Size Effect on the Thermal Conductivity of a Type-I Clathrate

• Published in: Crystals (Basel) 2023-03, Vol.13 (3), p.453
• Main Authors: Lužnik, Monika, Lientschnig, Günther, Taupin, Mathieu, Steiger-Thirsfeld, Andreas, Prokofiev, Andrey, Paschen, Silke
• Format: Article
• Language: English
• Subjects: Acoustics; Clathrate compounds; Clathrates; Conductivity; Diameters; Energy; Heat conductivity; Heat transfer; Hot pressing; Hypotheses; Ion beams; Lanthanum; mesowires; Microscopy; Phonons; Radiation; Room temperature; Single crystals; Size effects; Spectrum analysis; Temperature; Thermal conductivity; thermoelectrics
• ISSN: 2073-4352; 2073-4352
• DOI: 10.3390/cryst13030453

Clathrates are a materials class with an extremely low phonon thermal conductivity, which is a key ingredient for a high thermoelectric conversion efficiency. Here, we present a study on the type-I clathrate La1.2Ba6.8Au5.8Si38.8□1.4 directed at lowering the phonon thermal conductivity even further by forming mesoscopic wires out of it. Our hypothesis is that the interaction of the low-energy rattling modes of the guest atoms (La and Ba) with the acoustic modes, which originate mainly from the type-I clathrate framework (formed by Au and Si atoms, with some vacancies □), cuts off their dispersion and thereby tilts the balance of phonons relevant for thermal transport to long-wavelength ones. Thus, size effects are expected to set in at relatively long length scales. The structuring was carried out using a top-down approach, where the wires, ranging from 1260 nm to 630 nm in diameter, were cut from a piece of single crystal using a focused ion beam technique. Measurements of the thermal conductivity were performed with a self-heating 3ω technique down to 80 K. Indeed, they reveal a reduction of the room-temperature phonon thermal conductivity by a sizable fraction of ∼40 % for our thinnest wire, thereby confirming our hypothesis.