The atomic-scale structure of nanocrystalline ZrO2 obtained by ball milling has been studied using highenergy x-ray diffraction and the atomic pair distribution function technique. The studies show that, upon relatively short milling times, the parent crystalline material, monoclinic ZrO2, evolves into a nanocrystalline phase that is locally similar to monoclinic zirconia but shows a cubic-type...