Crystal Chemistry of Mo_xW_1-xO₂ Rutile Solid Solutions

Binary distorted rutile oxides with metal-metal bonds continue to attract significant interest due to the intriguing metal-to-insulator transitions observed in VO₂ and NbO₂, which involve an electro-structural phase transition, and MoO₂ and WO₂, which exhibit notable topological features, exceptional transport properties, and pronounced magnetoresistance. The mixed-metal solid solutions (e.g. Mo_xV₁₋ₓO₂ or Nb_xV₁₋ₓO₂) exhibit even greater complexity, which has led to open questions on how parameters such as metal size and valence impact the electrostructural coupling.

This study reports the Mo_xW₁₋ₓO₂ (0 ≤ x ≤ 1) solid solution for the first time. By developing a method for growing high-quality single crystals of Mo_xW₁₋ₓO₂, we found that the M1 monoclinic symmetry is maintained across the compositional range. This sets the Mo-W system apart from any of the other known solid solutions (Nb-V, Mo-V, W-V, and Nb-Mo), where metal-metal dimer formation is suppressed for some range of compositions. Details in the smoothly varying bonding interactions in the Mo_xW_1-xO₂ system provide unique insights into how subtle changes in the metal-oxygen environment influence the electronic properties.