RAMAN-SCATTERING AND PHASE-TRANSITION OF AMMONIUM NITRATES

Polarized Raman spectra of single crystals are observed for the room-temperature phase NH//4NO//3(IV). The spectra in the low-frequency lattice vibration region as well as in the internal vibration region are well interpreted based on D//2//h**1**3 structure. Raman spectra of powdered samples are measured at various temperatures covering whole phases NH//4NO//3 I-V. A characteristic feature of Raman spectrum is clarified for each phase. An abrupt spectral change occurs in the phase transition, indicating a first order transition. The splitting of A//g and B//1//g components of v//3 (asymmetric stretching mode of NO//3** minus ion) in NH//4NO//3 is quite large, due to the strong interionic interaction between NH//4** plus and NO//3** minus ions.