Structure and High-Temperature Thermoelectric Properties of the n-Type Layered Oxide Ca 2x Bi xd MnO 4c F. KAWASHIMA, 1,3 X.Y. HUANG, 2 K. HAYASHI, 1 Y. MIYAZAKI, 1 and T. KAJITANI 1 1.—Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. 2.—Core Research for Evolution Science and Technology (CREST), Japan Science and Technology Agency, Kawaguchi 333-0012, Japan. 3.—e-mail: kawashima@crystal. apph.tohoku.ac.jp We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca 2 MnO 4c . The electrical conductivity r and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity j of doped Ca 2 MnO 4c is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = rS 2 T/j, increases with Bi doping. The maximum ZT is 0.023 for Ca 1.6 Bi 0.18 MnO 4c at 877°C, which is ten times higher than that of the end member, Ca 2 MnO 4c . The increase of ZT mainly results from the considerable increase of r, which can be explained in terms of structural change. The Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO 3 layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility. Key words: n-Type layered oxide, Ca 2x Bi xd MnO 4c , thermoelectric properties, crystal structure INTRODUCTION Layered oxide semiconductors have been focused on as important thermoelectric materials since the discovery of high-performance cobaltates. 1–4 Compared with p-type oxides, potential n-type thermoelectric oxides for practical application have not been found. In the present study, we focused on the n-type layered oxide Ca 2 MnO 4 , the crystal structure of which is shown in Fig. 1a. At room temperature, Ca 2 MnO 4 belongs to the I4 1 /acd space group (no. 142) with lattice parameters a = b = 5.1890(1) A ˚ and c = 24.130(1) A ˚ . 5 CaO block layers and perovskite-type CaMnO 3 layers are alternately stacked along the c-axis. Figure 1b shows the Mn-O bonds of a MnO 6 octahedron separated into two groups: the Mn-O(1) bond in the c-direction and the Mn-O(2) bond in the ab-plane. It has been reported that Ca 2 MnO 4 has a low electrical conductivity r and that electrical conductance occurs in the CaM- nO 3 layers. 6 The thermal conductivity j of Ca 2 MnO 4 is believed to be low due to the layered structure. In this work, we aimed to improve the thermoelectric performance of Ca 2 MnO 4 by doping Ca 2+ sites with Bi 3+ ions. The valence state of Mn ions can be changed by such Bi doping, i.e., electron carriers are introduced into the CaMnO 3 layers. We herein report the crystal structure and high-temperature thermoelectric properties of Bi-doped Ca 2 MnO 4 . EXPERIMENTAL PROCEDURE Polycrystalline samples of Ca 2x Bi x MnO 4 (x = 0.0, 0.01, 0.1, 0.2, 0.3, and 0.4) were prepared by a solid- state reaction. Starting materials, CaCO 3 (4 N), Bi 2 O 3 (3 N), and Mn 3 O 4 (3 N), were ground in an (Received July 29, 2008; accepted January 29, 2009; published online March 3, 2009) Journal of ELECTRONIC MATERIALS, Vol. 38, No. 7, 2009 Special Issue Paper DOI: 10.1007/s11664-009-0700-z Ó 2009 TMS 1159