Retreat from Stress: Rattling in a Planar Coordination

Abstract

Thermoelectric devices convert heat flow to charge flow, providing electricity. Materials for highly efficient devices must satisfy conflicting requirements of high electrical conductivity and low thermal conductivity. Thermal conductivity in caged compounds is known to be suppressed by a large vibration of guest atoms, so-called rattling, which effectively scatters phonons. Here, the crystal structure and phonon dynamics of tetrahedrites (Cu,Zn)₁₂(Sb,As)₄S₁₃ are studied. The results reveal that the Cu atoms in a planar coordination are rattling. In contrast to caged compounds, chemical pressure enlarges the amplitude of the rattling vibration in the tetrahedrites so that the rattling atom is squeezed out of the planar coordination. Furthermore, the rattling vibration shakes neighbors through lone pairs of the metalloids, Sb and As, which is responsible for the low thermal conductivity of tetrahedrites. These findings provide a new strategy for the development of highly efficient thermoelectric materials with planar coordination.

The rattling of Cu atoms in tetrahedrites originates from the chemical pressure inherent in the trigonal-planar environment. When the chemical pressure is increased with reduction of the S₃ triangle area, the rattling amplitude is enlarged. The rattling vibration can shake neighboring metalloid atoms via assistance from the lone pairs of the metalloids.

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