High thermoelectric performance near the Mott-Ioffe-Regel limit in CuxS0.6Te0.4 meta-phases

The meta-phase, characterized by crystal-amorphicity duality in the extremely special case, has garnered considerable interests due to its glass-like thermal conductivity and tunable conduction mechanisms for carriers. However, the impact of crystal-amorphicity duality on the electrical and thermal transports remains elusive, and the carrier concentration in the meta-phase is rarely tuned in a wide range for optimized thermoelectric properties. Herein, through a combination of molecular dynamic simulations, chemical bonding analysis, and structural characterization, we reveal the glass-like distribution of Cu cations modulated by the mismatched S/Te anions in CuxS0.6Te0.4 meta-phase. Such unique atomic structure leads to hopping conduction of carriers and extremely low thermal conductivity. The carrier concentration is effectively optimized by tuning the Cu content in CuxS0.6Te0.4. Finally, we achieve a high zT of 1.4 at 900 K for Cu2.01S0.6Te0.4 sample, which is three times larger than those of Cu2S and Cu2Te matrixes. Remarkably, the high zT value is attained near the Mott-Ioffe-Regel Limit where the carrier mean free path is close to the average atomic distance. Our findings shed light on a new route to discovering high-performance thermoelectric materials.