Structural Relationships among La-Ni-O Ruddleston-Popper Phases
Abstract
The ternary phase diagram La-Ni-O contains a series of Ruddleston-Popper (RP) phases of general composition (LaO)(LaNiO_3)_n. Using neutron powder diffraction, we have reinvestigated the structure of the n=2 member of this series and found it to be of lower symmetry (Amam) than previously reported (Fmmm). The symmetry-lowering mode involves a rotation of MnO6 octahedera. This was observed for samples annealed both in air (La_3Ni_2O_7-δ, δ=0.08) and in oxygen (δ=0), i.e. symmetry lowering is not a direct consequence of oxygen vacancies. Analogous rotational modes reduce the symmetry of the n=1 and n=3 phases from Fmmm to Bmab. The n=1 phase undergoes a further structural transition at low temperatures, an analogue of which is not observed in the n=2 or n=3 phases. Furthermore, our refinement of the n=3 structure indicates considerable strain within the perovskite-type units of the RP structure. The origins of this strain, and its relationship to n, will be discussed in terms of competing crystal-chemical interactions within and between the perovskite-type and NaCl-type units of these RP phases. Measurements of electrical resisitivity and magnetic susceptibility for these samples will also be presented. A thorough understanding of the structures of such layered mixed-valent perovskites is a necessary precursor to an understanding of their electronic and magnetic properties.
- Publication:
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APS March Meeting Abstracts
- Pub Date:
- March 2000
- Bibcode:
- 2000APS..MARI27012L