The crystallographic and magnetic phase diagram of the nϭ2 layered manganite La 2Ϫ2x Sr 1ϩ2x Mn 2 O 7 in the region xу0.5 has been studied using temperature-dependent neutron powder diffraction. The magnetic phase diagram reveals a progression of ordered magnetic structures generally paralleling that of three-dimensional ͑3D͒ perovskites with similar electronic doping: A (0.5рxр0.66)→C (0.75рxр0.90)→G (0.90рxр1.0). However, the quasi-2D structure amplifies this progression to expose features of manganite physics uniquely accessible in the layered systems: ͑i͒ a ''frustrated'' region between the A and C regimes where no long-range magnetic order is observed; ͑ii͒ magnetic polytypism arising from weak interbilayer magnetic exchange in the type-C regime; and ͑iii͒ a tetragonal-to-orthorhombic phase transition whose temperature evolution directly measures ordering of d 3y 2 Ϫr 2 orbitals in the a-b plane. This orbital-ordering transition is precursory to type-C magnetic ordering, where ferromagnetic rods lie parallel to the b axis. These observations support the notion that e g orbital polarization is the driving force behind magnetic spin ordering. Finally, in the crossover region between type-C and type-G states, we see some evidence for the development of local type-C clusters embedded in a type-G framework, directly addressing proposals of similar short-range magnetic ordering in highly doped La 1Ϫx Ca x MnO 3 perovskites.

The preparation and crystal structures of the n = 2 Ruddlesden−Popper phases Sr2 - x Ln1+ x Mn2O7 (0 ≤ x ≤ 0.5, Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, and Er) are described. The crystal chemistry and stability of this structure is governed by the size of the lanthanide cation. Partial ordering of the Sr2+ and Ln3+ cations occurs between the two available A cation (A = Ln3+, Sr2+) sites, with the smaller lanthanides preferring the site in the rock-salt layer over that in the perovskite block. This ordering is almost complete for the small lanthanides (Tb−Er), and these ordered compounds can be prepared as single phases. Cation disorder in compounds of the larger lanthanides is accompanied by a subtle separation into two n = 2 Ruddlesden−Popper phases, which is apparent only upon detailed inspection of Rietveld refinements of the X-ray profiles. In these cases, the two-phase model is found to be superior to a single phase model with strain broadening included. For a particular lanthanide, both the ease of synthesis of single phases and the extent of cation ordering depend on the manganese oxidation state.

The crystal and magnetic structures of the n = 3 Ruddlesden−Popper phase with the ideal composition Ca4Mn3O10 have been studied using X-ray and neutron powder diffraction. The crystal structure at 293 K is relatively insensitive to the partial pressure of oxygen used in sample preparation. A sample prepared in air showed an orthorhombic distortion (space group Pbca, a = 5.26557(12), b = 5.26039(11), c = 26.8276(5) Å) from the ideal n = 3 RP structure, as did a sample prepared under 800 atm of O2 pressure (a = 5.26005(4), b = 5.25569(4), c = 26.83543(20) Å). Both samples showed a magnetic phase transition at 115 K from a paramagnetic phase with extensive short-range spin ordering to a weakly ferromagnetic (μferro = 2 × 10-3 μB per Mn) low-temperature phase. The antiferromagnetic components of the atomic magnetic moments (2.23(2) μB per Mn) order in a G-type manner within each perovskite block, and the interblock coupling reflects the orthorhombic symmetry of the structure.

Magnetization and magnetotransport measurements have been used to study the composition dependence of the electronic properties of the Ruddlesden-Popper phases Sr 2 NdMn 2 O 7 and Sr 1.9 Nd 1.1 Mn 2 O 7 . Although their behaviour differs in detail, both compounds show a colossal magnetoresistance (CMR) effect (>10 000% in 14 T) in the temperature range 4.2 T /K 100. However, neither material shows a transition to a ferromagnetic state above 4.2 K, and both materials have higher resistivities (>10 3 cm for 4.2 T /K 100) than the metallic oxides previously found to show CMR. In view of the low conductivity and the absence of ferromagnetism, the CMR of these phases is not readily explained by a doubleexchange mechanism.

Neutron powder diffraction experiments on layered manganites La 222x Sr 112x Mn 2 O 7 ͑0.32 # x # 0.40͒ show that coherent lattice anomalies observed at the Curie temperature (lattice parameters, Mn-O bond lengths) reverse sign at x ഠ 0.36, coinciding with the maximum T C ഠ 131 K. Remarkably, the anomalies are completely suppressed at this crossover composition. Incoherent distortions, as measured by anisotropic Debye-Waller factors, undergo an abrupt decrease below T C . The magnitude of this anomaly changes very little with x and does not display any sign reversal for x ഠ 0.36. PACS numbers: 75.30.Vn, 61.50.Ks Since the discovery of colossal magnetoresistance (CMR) in perovskite-related manganese oxides [1], there is increasing experimental and theoretical evidence pointing to the need to augment the simple double exchange picture for explaining the magnitude of the MR [2]. Because of the lattice anomalies observed pervasively in CMR materials, interest has focused on the role of the lattice degrees of freedom in the CMR mechanism. The relevance of this point, first raised by Millis and co-workers in 1995 [3], has been recognized by many subsequent theoretical [4] and experimental studies including dilatometry [5], neutron and x-ray diffraction [6], pair density function (PDF) analysis [7], EXAFS [8], x-ray diffuse scattering [9], and oxygen-isotope substitutions [10]. As a result of these studies, a connection has been drawn between the paramagnetic insulatorferromagnetic metal transition at T C (PI-FM transition) and anomalies in both the average and the local structure of CMR materials [6,9]. In the case of the 3D (threedimensional) perovskites, the anomalies in the coherent (average structure) and incoherent (local structure) lattice distortions are of the same sign, reflecting a diminution of the Jahn-Teller distortion of the MnO 6 octahedra in the metallic state. The relevance of this picture to layered manganites was questioned immediately with the observation of an increased octahedral distortion in the metallic phase of La 1.2 Sr 1.8 Mn 2 O 7 [11].In this Letter we report full temperature-dependent structural data that clearly differentiate the involvement of coherent and incoherent lattice distortions in the PI-FM transition of layered manganites La 222x Sr 112x Mn 2 O 7 ͑0.32 # x # 0.40͒. We find that the maximum T C ͑ഠ 131 K for x ഠ 0.36͒ coincides with a complete suppression of the anomalies in the lattice parameters and the Mn-O distances seen for other doping levels at the transition. Remarkably, these anomalies change sign upon crossing through this critical doping concentration, demonstrating that the existence of a metal-insulator transition in layered manganites does not depend on the sign of the anomaly in the average structure at T C . On the other hand, incoherent lattice distortions-as measured by the anisotropic Debye-Waller factors-decrease through the transition for all compositions in much the same way as the 3D perovskites, and no sign reversal is observed. To explain these resul...