In our previous study, modest boron segregation at CoSi grain boundaries could drastically reduce the electrical resistivity while maintaining a large Seebeck coefficient. We attribute this behavior to a healing effect of B on microcracks without affecting the electronic structures. CoSi 0.98B 0.02 is the optimized sample possessing the highest power factor of 60 muW K -2 cm -1 at room temperature. To reduce the thermal conductivity while maintaining the high power factor, isoelectronic substitution, i.e., Rh on Co site, is adopted. Co 1-x Rh x Si 0.98B 0.02 alloys with x = 0.00, 0.05, 0.10, and 0.20 were prepared by an arc melting and annealing procedure. The lattice thermal conductivity decreases monotonically with increasing x, which is ascribed to enhanced phonon scattering by point defects. The lowest lattice thermal conductivity obtained for the x = 0.20 sample is less than 50% of that for CoSi at room temperature. However, all the samples still exhibit very low resistivity and large Seebeck coefficient and thus the power factor is not decreased much. The best sample is x = 0.10, showing a room-temperature power factor of 58 muW K -2 cm -1. The room-temperature ZT is raised by a factor of 2.3 compared with CoSi due to the power factor enhancement and simultaneous thermal conductivity reduction.
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