| Summary: | Hexagonal M<sub>2</sub>C<sub>3</sub> compound is a new predicted functional material with desirable band gaps, a large optical absorption coefficient, and ultrahigh carrier mobility, implying its potential applications in photoelectricity and thermoelectric (TE) devices. Based on density-functional theory and Boltzmann transport equation, we systematically research the TE properties of M<sub>2</sub>C<sub>3</sub>. Results indicate that the Bi<sub>2</sub>C<sub>3</sub> possesses low phonon group velocity (~2.07 km/s), low optical modes (~2.12 THz), large Grüneisen parameters (~4.46), and short phonon relaxation time. Based on these intrinsic properties, heat transport ability will be immensely restrained and therefore lead to a low thermal conductivity (~4.31 W/mK) for the Bi<sub>2</sub>C<sub>3</sub> at 300 K. A twofold degeneracy is observed at conduction bands along Γ-M direction, which gives a high n-type electrical conductivity. Its low thermal conductivity and high Seebeck coefficient lead to an excellent TE response. The maximum thermoelectric figure of merit (ZT) of n-type can approach 1.41 for Bi<sub>2</sub>C<sub>3</sub>. This work shows a perspective for applications of TE and stimulate further experimental synthesis.
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