Solid state syntheses of 12SrO·7Al₂O₃ and formation of high density oxygen radical anions, O_- and O ₂^-

A new synthetic route via a solid-state reaction, the crystal structure of 12SrO·Al₂O₃ (S₁₂A₇) incorporated with OH^- ions, and the formation of oxygen radical anions in S₁₂A₇ have been examined. Investigations using X-ray powder diffraction and thermogravimetric and evolved gas combined analyses demonstrate that homogenization promoted by molten Sr(OH)₂ hydrates and the presence of Sr-hydrogarnet precursors play crucial roles in the formation of S₁₂A₇. Rietveld analyses confirm the lattice framework of S₁₂A₇ has a structure identical to that of the nanoporous crystal 12CaO·7Al₂O₃ (C ₁₂A₇, a mayenite structure) and demonstrate that two Sr²⁺ ions (pole Sr²⁺ ions) on the S₄ axis of a cage displace toward the cage center owing to the accommodation of an OH ^- ion. An electron paramagnetic resonance study demonstrates that oxygen radical anions, O₂^- and O^-, are generated in the cages with concentrations on the order of 10²⁰ cm^-3 each by oxygen annealing. From the g-values observed at 77 K, the oxygen radical anions are interpreted as being sandwiched between the two pole Sr²⁺ ions rather than adsorbed on one side.