Phonon anharmonicity of thermoelectric material HfTe₅ studied by Raman spectroscopy

Abstract

The understanding of phonon–phonon anharmonic effect in HfTe5 is essential not only for fundamental scientific interest but also for its potential thermoelectric applications. Here, rectangular ribbon‐shaped HfTe5 single crystals have been grown. Raman spectroscopy is further utilized to investigate the phonon anharmonicity of HfTe5 by quantifying the temperature dependence of the phonon mode softening and broadening. We focus on the temperature range from 80 up to 400 K since obvious surface oxidation occurs at higher temperatures. It is found that four phonon anharmonic effects are non‐negligible. The phonon lifetime is greatly reduced by the enhancement of phonon scattering, resulting in the low thermal conductivity in HfTe5 single crystals.

Thermal conductivity and phonon anharmonicity of chemical vapor transport grown and mineral–FeS2single crystals: An optothermal Raman study

“…For the E g mode, it is below 126 (mineral-FeS 2 ) and below 136 K (CVT-FeS 2 ). Such a behavior (dominant three-phonon scattering at lower temperatures) is also reported for some modes of other materials such as HfTe 5 [39] and ZrTe 5 . [48] Furthermore, the temperature where the dominant anharmonic mechanism switches from three-phonon scattering to thermal expansion depends on the sample.…”

“…The phonon anharmonicity contribution derived from the Balkanski–Klemens model (Equation ) is expressed as three‐phonon (Term 1) and four‐phonon decay processes (Term 2) in which A and B are the cubic and quartic anharmonic constants. [ 39 ] The power parameters m and n in the Equation are and , respectively. The three‐phonon process assumes that an optical phonon decays into two acoustic phonons with equal wavenumbers ( ), whereas in the four‐phonon process an optical phonon decays into three acoustic phonons.…”

“…In Equation , is the Raman mode Grüneisen parameter, and is the temperature‐dependent linear thermal expansion coefficient. [ 35,39 ] Temperature‐dependent was provided by Zuñiga‐Puelles et al as ( a 1 + 2 a 2 T )/ a ( T ). [ 3 ] The a ( T ) is given as a second‐order polynomial in the form of a 0 + a 1 T + a 2 T 2 , and a 0 (5.41 Å), a 1 (8 × 10 −6 Å/K), and a 2 (4.7 × 10 −8 Å/K 2 ) are the coefficients of the polynomial.…”

Probing the Spatial Homogeneity of Exfoliated HfTe5 Films

“…We note that special care has to be taken in choosing an appropriate laser power, in particular for confocal microscopy with small laser spot sizes and high power densities. By comparing the temperature dependence to the laser power dependence, we find that under 1 mW excitation, which is a common value from literature, 26 , 27 the local temperature within the sample volume probed by the laser spot can be up to 100 K above the substrate temperature ( Figure S1 ). In turn, the determined positions of the Raman peaks can deviate by several wavenumbers from their true value because of the large discrepancy between the substrate and crystal temperature.…”