High-temperature phase transition, coordination mechanism and magnetism in multiferroic YMnO₃ nanopowders

Single phase YMnO₃ nanopowders were successfully synthesized by a modified polyacrylamide gel route at an optimal sintering temperature. High temperature is helpful for further crystallization and the formation of single hexagonal YMnO₃. The process was monitored by X-ray diffraction as well as by X-ray photoelectron spectroscopy, transmission electron microscope, Fourier transform infrared spectrum and magnetic measurements. A mixture of orthorhombic and hexagonal phases was found to exist in the 800 °C, while for the metastable orthorhombic YMnO₃ phase complete conversion to hexagonal phase occurs at 950 °C. In the synthesis process, the coordination mechanisms of EDTA anions and metal ions (Y³⁺ and Mn³⁺) have been discussed on the basis of the experimental and computational results. The synthesis route used in this case has been aggreed to be more convenient for obtaining single hexagonal YMnO₃ phase than by the combination of hydrothermal synthesisand additional thermal treatment or the glycine–nitrate process. The magnetic susceptibility indicates that the anti-ferromagnetic transition temperature increases from 58 to 71 K with increasing sintering temperature from 800 to 950 °C.