Defect engineering and triangular copper as key drivers for ultralow thermal conductivity in Cu2ZrS3 polytypes

Abstract

Copper-rich sulfides are promising materials for energy-conversion applications due to their environmental compatibility, cost-effectiveness, and earth abundance. Their structural diversity and chemical flexibility offer a fantastic playground for the discovery of new structures and compounds, with various properties. We demonstrate that the presence of triangular copper in the ordered CdI2-type Zr2/3Cu1/3S2 layers of Cu2ZrS3 induces a high potential of this sulfide for polytypism. Two new layered polytypes, P31c(ABCB) and R3(ABC), are synthesized. Based on a comprehensive analysis by single-crystal X-ray diffraction, 3D-ED and HRSTEM analyses, we highlight that these polytypes differ by their stacking sequences while sharing identical tetrahedral copper layers and ordered Zr2/3Cu1/3S2 layers. Both polytypes are characterized by large anisotropic vibration of copper in triangular coordination. Low-temperature heat capacity and Raman spectroscopy, supported by phonon calculations, identify low-energy optical phonons associated with Cu atoms, which couple strongly to acoustic modes, yielding to non-Debye behavior and extremely short phonon lifetimes. The R3 polytype displays intrinsically ultralow lattice thermal conductivity, reaching 0.35 Wm-1K-1 at 673K, among the lowest reported in sulfides. Our findings provide new insights into the complex crystal chemistry and polytypism of copper-rich sulfides in relation with vibrational properties, opening new perspectives for the exploration of these materials.