Synthesis and oxygen content dependent properties of hexagonal DyMnO3+δ

Oxygen deficient polycrystalline samples of hexagonal P6(3)cm (space group #185) DyMnO3+delta (delta < 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+delta (0 < delta < 0.4) exhibit unusually large excess oxygen content, which readily incorporates on heating near 300 degrees C in various partial-pressures of oxygen atmospheres. Neutron and synchrotron diffraction data show the presence of two new structural phases at delta approximate to 0.25 (Hex(2)) and delta approximate to 0.40 (Hex(3)). Rietveld refinements of the Hex(2) phase strongly suggest it is well modeled by the R3 space group (#146). These phases were observed to transform back to P6(3)cm above similar to 350 degrees C when material becomes stoichiometric in oxygen content (delta=0). Chemical expansion of the crystal lattice corresponding to these large changes of oxygen was found to be 3.48 x 10(-2) mol(-1). Thermal expansion of stoichiometric phases were determined to be 11.6 x 10(-6) and 2.1 x 10(-6) K-1 for the P6(3)cm and Hex(2) phases, respectively. Our measurements also indicate that the oxygen non-stoichiometry of hexagonal RMnO3+delta materials may have important influence on their multiferroic properties. (C) 2011 Elsevier Inc. All rights reserved.