We identify inter-valley scattering as the microscopic origin of anomalous charge transport in n-type $\mathrm{M}{\mathrm{g}}_3{\left(\mathrm{Sb},\mathrm{Bi}\right)}_2$ thermoelectrics. This mechanism depends critically on carrier concentration, as energy conservation governs phonon-assisted inter-valley transitions. We show that reducing the carrier density can eliminate the transport anomaly. A key prediction of this framework, namely the existence of a conductivity minimum below room temperature, had not been observed previously, while this study reveals clear minima at 190 K in ${\mathrm{Mg}}_{3.1}{\mathrm{Sb}}_{1.99}{\mathrm{Te}}_{0.01}$ and 150 K in ${\mathrm{Mg}}_{3.1}{\mathrm{Sb}}_{1.49}{\mathrm{Bi}}_{0.5}{\mathrm{Te}}_{0.01}$. The observed temperatures of these minima are in good agreement with the phonon spectra. Together, these results provide a complete and self-consistent explanation of the long-standing transport anomaly in $\mathrm{M}{\mathrm{g}}_3{\left(\mathrm{Sb},\mathrm{Bi}\right)}_2$.

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• Concepts & principles
• Electrical conductivity
• Energy materials
• Thermoelectrics
• Resistivity measurements