Intuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is an enigmatic material known for a wellcharacterized anisotropic normal state and unconventional superconductivity. Recent ab initio transport calculations of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, $S$ acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures.

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• Entropy
• Seebeck effect
• Thermopower
• Thermal techniques