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In this paper we describe a detailed neutron diffraction investigation of thecrystal and magnetic structure of two layered CMR manganitesLa1·2Sr1·8Mn2O7(x = 0·4) andLa1·4Sr1·6Mn2O7(x = 0·3). In these materials of reduceddimensionality compared to the 3D perovskites, we find competing effectsbetween charge-lattice and spin degrees of freedom. These effects can beinvestigated by studying the behaviour of crystal and magnetic structure as afunction of temperature, composition and hydrostatic pressure. We findopposite lattice responses to the onset of charge delocalisation and magneticordering in these two layered compounds. Below the insulator-to-metaltransition (TIM), the lattice response suggests that charge is transferred tod3z2-r2orbitals inLa1·2Sr1·8Mn2O7and todx2-y2orbitals inLa1·4Sr1·6Mn2O7.We argue that these changes are too large to be due to chemical differences.Instead we suggest that the orbital configuration of the Mn ion below TIM issensitive to electronic doping. InLa1·2Sr1·8Mn2O7we find that the lattice response at TIM to be driven by lattice displacementsthat relax below TIM, consistent with polaronic degrees of freedom. We alsonote that the competition between super- and double-exchange to be significantin reduced dimensions. This is manifested in the change in the sign of theapical Mn-O bond compressibilities above and below TIM. Finally, we describethe magnetic structure of these two different layered manganites. We find thatelectronic doping also results in significant changes to the orderedarrangement of Mn spins. Interestingly the magnetism in reduced dimensions inthese materials can be varied from relative simple structures that showferromagnetic inter-bilayer coupling as observed inLa1·2Sr1·8Mn2O7to structures with antiferromagnetic inter-bilayer coupling as found inLa1·4Sr1·6Mn2O7.

Structure and Magnetism in the Layered CMR Manganites La2-2xSr1+2xMn2O7 (x= 0·3, 0·4)