Sr_1.8Nd_1.2Mn₂O₇:  Synthesis, Crystal Structure, and Physical Properties

Abstract

A detailed investigation into the preparation of powder samples of the n = 2 Ruddlesden−Popper (RP) oxide Sr 1.8 Nd 1.2 Mn 2 O 7 is presented. This material is of interest as it displays colossal magntoresistance (CMR) without three-dimensional ferromagnetic long-range order. It is shown that Rietveld refinement of high-resolution powder X-ray and neutron diffraction data is essential to assess the outcomes of syntheses because of the potential coexistence of two very similar Ruddlesden−Popper phases. Phase coexistence or significant temperature-dependent anisotropic strain can be indicated by the broadening of the {0 0 10} reflection at 5 K. This suggests that a more subtle form of the previously reported phase separation at x = 0 is also difficult to avoid at the x = 0.2 composition. Precise attention to reaction temperature and time is required to prepare samples containing only one n = 2 Ruddlesden−Popper phase, and contamination by small quantities of the n = ∞ perovskite is a pervasive problem. Neutron powder diffraction structural analysis of the highest quality sample allows a comparison with the three-dimensional ferromagnet Sr 1.8 La 1.2 Mn 2 O 7 . The structure suggests that only slight changes in the Mn−O bond lengths are required to radically alter the electronic structure of n = 2 Ruddlesden−Popper phases.