Manganese ions in the monoclinic, tetragonal and cubic phases of zirconia: an XRD and EPR study

Mn-doped monoclinic, tetragonal and cubic zirconia samples were characterized by XRD and EPR. The cubic modification was obtained by doping hydrous zirconia with Y2O3 (YSZ). The ZrO2 structure influences the solid solution formation and the nature of manganese species. EPR analysis revealed the following manganese paramagnetic species: isolated Mn4+ and Mn2+ in the monoclinic phase; isolated Mn2+ in the tetragonal phase; isolated Mn2+ in the cubic YSZ; and clustered Mn2+ in Mn3O4 and in MnO particles on the zirconia surface. Quantitative EPR suggested Mn3+ in all zirconia phases. After heating in air, Mn3+ and Mn4+ ions entered the monoclinic zirconia phase at 1273–1623 K and surface Mn3O4 particles formed, whereas Mn3+ and Mn2+ entered the tetragonal zirconia phase at 973–1173 K and at 1623 K into cubic YSZ. In all zirconia phases, subsequent heating in H2 at 773–973 K reduced Mnn+ ions to Mn2+ and converted Mn3O4 particles on the surface into MnO. In monoclinic zirconia heated in air at 1623 K the Mn-solubility limit was 0.2 wt.%.