Thermal and vibrational properties of thermoelectric ZnSb: Exploring the origin of low thermal conductivity

Abstract

The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9--400 K) and powder x-ray diffraction (300--725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below $60\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{D}}=195\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{\ensuremath{\Theta}}}_{\mathrm{E}1}=78\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{E}2}=277\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as ${\mathrm{Zn}}_{4}{\mathrm{Sb}}_{3}$, CdSb, ${\mathrm{Cd}}_{4}{\mathrm{Sb}}_{3}, {\mathrm{Cd}}_{13\ensuremath{-}x}{\mathrm{In}}_{y}{\mathrm{Zn}}_{10}$, and ${\mathrm{Zn}}_{5}{\mathrm{Sb}}_{4}{\mathrm{In}}_{2\ensuremath{-}\ensuremath{\delta}}$.