Impact of noble-gas filler atoms on the lattice thermal conductivity of CoSb₃ skutterudites: first-principles modelling

Abstract

We present a systematic first-principles modelling study of the structural dynamics and thermal transport in the CoSb3 skutterudites with a series of noble-gas filler atoms. A range of analysis techniques are proposed to estimate the filler rattling frequencies, to quantify the separate impacts of filling on the phonon group velocities and lifetimes, and to show how changes to the phonon spectra and interaction strengths lead to suppressed lifetimes. The fillers are found to reduce the thermal conductivity of the CoSb3 framework by up to 15 % primarily by suppressing the group velocities of low-lying optic modes. Calculations show that the filler rattling frequencies are determined by a detailed balance of increasing atomic mass and stronger interactions with the framework, and are a good predictor of their impact on the heat transport. Lowering the rattling frequency below ~1.5 THz by selecting heavy fillers that interact weakly with the framework is predicted to produce a much larger suppression of the thermal transport, by inducing avoided crossings in the acoustic-mode dispersion and facilitating resonant scattering with a consequent large reduction in the lifetimes. Approximate rattling frequencies determined from the harmonic force constants may therefore provide a useful metric for selecting filler atoms to optimise the thermal transport in skutterudites and other cage compounds such as clathrates.

Approximate models for the lattice thermal conductivity of alloy thermoelectrics

“…8 reproduces the provides a useful metric for comparing the relative phonon-phonon interaction latt strengths in different materials. 34 From the relationships in Eqs. 2, 5 and 8, the is inversely latt proportional to , and a suitable value can be estimated from a simple linear fit (Fig.…”

Approximate Models for the Lattice Thermal Conductivity of Alloy Thermoelectrics

“…However, the temperature variation in the ̃ obtained by fitting, as in Fig. 4, was found to be on the order of 1 % between 300 and 600 K in CoSb3, 34 so we do not expect this to be a large source of error.…”

“…39 In this work, we extend these studies to quantify the thermal transport in the Se-rich Sn(S0.1875Se0.8125). Using analysis techniques developed in our previous work, 34 we show that the difference in the latt of the SnS and SnSe endpoints is a balance of reduced phonon group velocities and longer lifetimes. We find that Sn(S0.1875Se0.8125) shows near-ideal mixing behaviour, with the mixing free energy dominated by entropy and a near-homogenous distribution of chalcogen atoms over the lattice sites.…”

“…To investigate the difference in the latt of SnS and SnSe, we used the constant relaxationtime approximation (CRTA) model developed in Ref. 34 in which the are replaced with a constant…”

Structural Dynamics and Thermal Transport in Bismuth Chalcogenide Alloys

“…As for latt the CRTA ⁄ are tensors, and the CRTA may be taken as a scalar, like the , or as a tensor to provide some flexibility to account for anisotropy in the averaged mode lifetimes. 46,70 Table 5 Constant relaxation-time approximation (CRTA) analysis of the room-temperature averaged thermal conductivities ave of Bi2S3, Bi2Se3 and Bi2Te3 in the Pnma and R3 ̅ m phases according to Eq. 7 in the text.…”