Characterization of RMnO3 (R=Sc, Y, Dy-Lu): High-pressure synthesized metastable perovskites and their hexagonal precursor phases

A series of RMnO3 samples with R = Sc, Y, Dy-Lu was synthesized through a two-step route in which first the hexagonal oxides, h-RMnO3, were prepared under normal pressure using an EDTA method and then converted to orthorhombic perovskites, o-RMnO3, through a high-pressure treatment at 5 GPa and 800 deg C. Since all the aforementioned members but the R = Sc member were converted to an orthorhombic perovskite phase under the 5-GPa high-pressure condition, an additional sample of InMnO3 of the hexagonal structure was prepared and tested for the high-pressure conversion. The stability limit of the orthorhombic perovskite structure under the 5-GPa high-pressure condition was concluded to be located between InMnO3 and LuMnO3 in terms of the size of the A-site cation constituent, RIII or InIII. The decrease in the size of RIII was found to increase the distortion of o-RMnO3 with a decrease in the MnOMn bond angle from 144.3 deg for Dy to 140.3 deg for Lu. All the hexagonal and orthorhombic RMnO3 phases exhibited antiferromagnetic characteristics at low temperatures.