Transition-metal dipnictides $R{X}_2$ ($R=\mathrm{Nb}$ or Ta and $X=\mathrm{P}$, Sb, or As) represent a key class of topological semimetals that exhibit rich quantum transport phenomena owing to their nontrivial band topology. However, their thermal transport mechanisms remain poorly understood. Here, using first-principles calculations combined with the Boltzmann transport equation, we systematically investigate thermal transport in the topological semimetal ${\mathrm{NbP}}_2$, which hosts nodal lines in the absence of spin-orbit coupling. In contrast to common metals, phonons are found to dominate heat transport in ${\mathrm{NbP}}_2$, which is largely due to exceptionally weak phonon-electron scattering arising from the low electronic density of states near the Fermi level associated with its topological electronic states. More intriguingly, we observe a pronounced upward deviation from the Wiedemann-Franz law, with the Lorenz number ($L_{\mathrm{e}}$) exceeding the Sommerfeld value ($L_0$) by up to 40%. Band-resolved analysis reveals that this deviation originates from nearly perfect electron-hole compensation dictated by the nontrivial band structure, which facilitates strong bipolar diffusive transport. Furthermore, we find that the degree of electron-hole compensation, and consequently the deviation of $L_{\mathrm{e}}$ from $L_0$, is highly sensitive to the chemical-potential position. Our work provides important insights into unconventional thermal transport in topological semimetals and underscores the role of band topology in governing the interplay between charge and heat transport.

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• Electrical conductivity
• Thermal conductivity
• Topological materials
• Density functional theory
• First-principles calculations