Probing bipyramid microstructure of pure and Dy-doped HoMnO3 using high pressure and low temperature studies.

Bipyramidal distortions play the governing role in physical properties of RMnO₃ multiferroics. We have carried out angle-dispersive high pressure synchrotron X-ray diffraction experiments on hexagonal HoMnO₃ to find the evolution of bipyramidal parameters upon external compression. It is found that the average bond-lengths and bond-angles between Mn and O atoms depict significant reduction, which results in the distortion in MnO₅ bipyramidal structure at high pressures. Further, since subtle variations in these distortions are used to compare the properties of hexagonal RMnO₃ manganite series, in-situ low temperature infrared spectroscopic studies of 10% Dy-doped HoMnO₃ have been carried out to study the effect of further increase in R-site cationic radius on MnO₅ deformations. It is found that the phonon modes specifically related to the motion of Mn and O atoms in ab-plane display irregularities near magnetic transition temperatures (T_N and T_SR) via spin phonon coupling. Even a small increase in average ionic radius of rare-earth atom suppresses this anomalous behaviour and follows the trend of undoped hexagonal rare-earth manganites. [ABSTRACT FROM AUTHOR]