Understanding the thermal properties of solids is an important thrust in the solid state chemistry because thermally-controlled solids play important roles in many applications, such as thermoelectric and photonic materials, and thermal management composites. The Wiedemann-Franz Law, established 165 years ago and widely applied in physics and materials science to interpret thermal conductivity of solids, especially metals, elucidates that the electronic contribution to thermal conductivity is proportional to electrical conductivity, i.e. a good electrical conductor must be a good thermal conductor. Herein we report the first example of validation of the Wiedemann-Franz Law in a millimeter size single crystal of 3D clathrate with widest temperature range 10-400 K and highest electrical conductivity-thermal conductivity ratio. These properties outperform previously reported unique examples of metallic VO2 [1] and Li0.9Mo6O17.[2] A unique combination of high electrical conductivity and low thermal conductivity was confirmed on the grain boundaries-free ~3 mm3 single crystal of BaNi2P4 which was grown from seed-aid metal flux technique. The detailed characterization including low-temperature single crystal X-ray diffraction, heat capacity measurements, inelastic neutron scattering, and first-principles phonon modeling and Raman Spectroscopy was applied to study the thermal behavior of AM2P4. The covalent nature of 3D framework in BaNi2P4 responses to the good electrical conductivity, while the displacement of guest Ba atoms resulting in the significant hybridization of the guest Ba and Ni-P-framework vibrational modes play important role in suppression of its thermal conductivity.

Unprecedented violation Wiedemann-Franz law: A story of disobeying clathrate

2019 Spring National Meeting

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