Mixed anion control of negative thermal expansion in a niobium oxyfluoride

A significant change in thermal expansion with anion composition has been identified in the niobium oxyfluoride, NbO_2-F₁₊ from 0 < x < 0.6. Fluorine doping leads to a transition from positive thermal expansion to unusual zero and negative thermal expansion caused by transverse anionic vibrations. This work has consequences for the development of advanced technological materials with tuneable low thermal expansion and is the first example of the use of multiple anions to control thermal expansion.