Synthesis and oxygen content dependent properties of hexagonal DyMnO3+δ

Oxygen deficient polycrystalline samples of hexagonal P63cm (space group #185) DyMnO3+δ (δ<0) were synthesized in Ar by intentional decomposition of its perovskite phase obtained in air. The relative stability of these phases is in accord with our previous studies of the temperature and oxygen vacancy dependent tolerance factor. Thermogravimetric measurements have shown that hexagonal samples of DyMnO3+δ (0≤δ≤0.4) exhibit unusually large excess oxygen content, which readily incorporates on heating near 300 °C in various partial-pressures of oxygen atmospheres. Neutron and synchrotron diffraction data show the presence of two new structural phases at δ≈0.25 (Hex2) and δ≈0.40 (Hex3). Rietveld refinements of the Hex2 phase strongly suggest it is well modeled by the R3 space group (#146). These phases were observed to transform back to P63cm above ∼350 °C when material becomes stoichiometric in oxygen content (δ=0). Chemical expansion of the crystal lattice corresponding to these large changes of oxygen was found to be 3.48×10−2 mol−1. Thermal expansion of stoichiometric phases were determined to be 11.6×10−6 and 2.1×10−6 K−1 for the P63cm and Hex2 phases, respectively. Our measurements also indicate that the oxygen non-stoichiometry of hexagonal RMnO3+δ materials may have important influence on their multiferroic properties.

Thermogravimetric annealing in 21% O2 after initial synthesis of DyMnO3+δ in argon (curve 1) and subsequent reduction in 42% H2 to Dy2O3 and MnO (curve 2).Figure optionsDownload as PowerPoint slideHighlights
► Thermogravimetric measurements of hexagonal DyMnO3+δ exhibited large changes in δ near 300 °C.
► X-ray and neutron powder diffraction showed the presence of two new structural phases at δ≈0.25 and 0.40.
► The chemical expansion properties of these large changes in oxygen.
► Thermal expansion coefficient of stable oxygen content regions.