Luminescence of Mn⁴⁺ in the orthorhombic perovskites, AZrO₃ (A=Ca, Sr)

The spectroscopic properties of the Mn⁴⁺ (3d³) ion in the orthorhombic perovskites ABO₃ (A = Ca, Sr; B[dbnd]Zr⁴⁺) is the subject of this investigation. The optical properties of the Mn⁴⁺ ion in CaZrO₃ is sensitive to oxygen stoichiometry. Comparison with the reported optical properties of oxygen-stoichiometric CaZrO₃:Mn⁴⁺ obtained by synthesizing in oxygen atmosphere indicates that the air sintered sample represents Mn⁴⁺ luminescence in an oxygen deficient perovskite lattice. The luminescence of SrZrO₃:Mn⁴⁺ suggests that this perovskite is less prone to the formation of oxygen vacancies. The difference between the energies of the zero-phonon line (²E→⁴A₂ transition; R-line) in CaZrO₃ and SrZrO₃ is related to the deviation of the octahedral O–Mn–O bond angle from the ideal 90° value. Comparison with isostructural perovskites ASnO₃ (A = Ca, Sr) shows that the energy of the ²E level depends on the covalent character of B–O bonding. The stronger covalence of Zr⁴⁺ 4d⁰- O²⁻ 2p bonding compared to that of Sn⁴⁺ 4d¹⁰ - O²⁻ 2p bonding is responsible for the higher energy position of the Mn⁴⁺²E level in the zirconate perovskite. Mulliken population analysis and bond order calculations are made to understand the nature of chemical bonding in the perovskites ABO₃ (A = Ca, Sr; B= Zr⁴⁺, Sn⁴⁺).