Amorphous-crystalline transition in thermoelectric NbO₂

Density functional theory was employed to design enhanced amorphous NbO₂ thermoelectrics. The covalent-ionic nature of Nb–O bonding is identical in amorphous NbO₂ and its crystalline counterpart. However, the Anderson localisation occurs in amorphous NbO₂, which may affect the transport properties. We calculate a multifold increase in the absolute Seebeck coefficient for the amorphous state. These predictions were critically appraised by measuring the Seebeck coefficient of sputtered amorphous and crystalline NbO₂ thin films with the identical short-range order. The first-order phase transition occurs at approximately 550 °C, but amorphous NbO₂ possesses enhanced transport properties at all temperatures. Amorphous NbO₂, reaching  −173 μV K⁻¹, exhibits up to a 29% larger absolute Seebeck coefficient value, thereby validating the predictions. (paper)