Fully dense crystalline solids with extremely low lattice thermal conductivity (κ(L)) are of practical importance for applications including thermoelectric energy conversion and thermal barrier coatings. Here we show that lone-pair electrons can give rise to minimum κ(L) in chalcogenide compounds that contain a nominally trivalent group VA element. Electrostatic repulsion between the lone-pair electrons and neighboring chalcogen ions creates anharmonicity in the lattice, the strength of which is determined by the morphology of the lone-pair orbital and the coordination number of the group VA atom.

Experimental measurements have recently shown that Cu3SbSe3 exhibits anomalously low and nearly temperature independent lattice thermal conductivity, whereas Cu3SbSe4 does not exhibit this anomalous behavior. To understand this strong distinction between these two seemingly similar compounds, we perform density functional theory (DFT) calculations of the vibrational properties of these two semiconductors within the quasi-harmonic approximation. We observe strikingly different behavior in the two compounds: almost all the acoustic mode Grüneisen parameters are negative in Cu3SbSe4, whereas almost all are positive in Cu3SbSe3 throughout their respective Brillouin zones. The average of the square of the Grüneisen parameter for the acoustic mode in Cu3SbSe3 is larger than that of Cu3SbSe4, which theoretically confirms that Cu3SbSe3 has a stronger lattice anharmonicity than Cu3SbSe4. The soft frequency and high Grüneisen parameters in Cu3SbSe3 arise from the electrostatic repulsion between the lone s 2 pair at Sb sites and the bonding charge in Sb-Se bonds. Using our first-principles determined longitudinal and transverse acoustic mode Grüneisen parameters, zone-boundary frequencies, and phonon group velocities, we calculate the lattice thermal conductivity using the Debye-Callaway model. The theoretical thermal conductivity is good agreement with the experimental measurements.

We report thermoelectric properties of selected compounds from the Cu3SbSe4-Cu3SbS4 system. Additional phonon scattering due to the disordered arrangement of Se and S atoms reduces the lattice thermal conductivity to near its minimum possible value at high temperature. The hole concentration is optimized at approximately 2.0 × 1020 cm−3 by doping with 3% Ge on the Sb site. Compounds of the form Cu3Sb1−yGeySe4−xSx (x = 0.8 and 1.2, y = 0.02 and 0.03) all have dimensionless thermoelectric figure of merit in excess of 0.8 at 650 K, with a maximum value of 0.89 for x = 1.2, y = 0.03.

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