Superior Thermoelectric Performance of Ordered Double Transition Metal MXenes: Cr₂TiC₂T₂ (T = -OH or -F)

Using SCAN-rVV10+U, we show Cr₂TiC₂ and Cr₂TiC₂T₂ (T = -F and -OH) MXenes are moderate band gap semiconductors mostly in the antiferromagnetic state. All investigated MXene structures show large Seebeck coefficients (>400 μV/K), especially Cr₂TiC₂ (>800 μV/K) and Cr₂TiC₂F₂ (>700 μV/K). The hole relaxation time of p-type Cr₂TiC₂(OH)₂ is found to be ∼8 ps, ensuring its superior electron transport properties in comparison to other investigated MXenes. It is also discovered that the surface functionalization could decrease the phonon thermal conduction and that Cr₂TiC₂(OH)₂ has the smallest lattice thermal conductivity (∼6.5 W/m·K) and the largest electron thermal conduction (>50 W/m·K with n = 10¹⁹ cm^-3). We predict the ZT value of p-type Cr₂TiC₂(OH)₂ can reach 3.0 at 600 K with the maximum thermoelectric conversion efficiency of 20%. Overall, the thermoelectric property of Cr-based ordered double transition metal MXenes is far superior to that of any known two-dimensional structures in the MXene family.