First-principles description of anomalously low lattice thermal conductivity in thermoelectric Cu-Sb-Se ternary semiconductors

Experimental measurements have recently shown that Cu ₃SbSe ₃ exhibits anomalously low and nearly temperature-independent lattice thermal conductivity, whereas Cu ₃SbSe ₄ does not exhibit this anomalous behavior. To understand this strong distinction between these two seemingly similar compounds, we perform density functional theory calculations of the vibrational properties of these two semiconductors within the quasiharmonic approximation. We observe strikingly different behavior in the two compounds: almost all the acoustic-mode Grüneisen parameters are negative in Cu ₃SbSe ₄, whereas almost all are positive in Cu ₃SbSe ₃ throughout their respective Brillouin zones. The average of the square of the Grüneisen parameter for the acoustic mode in Cu ₃SbSe ₃ is larger than that of Cu ₃SbSe ₄, which theoretically confirms that Cu ₃SbSe ₃ has a stronger lattice anharmonicity than Cu ₃SbSe ₄. The soft frequency and high Grüneisen parameters in Cu ₃SbSe ₃ arise from the electrostatic repulsion between the lone s2 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 in good agreement with the experimental measurements.