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) (Sb,As) S 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.

Low-energy optical phonons induce glassy-like vibrational and thermal anomalies in ordered crystals

“…In particular, [21] has reported experimental observations of a strong boson peak in perfectly ordered crystals of halomethanes due to low-lying optical modes, whereas [22,23] experimentally observed the upturn of the VDOS predicted here in organic molecular systems with high degree of crystallinity (and presumably very soft optical modes) such as starch and glucose. Similar behaviors have been measured also in thermoelectric crystals [24,25] where a boson peak in the VDOS is also observed, and where, interestingly, low-energy vibrations (so-called rattling) of caged compounds give rise to an upturn in the VDOS at vibrational energies below the boson peak [18].…”

“…In conclusion, the above theoretical calculation shows that low-energy optical phonons in ordered crystals are responsible for glassy-like anomalies in the VDOS (the so-called boson peak) and in the low-T specific heat. Also, the model shows that, for low-lying optical modes, the normalized VDOS may no longer be flat at w  0 but may exhibit an upturn, as shown recently in some systems [18,22,23], including thermoelectric crystals [18] where low-energy optical modes are important.…”

An Integrated Approach to Thermoelectrics: Combining Phonon Dynamics, Nanoengineering, Novel Materials Development, Module Fabrication, and Metrology

“…[19,20] (Ba,Sr) 8 Ga 16 Ge 30 comprises (Ga,Ge) 24 tetrakaidecahedral cages and (Ga,Ge) 20 dodecahedral cages filled with (Ba,Sr) atoms. In particular, the (Ga,Ge) 24 cages are oversized compared to the sizes of Ba and Sr atoms. The interatomic force constants of Ba(Ga,Ge) and Sr(Ga,Ge) pairs are 0.011-0.014 and 0.007-0.009 mdyn Å −1 , respectively, which are dramatically smaller than those of (Ga,Ge)(Ga,Ge) pairs exhibiting 0.6-0.8 mdyn Å −1 .…”

“…In particular, the (Ga,Ge) 24 cages are oversized compared to the sizes of Ba and Sr atoms. Clearly, Ba and Sr atoms are loosely bounded within the (Ga,Ge) 24 oversized cages and behave as rattlers. Clearly, Ba and Sr atoms are loosely bounded within the (Ga,Ge) 24 oversized cages and behave as rattlers.…”

“…[21,22] In the compounds, Cu atoms located in S 3 triangles vibrate largely out of plane, although there is no oversized cage. [24] In tetrahedrites, the rattling motions are observed around E = 3 meV. [21,23] However, it is not obvious whether the rattling dynamics are similar to those of cage compounds, because the local lattice structure around the rattling atoms is quite different.…”