Studies of phase transformations in nitrates and nitrites II. Changes in ultra-violet absorption spectra accompanying thermal transformations in the crystals

Abstract

Changes in ultra-violet absorption spectra have been measured for nitrates and nitrites of the alkali metals and for ammonium, silver and thallous nitrates, in relation to thermal transformations in these solids. A gradual shift of absorption bands is found to accompany thermal expansion of all the crystals. More abrupt changes in absorption spectra are observed at transformation points from one crystal structure to another; in many instances these two effects of temperature can be correlated. In cases where crystal structures are known, shifts in absorption maxima due to changes in temperature have been interpreted. in relation to changes in the distances between cations and anions. By means of the techniques described, ultra-violet absorption spectroscopy provides a sensitive means for studying various thermodynamic effects in phase transformations. Thus with ammonium nitrate, hysteresis with both superheating and supercooling is observed for the transitions IV ^ III ( T c ~ 32 °C) and III ^ II ( T c ~ 88 °C), but not II ^ I ( T c ~ 125 °C). With potassium nitrate during cyclic changes of temperature a (metastable) crystal form III appears. With potassium nitrite, a new crystal transformation showing hysteresis ( T c ~ 40 °C) is readily detected by ultra-violet spectroscopy.