Design of an Oxide Monolayer with High ZT 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 XO 2 (X = Ti, Zr, Hf) monolayer is replaced by the [Ag 6 ] 4+ cluster and a stable structure Ag 6 O 2 is achieved. Due to the abundant valence electrons in the [Ag 6 ] 4+ cluster unit, n-type Ag 6 O 2 has high electrical conductivity, which leads to a satisfactory power factor. More importantly, Ag 6 O 2 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 [Ag 6 ] 4+ cluster unit. Due to the satisfactory power factor and ultralow phonon thermal conductivity, Ag 6 O 2 has high ZT at 300-700 K, and the maximum ZT 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.