NEUTRON-DIFFRACTION STUDY OF THE STRUCTURAL AND ELECTRONIC-PROPERTIESOF SR2HOMN2O7 AND SR2YMN2O7

The crystal structures of Sr2HoMn2O7 and Sr2YMn2O7 have been determine d at 290 and 1.7 K from neutron and X-ray powder diffraction data, Bot h are distorted Ruddlesden-Popper structures formed by the intergrowth of rock-salt-like layers and perovskite-like blocks of tilted MnO6 oc tahedra (space group P4(2)/mnm; for Ho at 290 K alpha = 5.40388(5), c = 19.9050(2) Angstrom). The majority of the lanthanide cations are loc ated in the rock-salt layers, Neither structure changes significantly on cooling to 1.7 K. There is evidence for neither charge ordering of Mn3+ and Mn4+ cations nor for long-range magnetic ordering, The magnet ic susceptibility maximum observed previously is thus confirmed as sig nifying a transition to a spin-glass phase, The behavior of these comp ounds is contrasted with that of other Mn oxides which show long-range magnetic order and colossal magnetoresistance.