SnSe exhibits exceptionally high thermoelectric (TE) figure of merit zT mainly due to its ultralow lattice thermal conductivity (¬ lat) [L.-D. Zhao et al.: Nature 508 (2014) 373.]. It is considered that strong lattice anharmonicity caused by the lone pair electrons of Sn 2+ results in the ultralow ¬ lat. Here, we focus on SnO because it has the lone pair electrons of Sn 2+ like SnSe. Bulk samples of SnO were synthesized by low-temperature high-pressure spark plasma sintering and their TE properties were examined. The present study revealed that SnO exhibits very low ¬ lat (1.44 Wm ¹1 K ¹1 at 573 K) compared with SnO 2 which has no lone pair electrons. The Grüneisen parameter (£) of SnO was evaluated to be 1.70 and this high £ leads to large lattice anharmonicity and thereby low ¬ lat. Even though SnO has low ¬ lat , the zT values were significantly low compared with SnSe. The maximum zT value of SnO was 0.00141 at 573 K. Since the main reason of this low zT is its non-optimized carrier concentration, the zT of SnO can be enhanced through the carrier concentration optimization.
Nowotny Chimney-Ladder (NCL) compounds are attracting increased attention as good thermoelectric (TE) materials. Although the TE properties of Si-based NCL compounds such as higher manganese silicides are well investigated, those of Ge-based ones are scarcely studied. Here, we demonstrate that a series of the Ge-based NCL compounds in the Mo13Ge23-Ru2Ge3 pseudobinary system, i.e., Mo1-xRuxGe1.769, shows quite low lattice thermal conductivity as well as good electrical properties like a material called phonon-glass electron-crystal and thus it shows good TE properties. By tuning the valence electron concentration, the TE properties are optimized and the maximum zT, called materials dimensionless figure of merit, reaches 0.23 for x = 0.6 in Mo1-xRuxGe1.769, which is approximately 15 times higher than that of x = 0 in Mo1-xRuxGe1.769, i.e., Mo13Ge23.
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