Dimer rattling mode induced low thermal conductivity in an excellent acoustic conductor

TL;DR

This study reveals that CuP2 exhibits unexpectedly low lattice thermal conductivity despite high sound speeds, due to a rattling mode from Cu dimers that strongly scatters phonons, offering new insights into thermal transport control.

Contribution

It uncovers a novel mechanism where dimer rattling modes in layered structures suppress thermal conductivity without atomic disorder.

Findings

CuP2 has high sound speeds similar to GaAs.

CuP2 exhibits very low thermal conductivity (~4 W/m·K).

Dimer rattling modes strongly scatter phonons, reducing thermal transport.

Abstract

A solid with larger sound speeds exhibits higher lattice thermal conductivity (k_{lat}). Diamond is a prominent instance where its mean sound speed is 14400 m s-1 and k_{lat} is 2300 W m-1 K-1. Here, we report an extreme exception that CuP2 has quite large mean sound speeds of 4155 m s-1, comparable to GaAs, but the single crystals show a very low lattice thermal conductivity of about 4 W m-1 K-1 at room temperature, one order of magnitude smaller than GaAs. To understand such a puzzling thermal transport behavior, we have thoroughly investigated the atomic structure and lattice dynamics by combining neutron scattering techniques with first-principles simulations. Cu atoms form dimers sandwiched in between the layered P atomic networks and the dimers vibrate as a rattling mode with frequency around 11 meV. This mode is manifested to be remarkably anharmonic and strongly scatters acoustic phonons to achieve the low k_{lat}. Such a dimer rattling behavior in layered structures might offer an unprecedented strategy for suppressing thermal conduction without involving atomic disorder.