The layered delafossite ${\mathrm{PdCoO}}_2$ has been predicted to be one of very few materials with a thermopower that is highly anisotropic and switches sign between different crystallographic directions. These properties are of interest for various applications, but have been difficult to verify because sufficiently large crystals have not been available. We report measurements of the high-temperature electrical resistivity, thermal conductivity, and thermopower of phase-pure ${\mathrm{PdCoO}}_2$ powder compacts prepared by a highly Pd-efficient synthesis route. While the electronic transport of the polycrystalline samples is dominated by that of the Pd planes, the thermopower exhibits a well-defined deviation from the in-plane character at temperatures above $600\mathrm{K}$, which is indicative of opposing trends in the Seebeck coefficients within and perpendicular to the delafossite layers. The experimental data are consistently described by a combination of effective-medium models based on the main axes transport quantities. The results support the predicted ambipolar thermopower anisotropy in ${\mathrm{PdCoO}}_2$.

中文翻译：

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多晶PdCoO2的高温电和热输运性质

层状褐铁矿 ${\mathrm{钯钴}}_2$据预测，这种材料是极少数具有高度各向异性并在不同晶体学方向之间切换符号的热电材料。这些特性对于各种应用都是令人感兴趣的，但是由于没有足够大的晶体而难以验证。我们报告了纯相的高温电阻率，热导率和热功率的测量结果${\mathrm{钯钴}}_2$通过高效Pd合成途径制备的粉末压块。虽然多晶样品的电子传输受Pd平面的电子传输支配，但在高于20℃的温度下，热功率与平面内特征的偏差明显$600\mathrm{ķ}$，这表示在铜铁矿层内和垂直于铜铁矿层的塞贝克系数呈相反趋势。通过基于主轴传输量的有效介质模型的组合来一致地描述实验数据。结果支持了预测的双极性热电各向异性${\mathrm{钯钴}}_2$。