicals Co., Ltd.), and HAuCl 4 ·4H 2 O (99.0%, Kishida Chemical Co., Ltd.) as starting materials in stoichiometric quantities. The n value in Ga(NO 3 ) 3 ·nH 2 O was determined by thermogravimetric analysis. The starting reagents were dissolved into deionized water and dried with stirring. The obtained powder was heated at 673 K to remove NO x . The obtained powder was calcined at 1073 K for 6 h and pressed into a disk 20 mm in diameter and 1 mm thick. The prepared disks were sintered at 1523 K for 6 h and were sufficiently dense as obtained with a relative density higher than 90%. The crystal structures of the prepared disks were measured Received: April 30, 2014 Revised: December 11, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp504220y | J. Phys. Chem. C XXXX, XXX, XXX-XXX