New RMnO3+δ (R=Y, Ho; δ≈0.35) phases with modulated structure

• Hexagonal RMnO3+δ phases are commonly taken as oxygen-stoichiometric.
• For R=Y and Ho phases excess-oxygen loading is possible.
• Single-phase samples with a new composite structure are obtained at δ≈0.35.
• The new structure is composed of hexagonal- and pyrochlore-like units.

A series of hexagonal RMnO3 (R=Y, Ho–Lu) compounds are first annealed in various highly oxidizing conditions to show that for the largest R constituents, Y and Ho, it is possible to load the phases with large amounts of excess oxygen. Within the oxygen-annealed samples we identify new heavily-oxygenated phases, that is, YMnO3.35 and HoMnO3.34. The former is then thoroughly characterized by thermogravimetric analysis, X-ray diffraction and electron diffraction. These studies evidence a hexagonal sub-cell (aS≈3.578 Å (aH/3), cS≈11.18 Å, space group P63mc  ) and satellites, associated with two modulation vectors q→1⁎≈0.41a→S⁎+0.41b→S⁎+1/3c→S⁎andq→2⁎=(1/2). The results are interpreted by the formation of a modulated composite structure, resulting from the evolution of the hexagonal parent YMnO3+δ structure toward a reduced pyrochlore-type structure Y2Mn2O7−x, in long- and short-range ordering.

Hexagonal RMnO3+δ phases are commonly taken as oxygen-stoichiometric. Here we show that the R=Y and Ho phases can be loaded with excess oxygen up to δ≈0.35 through high-pressure oxygenation annealing. Formation of a new composite structure composed of hexagonal-like RMnO3+δ and pyrochlore-like RMnO3.5−δ constituents is demonstrated in the case of R=Y.Figure optionsDownload as PowerPoint slide