Low thermal conductivity in bournonite PbCuSbS3 : A comprehensive study

Two natural bournonite specimens have been characterized by structural, chemical, spectroscopical, magnetic, and thermodynamic analyses. This study confirms a stoichiometric composition of ${\mathrm{PbCuSbS}}₃$ and the crystal to be of an outstanding quality allowing us to consider its properties as being intrinsic for this material. Electronic structure calculations, electrical resistivity measurements, and spectroscopical characterizations reveal ${\mathrm{PbCuSbS}}₃$ to be a direct $n$-type semiconductor with an optical energy gap ${E}_g^{\mathrm{opt}}$ = 1.69 eV, huge Seebeck coefficient and electrical resistivity (e.g., $\ensuremath{\sim}\ensuremath{-}1200 \ensuremath{\mu}\mathrm{V}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and $\ensuremath{\sim}1000 \mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ at room temperature, respectively). The thermal conductivity in ${\mathrm{PbCuSbS}}₃$ is found to be very low [$\ensuremath{\kappa}(T)\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}2--0.5 \mathrm{W}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in the temperature range 100--600 K] and is 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, large atomic displacement parameters and the presence of low-energy optical phonons have been identified, evidencing ``rattling'' effects associated to Cu and Pb atoms in the crystal structure. All these result in high Gr\"uneisen parameters (${\ensuremath{\gamma}}_{\mathrm{G}}$ = 4.8--3.2) and very short phonon mean free path (${l}_{\mathrm{ph}}$ = 11--3 \AA{} for 100--300 K). Thus, the low thermal conductivity in bournonite reflects a combination of different factors leading to a huge phonon anharmonicity.