The identification of each phase by x-ray diffraction can be difficult, especially whentryingto dis- tinguish between the tetragonal and cubic structures, because of the overlapping peaks (the difference between the main peaks of each phase is only 0.079 in 20 degrees) One of the most accurate methods ofstructurerefinementand quan- titative analysis of Zro2 polymorphs is the Rietveld method (18], which was adopted in this case using the MAUD software. By using this tool, the contribution of each phase in the mixture is discriminated taking into account the crystallographic details earlier ascribed to the cubic and tetragonal structures. As mentioned earlier, the starting material was a mixture of monoclinic, tetragonal, and cubic structures, where the tetragonal and cubic phases were stabilized with 3 %mol of yttria. It was observed that ball milling seemed to destabilize those phases since a structural change resulted, which is evident in the x-ray diffrac- tion pattern shown in Fig. 2. This last result can be demonstrated by tak- ing into account the main peak of tetragonal Zro2 (30.198 whose