High pressure and high temperature phase transformations in LiNbO3

A behavior of LiNbO3 under high pressure and temperature has been studied up to 90 GPa by means of high pressure in situ x-ray observations. Recovered samples were analyzed by transmission electron microscope (TEM). When the LiNbO3 was compressed at room temperature, a transformation occurred at about 25 GPa. The powder x-ray diffraction pattern of this "room-temperature and high-pressure" (RT-HP) phase was successfully explained by the NaIO3-type structure. No further transformation was observed at room temperature up to 90 GPa and reverse transition to starting phase occurred at about 10 GPa, thus this phase was unquenchable on release of pressure. When this RT-HP phase was heated at above 30 GPa, a phase appeared which can be recovered to ambient condition. X-ray diffraction and TEM analysis of this "high-temperature and high-pressure" (HT-HP) phase clarified that this phase has hexagonal symmetry with a most likely space group of P6(3). The quenched sample reverts to the starting phase on heating above 650 K. This HT-HP phase is opaque, suggesting the change of electronic property. The density of these RT-HP and HT-HP phases are, respectively, 21% and 23% higher compared to the starting LiNbO3 phase at ambient condition. (C) 2003 American Institute of Physics.