Raman spectroscopic investigations of structural aspects of the different phases of lithium sodium and potassium nitrate

Raman spectra have been obtained for LiNO₃, NaNO₃ and KNO₃ as oriented single crystals and polycrystalline solids over a wide range of temperatures up to the melting points. The anomalous component previously observed in the symmetric stretching region and attributed to a disordered nitrate group has been studied as a function of temperature. The results suggest that the peak may have a hot band origin such that transitions 0001 → 1001 and 0100→ 1100 may occur in addition to the ground state transition, 0000→ 1000 of nitrate. In the symmetric stretching region the shift of the hot band is sufficiently large to ensure that the modes of this origin are not coupled to the ground state lattice in the usual way. Hot bands with an external mode origin could not be detected. KNO₃(II), KNO₃(III), NaNO₃(II) and LiNO₃, have ordered structures with small amounts of disorder of a hot band origin. KNO₃(I) and NaNO₃(I) have similarly disordered structures but exhibit minor differences due to different relative populations and symmetries of the disordered sites. Raman studies of the disordered phases suggest that the crystal symmetry is lower than the “average” structure deduced from X-ray diffraction methods. A number of bands previously reported have not been detected while several bands were reassigned on the basis of ¹⁵N substitution.