Raman Scattering in KNO3Phases I, II, and III

The Raman scattering for a complete phase-transition cycle (II → I → III → II) in KN${\mathrm{O}}_3$ is reported for the first time, with the intention to correlate the Raman-active modes in different crystalline structures. In addition to the data reported in earlier works, experimental results are obtained for the ferroelectric phase in which we noted a significant increase of intensity for all the covalent modes. A broad band with its half-width depending closely upon the temperature is also observed at 120 ${\mathrm{cm}}^{-1\mathrm{}}$ in this phase. In addition, two new lines at 65 and 133 ${\mathrm{cm}}^{-1\mathrm{}}$ are found in the room-temperature phase. Furthermore, the variation of intensity with various temperatures allows us to follow the scattering of all the observed Raman modes through the different phase transitions during a complete cycle. Using group theory, a study is made to connect various optical modes in the simple ferroelectric phase with corresponding modes in the other two phases. The Raman data presented here, together with previous infrared results, enable us to verify this deduction satisfactorily, at least for the covalent modes.