Impact of oxygen partial pressure on the Ruddlesden-Popper series Nd2-2xSr1+2xMn2O7: Oxygen vacancy formation and ordering

The phase diagram of the Nd2-2xSr1+2xMn2O7+delta n = 2 Ruddlesden-Popper se ries has been investigated as a function of composition and oxygen partial pressure at 1400 degreesC. The regions of phase stability for 0.10 less tha n or equal to x less than or equal to 0.70 and 10(-6) less than or equal to p(O-2)/atm less than or equal to 1.0 have been mapped out and a systematic relationship between x and oxygen partial pressure established. At high p( O-2) (near 1 atm) the upper solubility limit is fixed by an average Mn oxid ation state of +3.5, while the lower limit of stability is a strong functio n of p(O-2). Phases synthesized at low p(Oa) are associated with the introd uction of oxygen vacancies that can be filled at low temperature to yield m etastable phases with formal Mn oxidation states beyond +3.5. The oxygen va cancy structures are highly dependent on the Nd/Sr ratio, with the location of the vacancies changing from in-plane to axial as a function of both x a nd temperature. Systematic study of the Mn-O bonding framework reveals a su btle dependence of the crystal structure on both the Mn oxidation state and vacancy location. The unusual crystal chemistry of these phases suggests a more general synthetic technique for accessing layered manganite phases as potential CMR candidates.