Gd<sub>12</sub>Co<sub>5.3</sub>Bi and Gd<sub>12</sub>Co<sub>5</sub>Bi, Crystalline Doppelgänger with Low Thermal Conductivities


Attempts to prepare Gd<sub>12</sub>Co<sub>5</sub>Bi, a member of the rare-earth (RE) intermetallics RE<sub>12</sub>Co<sub>5</sub>Bi, which were identified by a machine-learning recommendation engine as potential candidates for thermoelectric materials, led instead to formation of the new compound Gd<sub>12</sub>Co<sub>5.3</sub>Bi with a very similar composition. Phase equilibria near the Gd-rich corner of the Gd–Co–Bi phase diagram were elucidated by both lab-based and variable-temperature synchrotron powder X-ray diffraction, suggesting that Gd<sub>12</sub>Co<sub>5.3</sub>Bi and Gd<sub>12</sub>Co<sub>5</sub>Bi are distinct phases. The higher symmetry structure of Gd<sub>12</sub>Co<sub>5.3</sub>Bi (cubic, space group <i>Im</i>3̅, <i>Z</i> = 2, <i>a</i> = 9.713(6) Å), as determined from single-crystal X-ray diffraction, is closely related to that of Gd<sub>12</sub>Co<sub>5</sub>Bi (tetragonal, space group <i>Immm</i>). Single Co atoms and Co–Co dumbbells are disordered with occupancies of 0.78(2) and 0.22(2), respectively, in Gd<sub>12</sub>Co<sub>5.3</sub>Bi, but they are ordered in Gd<sub>12</sub>Co<sub>5</sub>Bi. Consistent with this disorder, the electrical resistivity shows less dependence on temperature for Gd<sub>12</sub>Co<sub>5.3</sub>Bi than for Gd<sub>12</sub>Co<sub>5</sub>Bi. The thermal conductivity is low and reaches 2.8 W m<sup>–1</sup> K<sup>–1</sup> at 600 °C for both compounds; however, the temperature dependence of the thermal conductivity differs, decreasing for Gd<sub>12</sub>Co<sub>5.3</sub>Bi and increasing for Gd<sub>12</sub>Co<sub>5</sub>Bi as the temperature increases. The unusual trends in thermal properties persist in the heat capacity, which decreases below 2<i>R</i>, and in the thermal diffusivity, which increases at higher temperatures

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oai:figshare.com:article/3436871Last time updated on 2/12/2018

This paper was published in FigShare.

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