Rutile ${\mathrm{GeO}}_2$ is an emerging ultra-wide band gap semiconductor (UWBG) that has demonstrated excellent potential for applications in power electronic devices. Alloys of rutile ${\mathrm{SnO}}_2$, a well-established UWBG semiconducting oxide, with ${\mathrm{GeO}}_2$ are promising for tuning the material properties for applications. The thermal conductivity, in particular, is a key property, which is significantly impacted by alloy disorder, but which is also essential in assessing the operation and degradation of materials in high-power electronic applications. Here, we present first-principles calculations of the thermal conductivity of rutile ${\mathrm{GeO}}_2, {\mathrm{SnO}}_2$, and their alloys, and quantify the effects of scattering by alloy disorder, temperature, and isotope mass distribution. We show that the relatively high thermal conductivity of the binary compounds is reduced by alloying and isotope disorder. We further assess grain boundary effects via quantifying the impact of phonon mean free path. However, we also find that the room-temperature thermal conductivity of the alloys is still comparable to or surpasses the values for $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, an established UWBG semiconducting oxide. Our findings provide a roadmap for the co-design of the thermal properties of rutile ${\mathrm{Ge}}_x{\mathrm{Sn}}_{1-x}{\mathrm{O}}_2$ alloys for electronic device applications.

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• Thermal conductivity
• Wide band gap systems
• Ab initio calculations