Vibrational spectroscopy at very high pressures. Part 7.—I.r. spectra of potassium nitrate

Abstract

I.r. spectra of KNO₃ have been obtained at pressures up to 40 kbar and 80°C and, as a function of temperature alone, up to 153°C, thereby covering the phase transition cycles II → IV → II and II → I → III → II respectively. The i.r. spectrum of KNO₃(IV) has been obtained for the first time and studied as a function of temperature and pressure. A general increase in the number and intensity of the anion modes in phase IV as compared with phase II suggests an increase in environmental polarity and is consistent with a unit cell of high occupancy and low symmetry. New information was also obtained for phases I, II and III in the ν₃ and (ν₁+ν₄)(1 200–1 800 cm^–1) region, and the assignment of ν₃ for KNO₃(I) finally settled. The spectra of KNO₃(II) and KNO₃(III) are consistent with their known structures but there are difficulties in relating the spectrum of KNO₃(I) to its commonly accepted structure. A small amount of moisture has a marked effect upon the ν₃ region under pressure, making the II → IV transition sluggish and apparently supressing the II → III transition.