Phonon anharmonicity in Cu-based layered thiophosphates

TL;DR

This study investigates phonon anharmonicity in layered Cu-based thiophosphates using temperature-dependent Raman spectroscopy, revealing dominant anharmonic effects and rapid phonon decay, with implications for thermoelectric applications.

Contribution

It provides the first detailed analysis of phonon anharmonicity and decay mechanisms in Cu-based layered thiophosphates, highlighting the role of three- and four-phonon processes.

Findings

Phonon decay dominated by anharmonic effects rather than thermal expansion.

Optical phonon lifetimes less than 1 ps indicate high scattering rates.

Different phonon decay processes observed in various compounds.

Abstract

In this work, we performed temperature-dependent Raman spectroscopy studies on three layered metal thiophosphates: CuInP2S6, CuCrP2S6, and CuInP2S6-In4/3P2S6 heterostructures. These materials are emerging multiferroics, with CuInP2S6 and CuInP2S6-In4/3P2S6 exhibiting room temperature ferrielectric polarization while CuCrP2S6 is both anti-ferroelectric and antiferromagnetic at low temperatures (<145 and <30 K, respectively). We studied two prominent peaks in the Raman spectra of these compounds, namely their P-P and P-S stretching modes. First order linear fits to the mode softening yielded coefficients similar to other layered materials. Furthermore, analysis of the phonon decay revealed the dominant role of anharmonic effects rather than thermal expansion. In particular, the decay of the optical phonons was found to be governed by three-phonon processes in CuInP2S6 and CuCrP2S6 and by the four-phonon process in the CuInP2S6-In4/3P2S6 heterostructures. These phonon lifetimes were found to be less than 1 ps, indicating high scattering rates and showing the potential for these materials to be used in thermoelectric applications.