Design of an Oxide Monolayer with High <i>ZT</i> by a Strong Anharmonicity Unit

In this paper, a new strategy to obtain a transition-metal oxide (TMO) thermoelectric monolayer is demonstrated. We show that the TMO thermoelectric monolayer can be achieved by the replacement of a transition-metal atom with a cluster, which is composed of heavy transition atoms with abundant valence electrons. Specifically, the transition-metal atom in the XO₂ (X = Ti, Zr, Hf) monolayer is replaced by the [Ag₆]⁴⁺ cluster and a stable structure Ag₆O₂ is achieved. Due to the abundant valence electrons in the [Ag₆]⁴⁺ cluster unit, n-type Ag₆O₂ has high electrical conductivity, which leads to a satisfactory power factor. More importantly, Ag₆O₂ has an extremely low phonon thermal conductivity of 0.16 W·m^-1·K^-1, which is one of the lowest values in thermoelectric materials. An in-depth study reveals that the extremely low value originates from the strong phonon anharmonicity and weak metal bond of the [Ag₆]⁴⁺ cluster unit. Due to the satisfactory power factor and ultralow phonon thermal conductivity, Ag₆O₂ has high <i>ZT</i> at 300-700 K, and the maximum <i>ZT</i> is 3.77, corresponding to an energy conversion efficiency of 22.24%. Our results demonstrate that replacement of the transition-metal atom by an appropriate cluster is a good way to obtain a TMO thermoelectric monolayer.