Intense Turquoise and Green Colors in Brownmillerite-Type Oxides Based on Mn⁵⁺ in Ba₂In_2–<i>x</i>Mn_<i>x</i>O_5+<i>x</i>

Brownmillerite-type oxides Ba2In2–xMnxO5+x (x = 0.1–0.7) have been prepared and characterized. Magnetic measurements indicate that manganese in as-prepared samples is substituting predominantly as Mn5+ for all values of x with observed paramagnetic spin-only moments close to values expected for two unpaired electrons. Electron paramagnetic resonance measurements indicate that this Mn5+ is present in a highly distorted tetrahedral environment. Neutron diffraction structure refinements show that Mn5+ occupies tetrahedral sites for orthorhombic (x = 0.1) and tetragonal (x = 0.2) phases. For Mn ≥ 0.3 samples, neutron refinements show that the phases are cubic with disordered cations and oxygen vacancies. The colors of the phases change from light yellow (x = 0) to intense turquoise (x = 0.1) to green (x = 0.2, 0.3) or to dark green (x ≥ 0.4). Under reducing conditions, Mn5+ is reduced to Mn3+, and Ba2In2–xMnxO5+x phases become black Ba2In2–xMnxO5 phases still with the brownmillerite structure.