Recent research revealed that magnetic semiconductors, such as CuFeS2 and Mn-doped CuGaTe2 with chalcopyrite structure, could be considered as promising power-generation materials due to their excellent transport properties [1, 2, 3]. Here we report the preparation and thermoelectric properties of Cu4Mn2Te4, which is antiferromagnetic with TN = 50 K. 
Cu4Mn2Te4 adopts a spinel-related structure. Each unit cell contains eight formula units (Z = 8). The Te ions form a cubic closest-packing (ccp) with Cu occupying half of the tetrahedral sites and Mn half of the octahedral sites. When temperature is over 723 K, Cu and Mn will statistically occupy half of the tetrahedral sites and half of the octahedral sites, respectively.  Thermoelectric properties measurements show that Cu4Mn2Te4 displays an electrical conductivity 2500 Ω–1cm–1 and Seebeck coefficient 20 μV K–1 at 325 K . Its thermoelectric performance might be further improved through electron doping.
In this work, we have prepared various modifications of Cu4Mn2Te4 by directly reacting the elements followed by spark plasma sintering (SPS). And then investigated the effects of the contents of extra Mn and In on thermoelectric properties. All samples presented are mainly composed of the Cu4Mn2Te4 phase, as observed from powder X-ray diffraction (XRD). According to the thermoelectric transport properties measured, we demonstrate that the thermoelectric figure-of–merit for Cu4Mn2Te4 could be enhanced through the addition of excessive Mn or In. Benefiting from the increased power factor and reduced thermal conductivity, zT is improved by 125% from 0.12 to 0.27 through excess Mn doping in Cu4Mn2+xTe4, further to 0.52 in Cu4-yInyMn2Te4 with In2Te3 precipitates, and finally to zT = 0.65 in Mn\In co-added Cu4-yInyMn2+xTe4 at around 680 K. This value (zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides. It is worth noting that the effective mass of the carriers for samples with excessive Mn are estimated to be around 1.93m0, which perhaps is responsible for the high Seebeck coefficient and power factor of the samples. Another interesting feature is their low thermal conductivity values, which could be understood based on the low Debye temperature, a very low speed of the sound and a high Grüneisen parameter. 
Thus we believe that chalcogenides with spinel\spinel-related structure have great potential for future application and exploring magnetic semiconductors is a novel direction for developing thermoelectrics.
This work was supported by JST CREST Grant Number JPMJCR15Q6, Japan.
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