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

Abstract

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.%.