Design\nof an Oxide Monolayer with High <i>ZT</i> by a Strong Anharmonicity\nUnit

In\nthis paper, a new strategy to obtain a transition-metal oxide\n(TMO) thermoelectric monolayer is demonstrated. We show that the TMO\nthermoelectric monolayer can be achieved by the replacement of a transition-metal\natom with a cluster, which is composed of heavy transition atoms with\nabundant valence electrons. Specifically, the transition-metal atom\nin the XO<sub>2</sub> (X = Ti, Zr, Hf) monolayer is replaced by the\n[Ag<sub>6</sub>]<sup>4+</sup> cluster and a stable structure Ag<sub>6</sub>O<sub>2</sub> is achieved. Due to the abundant valence electrons\nin the [Ag<sub>6</sub>]<sup>4+</sup> cluster unit, n-type Ag<sub>6</sub>O<sub>2</sub> has high electrical conductivity, which leads to a\nsatisfactory power factor. More importantly, Ag<sub>6</sub>O<sub>2</sub> has an extremely low phonon thermal conductivity of 0.16 W·m<sup>–1</sup>·K<sup>–1</sup>, which is one of the lowest\nvalues in thermoelectric materials. An in-depth study reveals that\nthe extremely low value originates from the strong phonon anharmonicity\nand weak metal bond of the [Ag<sub>6</sub>]<sup>4+</sup> cluster unit.\nDue to the satisfactory power factor and ultralow phonon thermal conductivity,\nAg<sub>6</sub>O<sub>2</sub> has high <i>ZT</i> at 300–700\nK, and the maximum <i>ZT</i> is 3.77, corresponding to an\nenergy conversion efficiency of 22.24%. Our results demonstrate that\nreplacement of the transition-metal atom by an appropriate cluster\nis a good way to obtain a TMO thermoelectric monolayer.