Gd$_{12}$Co$_{5.3}$Bi and Gd$_{12}$Co$_{5}$Bi, Crystalline Doppelgänger with Low Thermal Conductivities

Abstract

Attempts to prepare Gd$_{12}$Co$_{5}$Bi, a member of the rare-earth (RE) intermetallics RE$_{12}$Co$_{5}$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$_{12}$Co$_{5.3}$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$_{12}$Co$_{5.3}$Bi and Gd$_{12}$Co$_{5}$Bi are distinct phases. The higher symmetry structure of Gd$_{12}$Co$_{5.3}$Bi (cubic, space group Im3̅, Z = 2, a = 9.713(6) Å), as determined from single-crystal X-ray diffraction, is closely related to that of Gd$_{12}$Co$_{5}$Bi (tetragonal, space group Immm). Single Co atoms and Co-Co dumbbells are disordered with occupancies of 0.78(2) and 0.22(2), respectively, in Gd$_{12}$Co$_{5.3}$Bi, but they are ordered in Gd$_{12}$Co$_{5}$Bi . Consistent with this disorder, the electrical resistivity shows less dependence on temperature for Gd$_{12}$Co$_{5.3}$Bi than for Gd$_{12}$Co$_{5}$Bi . The thermal conductivity is low and reaches 2.8 W m$^{-1}$ K$^{-1}$ at 600 °C for both compounds; however, the temperature dependence of the thermal conductivity differs, decreasing for Gd$_{12}$Co$_{5.3}$Bi and increasing for Gd$_{12}$Co$_{5}$Bi as the temperature increases. The unusual trends in thermal properties persist in the heat capacity, which decreases below 2R, and in the thermal diffusivity, which increases at higher temperatures.This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) (A.O.O. and A.M). M.W.G. is grateful for support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant No. 659764. T.D.S. also acknowledges resources from the DARPA SIMPLEX program N66001-15-C-4036. We thank Dr. S. Lapidus for assistance with the high-resolution synchrotron XRD experiments, made possible through the mail-in powder diffraction service, at 11-BM at the Advanced Photon Source at Argonne National Laboratory. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357