*

Non-stoichiometric oxides have attracted tremendous interest for their promising potential for applications as materials with reversible oxygen storage/release capabilities. The development of oxygen storage materials is critical for the growth of new energy related technologies such as oxy-fuel and chemical looping combustion and reforming for clean coal energy and synthesis gas production.  We have recently reported superior oxygen storage/release performances displayed by rare-earth hexagonal manganites, namely RMnO_3+d,(where R is a rare earth elements or Y).  These materials are capable of storing as much as 2000 μmol-O/g when annealed in air.  Large changes of oxygen content occur on cycling at unusually low but desirable temperatures between 250 and 350° C.  Tb-substituted hexagonal Y_1-xTb_xMnO_3+d materials are singled out because of their capability to hold even more oxygen (up to 0.5 oxygen atom per unit formula) with the drastic enhancement causing significant distortions of the structural symmetry.  Neutron data enabled the determination of an elegant solution of a previously unseen superlattice which is four times larger than that of the pristine YMnO₃ parent material.  Detailed discussions of the structure and materials properties will be presented.