We report powder neutron diffraction results on the crystal and magnetic st
ructures of the bilayer Ruddlesden-Popper phase Sr3Mn2O7-delta(delta = 0.0,
0.45) and correlate these structures with their magnetic and transport pro
perties. The delta = 0.45 compound contains a large number of oxygen vacanc
ies that are disordered in the MnO2 planes. As a result of this disordered
vacancy structure, Sr3Mn2O6.55 is a nonmagnetic insulator. Sr3Mn2O7.0(delta
= 0) is an antiferromagnetic insulator whose magnetic structure is related
to that of the SrMnO3 perovskite. Comparison of this end-member compound t
o its doped congeners in the La2-2xSr1+2xMn2O7 series highlights the extrem
e sensitivity of magnetic structure to dopant concentration in these layere
d materials. (C) 1999 American Institute of Physics. [S0021-8979(99)18608-0
].