Metal-insulator transition and anomalous lattice parameters changes in Ru-doped VO2

VO2, of interest for decades due to both its phenomenology and its potential applications, has a monoclinic distortion of the rutile crystal structure at ambient temperature that is coupled to its metal-insulator transition. In contrast, RuO2 has three electrons more per formula unit, is a metallic conductor, and has an undistorted rutile structure. Here, we report a systematic study of Ru-doped VO2 (V1-xRuxO2, 0.01≤x≤0.9), generally characterizing its crystal structure, magnetic and electronic properties, and heat capacity. The composition-dependent Wilson ratio is determined. We find that an unusually high Ru doping value (80%, x=0.8) is required to achieve a metallic state in V1-xRuxO2. No superconductivity was observed down to 0.1 K in the metallic materials. We propose a possible understanding for how the insulating state can exist in V1-xRuxO2 at high Ru contents.