Low thermal conductivity in bournonite PbCuSbS3: A comprehensive study

Two natural bournonites have been characterized by structural, chemical, spectroscopical, magnetic and thermodynamic analyses. This study confirmed them to possess the PbCuSbS3 stoichiometric composition and to be of an outstanding quality allowing us to consider their properties as being intrinsic for this material. Electronic structure calculations, electrical and spectroscopical characterizations reveal PbCuSbS3 to be a direct n-type semiconductor with an energy gap Eopt g = 1.69 eV, huge Seebeck coefficient and electrical resistivity (e.g. ∼-1200 μV K−1 and ∼ 1000 Ω m at RT, respectively). The thermal conductivity in PbCuSbS3 is found to be very low [κ(T) ∼ 2-0.5 W m−1 K−1 in temperature range 100-600 K] and to be dominated by optical phonons (T > 100 K), which poorly transport heat, strongly scatter the acoustic ones and substantially intensify the phonon-phonon umklapp processes. Additionally, strong phonon-phonon scattering in bournonite is caused by many empty voids in its structural arrangement as well as by more than 60 Raman modes appearing in the frequency range of 150-250 cm−1 (∼5-8 THz) at RT. All these result in high Gruneisen parameter (Γ = 4.8-3.2) and very short phonon mean free path (lph = 11-3 ˚A for 100-300 K). Thus, the low thermal conductivity in bournonite is a reflection of combination of many different factors leading to a huge phonon anharmonicity.