High-pressure structural stability of multiferroic hexagonal RMnO ₃ (R=Y, Ho, Lu)

Structural changes in RMnO₃ (R= Y, Ho, Lu) under high pressure were examined by synchrotron x-ray diffraction methods at room temperature. Compression occurs more readily in the ab plane than along the c axis. With increased pressure, a pressure-induced hexagonal to orthorhombic phase transition was observed starting at ∼22 GPa for Lu(Y)MnO₃. When the pressure is increased to 35 GPa, a small volume fraction of Lu(Y)MnO ₃ is converted to the orthorhombic phase and the orthorhombic phase is maintained on pressure release. High-pressure infrared absorption spectroscopy and Mn K-edge near-edge x-ray absorption spectroscopy confirm that the hexagonal P6₃cm structure is stable below ∼20 GPa and the environment around the Mn ion is not changed. Shifts in the unoccupied p-band density of states with pressure are observed in the Mn K-edge spectra. A schematic pressure-temperature phase diagram is given for the small ion RMnO₃ system.