## 3 Results
Figure 5 shows the temperature dependent \(\kappa\) of all samples (the uncertainty of \(\kappa\) would be around \(\pm\)3%), while the inset shows the lattice thermal conductivity (\(\kappa_{\rm latt}\)), calculated by subtracting the electronic contribution (\(\kappa_{\rm elec}\)) calculated by the Wiedemann-Frantz law (\(\kappa_{\rm elec}\) = \(L\)-\(\sigma\)-\(T\)), where \(L\) is Lorenz number, which can be estimated with Equation (2) (where \(L\) is in 10\({}^{-8}\) W\(\Omega\)K\({}^{-2}\) and \(S\) in \(\mu\)V/K) [20],  
\[L\ =\ 1.5+exp\left(-\frac{|S|}{116}\right) \tag{2}\]  
Figure 4: Carrier concentration and mobility for **(a)** Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{x}\) (\(x\) = 0, 0.003, 0.01) and **(c)** Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{x}\) (\(x\) = 0, 0.005, 0.01). [PERSON] plots (\(S\) as a function of carrier concentration) for **(b)** Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{x}\) (\(x\) = 0, 0.003, 0.01) and **(d)** Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{x}\) (\(x\) = 0, 0.005, 0.01) at room temperature. For Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{\rm x}\) samples, a gradual decrease in \(\kappa_{\rm latt}\) (0.81, 0.78, and 0.75 W/mK for x = 0, 0.003, and 0.01) was seen at room temperature (inset of Figure 5a), while there seems to be no distinct change in \(\kappa\) in Figure 5a. At high temperatures, the different influences from the bipolar conduction may affect the \(\kappa_{\rm latt}\). The second phases of NiTe\({}_{2}\) may scatter the phonons further to reduce room temperature \(\kappa_{\rm latt}\). For Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{0.01}\), the \(\kappa\) (and \(\kappa_{\rm latt}\)) decreased to 0.92 W/mK (0.70 W/mK) at room temperature, compared to 1.03 W/mK (0.81 W/mK) of Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\). The estimated \(zT\) values of the samples are shown in Figure 6. For Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{\rm x}\) samples (Figure 6a, the uncertainty of \(zT\) would be around \(\pm\)17%), the \(zT\) was enhanced from 0.79 to 0.82 and 0.98 for \(x\) = 0.003 and 0.01, respectively, as the power factor increased due to increases in \(S\), power factor, and \(m^{\star}\) (Figures 3b and 4b). We speculate that the interface barrier height of 0.26 eV between Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) and NiTe\({}_{2}\) (Figure 2a) induced the carrier filtering effect, while there was slight systematic \(\kappa_{\rm latt}\) reduction as well (inset of Figure 5a). For Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{\rm x}\) samples (Figure 6b), with the much smaller barrier height of 0.08 eV between Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\) and NbTe2 (Figure 2b), no increases in \(S\), power factor, and \(m^{\star}\) (Figures 3d and 4d) were seen. However, the reduction in \(\kappa_{\rm latt}\) to 0.70 W/mK at room temperature (inset of Figure 5b) was achieved for x = 0.01. Therefore, the \(zT\) was also enhanced slightly from 0.79 to 0.83 and 0.94 for \(x\) = 0.005 and 0.01; however, this was mainly due to decreased \(\kappa\) (and \(\kappa_{\rm latt}\)) for \(x\) = 0.01 (Figure 5b). For Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NiTe\({}_{2}\))\({}_{\rm x}\) samples, the improved electronic transport properties, caused by the possible carrier filtering effect, enhanced the \(zT\), while the reduced thermal conductivity enhanced the \(zT\) in Bi\({}_{0.5}\)Sb\({}_{1.5}\)Te\({}_{3}\)(NbTe\({}_{2}\))\({}_{\rm x}\) samples.