Pressure dependent structural changes and predicted electrical polarization in perovskite RMnO₃

High pressure x-ray diffraction measurements on perovskite RMnO₃ (R = Dy, Ho and Lu) reveal that varying structural changes occur for different R ions. Large lattice changes (orthorhombic strain) occur in DyMnO₃ and HoMnO₃ while the Jahn-Teller (JT) distortion remains stable. Conversely, in the small R-ion system LuMnO₃, Mn-O bond distortions are observed between 4 and 8 GPa with a broad minimum in the JT distortion. High pressure infrared measurements indicate that a phonon near 390 cm^-1 corresponding to the complex motion of the Mn and O ions changes anomalously for LuMnO₃. It softens in the 4-8 GPa region, which is consistent with the structural change in Mn-O bonds and then hardens at higher pressures. By contrast, the phonons continuously harden with increasing pressure for DyMnO₃ and HoMnO₃. Density functional theory methods show that E-phase LuMnO₃ is the most stable phase up to the 10 GPa pressure examined. Simulations indicate that the distinct structural change under pressure in LuMnO₃ can possibly be used to optimize the electric polarization by pressure/strain.