J. Chem. Soc., Faraday Trans. 1, 1982,78, 3691-3692 Solid-state Studies Part 26.-Raman Spectroscopic Evidence for a Phase 11-like Intermediate during the Course of the IV-I11 Phase Transition in Ammonium Nitrate BY GORDON J. KEARLEY AND SIDNEY F. A. KETTLE* School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ Received 23rd April, 1982 Raman evidence is presented which supports the view that the intermediate occurring in the IV + I11 phase transition of ammonium nitrate has a structure similar to that of phase 11. The room-temperature, stable form of ammonium nitrate, phase IV, converts into phase I11 at ca. 32 OC. This transformation is of technical importance because if a sample is repeatedly cycled through the transition, crystalline samples crumble, giving rise to a variety of problems.The crystal structure of both phase IV1 and phase 1112 are known and bear few similarities to each other. For instance, the nitrate-ion molecular planes are parallel in phase IV but in phase I11 these ions lie in planes at ca. 90'. In addition, on proceeding from phase IV to phase I11 there is an increase in volume which is calculated to be 4.6% from unit-cell dimensions1*2 but found to be 3.6% by density determination^.^ This discrepancy may be due to expansion of the lattice into voids which are commonly found in crystallites of phase IV or alternatively may reflect local compression and lattice distortions in phase I11 arising from the phase transition. It has recently been reported that a transient intermediate phase exists in the IV-I11 tran~formation.~* X-ray data have been recorded for this transient4 and it has been suggested that it has a structure akin to that of phase I1 (a phase stable between 82 and 125 OC).Since this report we have obtained photographic evidence for the existence of this inte~nediate;~ in our hands it has proved to be stable for a maximum of ca. 1 min. There may be considerable hysteresis on the IV-I11 transition; the higher the temperature at which it occurs the longer the period of stability of the intermediate. Thus, the ca. 1 min stability period was obtained for a sample for which the transition occurred at ca. 45 OC. This limited temporal stability is such as to preclude complete spectral coverage with the conventional apparatus available to us.We have therefore studied the Raman spectrum of the intermediate in the 720 cm-l region, a region in which phase 11 has a very different spectrum to phase IV. Although complete Fourier-transform infrared studies are possible and superficially attractive, many infrared bands are broad and not useful for characterisation of the intermediate. Although the infrared band at ca. 720 cm-l could be used for this purpose, it has considerable intensity and structure in phases IV and 111; very strangely it is of negligible intensity in phase 11. We now report spectroscopic evidence that the intermediate occurring between phases IV and I11 resembles phase 11. In fig. 1 we show Raman spectra recorded over a range of ca. 30 cm-l of the nitrate-ion in-plane deformation mode, v4, recorded as a sample passed through the 111-11 transition, the spectrometer scan direction being reversed at the points indicated by arrows.We have recorded similar data for the other transitions and for the IV-I11 transition, on rare occasions, have obtained a spectrum 369 It > E e: 4- .C U .- !6Y2 IV-111 PHASE TRANSITION I N AMMONIUM NITRATE I crn-' m 71 7 I Lp 71 6 100 7 5 50 46 45 44 T/'C T/'C FIG. 1. FIG. 2. FIG. 1 .-Consecutively recorded Raman spectra of the 720 cm-' region of poiycrystalline ammonium nitrate illustrating the spectral consequences of a 111-11 transition. Whilst the sample was heated from 50 to 100°C the spectrometer was scanned over a ca. 50cm-' range eight times. The spectrometer scan direction was reversed at the points designated by arrows so that a wavelength decreasing scan is followed by one with wavelength increasing.Temperature calibration is approximate only. FIG. 2.-Spectra recorded as in fig. 1 but at a lower temperature illustrating a two-step IV-I11 transition occurring at ca. 45 O C with the intermediate giving rise to a doubled v4 feature analogous to that obtained for phase I1 (see fig. 1). Arrows indicate reversal of spectrometer scan direction. which must be that of the intermediate. The best example is shown in fig. 2, unfortunately run at a different chart speed to fig. 1 but over a similar frequency range, where the single v4 features in phases IV and I11 are to be contrasted with the double feature of the intermediate. In both frequency and intensity pattern the spectrum of the intermediate resembles that of phase I1 (fig.l), consistent with the two phases having very similar structures. Because of the very different nitrate-ion orientations in phases IV and I11 it seems probable that in any intermediate they would be free to reorientate from the one orientation to the other; note that in phase I1 the nitrate ions are disordered.6 EXPERIMENTAL Raman spectra were recorded using a Spex 1401 double monochromator, an Ortec photon counting system 5C1 and a Spectra Physics Ar+/Kr- laser. The exciting line used was 20490 cm-l and the laser power, measured at the sample, was ca. 30 mW. Polycrystalline samples of AnalaR grade ammonium nitrate were sealed in capillary tubes and temperature control was achieved using a modified microscope hot-stage arrangement. G. J. K. thanks the S.E.R.C. and Fisons Ltd for financial support. C. S. Choi, J. E. Mapes and E. Prince, Acta Crystallogr., Sect. B, 1972, 28, 1357. B. W. Lucas, M. Ahtee and A. W. Hewat, Acta Crystallogr., Sect. B, 1980, 36, 2005. S. B. Hendncks, E. Posnjak and F. C. Kracek, J. Am. Chem. SOC., 1932, 54, 2766. W. Engel and P. Charbit, J. Therm. Anal., 1978, 13, 275. G. J. Kearley and S. F. A. Kettle, J. Chem. SOC., Faraday Trans. I , 1982, 78, 1817. B. W. Lucas, M. Ahtee and A. E. Hewat, Acta Crystallogr., Sect. B, 1979, 35, 1038. (PAPER 2/676)