Prediction of NbXGe (X = Rh, Ir) half-Heusler semiconducting compounds with promising thermoelectric property using 18-electron rule

The electronic structure, phonons, and thermoelectric transport characteristics of NbXGe (X = Rh, Ir) half-Heusler compounds have been investigated using the first principles based on density functional theory. These alloys show semiconducting character with 18-valence electrons per unit cell following the Slater-Pauling rule. The band gaps of NbRhGe and NbIrGe are 0.65 and 0.63 eV, respectively. The good thermoelectric properties are achieved by applying lattice strains. NbRhGe obtains the ZT value of 0.87 at 300 K at the strain of - 6% gaining a double enhancement compared with the unstrained case, and NbIrGe obtains the ZT value of 0.74 at 300 K at the strain of 6% gaining a 12% enhancement compared with the unstrained case. Moreover, the NbXGe (X = Rh, Ir) half-Heusler compounds with a special quasi-random structure have been modeled to simulate the inevitable atomic occupation disorder between two sublattices. The maximum ZT values of NbIrGe and NbRhGe with SQS are 1.01 (300 K) and 0.83 (1250 K), respectively, which are higher than their corresponding structure of ordered occupation (0.66 and 0.78, respectively). Our results indicate that NbXGe (X = Rh, Ir) should be good thermoelectric candidate materials.