We demonstrate that the KPb_mSbTe_(2+m) system (PLAT-m for tellurium, antimony, lead potassium, m = 19-21) of materials exhibits high thermoelectric performance. Samples with compositions K_(1-x)Pb_(m+δ)Sb_(1+γ)Te_(m+2) were prepared using several combinations of x, δ, γ and m and their thermoelectric properties were investigated in the temperature range of 300 - 800 K. All K_(1-x)Pb_(m+δ))Sb_(1+γ)Te_(m+2) samples exhibited n-type conduction over the measured temperature range. Their lattice thermal conductivities were found to be significantly reduced when compared to PbTe and even AgPb_mSbTe_(m+2). For example, for K_(0.95)Pb_(20)Sb_(1.2)Te_(22) a lattice thermal conductivity as low as 0.4 W/(m · K) was estimated at 650 K (based on a Lorenz number of 1.25 x 10~(-8) W · Ω/K~2). High resolution transmission electron microscopy on several samples revealed a widely dispersed nanoscale particle with varying size and shape endotaxially embedded inside a PbTe-rich matrix which is believed to be responsible for the reduced lattice thermal conductivity of K_(1-x)Pb_(m+δ)Sb_(1+γ)Te_(m+2) materials. Because of their small size, the nanoinclusions are coherent with the matrix and therefore do not markedly degrade the electrical conductivity of the materials. As a result, very high figures of merit are achieved at high temperature for several compositions. For K_(0.95)Pb_(20)Sb_(1.2)Te_(22), a maximum figure of merit ZT ～1.6 was obtained around 750 K. This value is similar to that of n-type LAST-18 and is two times larger than that of the-state-of-the-art n-type PbTe.
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