Continuous and discontinuous phase transitions in ammonium halides

Abstract

Raman spectra of NH 4Cl and NH 4Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH 4Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH 4Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν 2 and ν 5 Raman modes of NH 4Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν 5 Raman mode of NH 4Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.