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https://doi.org/10.1007/s11664-020-08439-6 Ó 2020 The Minerals, Metals & Materials Society

Thermoelectric Properties of La1x Srx ZnAsO Y. KIMURA,1,2 H. KUNIOKA,1,2 K. KIHOU,1 H. NISHIATE,1 H. USUI,3 Y. TOKUNAGA,4 T. IIDA,2 K. KUROKI,4 and C.H. LEE 1,2,5 1.—National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan. 2.—Department of Materials Science and Technology, Tokyo University of Science (TUS), Katsushika, Tokyo 125-8585, Japan. 3.—Department of Physics and Materials Science, Shimane University, Matsue-shi, Shimane 690-8504, Japan. 4.—Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan. 5.—e-mail: [email protected]

Polycrystalline La1x Srx ZnAsO (0.00  x  0.075 ) was synthesized by solidstate reactions followed by hot pressing. The sign of the Seebeck coefficient changed from negative to positive by Sr doping, indicating that holes were successfully doped as a result of Sr substitutions. The maximum power factor of 0.118 mW/m K2 was attained at T ¼ 747 K in x ¼ 0:05. The thermal conductivity was relatively low at T ¼ 773 K, with a value of approximately 1.9 W/m K independent of Sr doping. These results led to a dimensionless figureof-merit ZT of 0.044 at T ¼ 747 K in x ¼ 0:05. Key words: Thermoelectric compounds, layered crystal structure, pnictides

INTRODUCTION The 1111-system possessing a ZrCuSiAs-type structure has attracted considerable attention not only as a high-Tc superconductor but also as a highperformance thermoelectric compound. Spurred by the discovery of superconductivity in LaFeAsO,1 iron pnictides and other 1111-systems have been extensively studied. For example, the thermoelectric properties of the Sb-based 1111-system were studied.2–5 It has been found that (Nd,Sr)ZnSbO exhibits a dimensionless figure-of-merit (ZT) value of 0.2 at 700 K.4 The highest reported ZT value for a 1111-system was for BiCuSeO with a ZT value of 1.4 at 923 K,6–9 demonstrating the strong potential of 1111-systems as thermoelectric compounds. LaFeAsO was found to exhibit a high power factor (S2 =q, where S is the Seebeck coefficient and q is the electrical resistivity) of 4.1 mW/m K2 at 75 K.10,11 The ZT value at 110 K was 0.08, which is relatively high for the low-temperature region, demonstrating that the As-based 1111-system is also a potential thermoelectric material. As-based compounds such

as the 122-systems of (Ba,K)Zn2 As2 and (Ba,K)Cd2 As2 exhibit relatively high ZT values,12,13 demonstrating the potential of As-based compounds as thermoelectric materials. Nevertheless, the Asbased 1111-system has not been sufficiently explored. Therefore, we have studied the thermoelectric properties of (La,Sr)ZnAsO in the present work. LaZnAsO forms a lamellar crystal structure, with the LaO and ZnAs layers stacked in an alternating fashion.14–17 Sr doping in LaO layers introduces holes in the ZnAs layers, causing carrier flow through the ZnAs layers. The two-dimensional nature of their electronic states is advantageous for achieving a high power facto