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Gd12Co5.3Bi and Gd12Co5Bi, Crystalline Doppelgänger with Low Thermal Conductivities
journal contribution
posted on 2016-06-14, 17:35 authored by Anton O. Oliynyk, Taylor D. Sparks, Michael W. Gaultois, Leila Ghadbeigi, Arthur MarAttempts
to prepare Gd12Co5Bi, a member of the rare-earth
(RE) intermetallics RE12Co5Bi, which were identified
by a machine-learning recommendation engine as potential candidates
for thermoelectric materials, led instead to formation of the new
compound Gd12Co5.3Bi 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 Gd12Co5.3Bi and Gd12Co5Bi are distinct
phases. The higher symmetry structure of Gd12Co5.3Bi (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 Gd12Co5Bi (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 Gd12Co5.3Bi, but they are ordered in Gd12Co5Bi. Consistent
with this disorder, the electrical resistivity shows less dependence
on temperature for Gd12Co5.3Bi than for Gd12Co5Bi. 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 Gd12Co5.3Bi and increasing for Gd12Co5Bi 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.