Differential scanning calorimetry studies on structural phase transitions in crystals ( n = 1 - 5, X = Br or Cl)

Abstract

The structural phase transitions in (n=1 - 5, X = Br or Cl) were investigated by differential scanning calorimetry (DSC) between about 130 K and the melting points. All of the compounds showed at least one structural phase transition over the temperature range investigated. The results for the Cl compounds agreed well with the reported data obtained by DSC or adiabatic calorimetry measurements. The transition temperatures of the Br compounds are in good agreement with those determined by nuclear quadrupole resonance measurements. The values of the transition entropies obtained suggest that most of the phase transitions in the Br compounds are of the order - disorder type like those in the Cl analogues. The nature of the phase transitions in the Br compounds is discussed in comparison with that of those in the corresponding Cl compounds. The entropy changes due to the melting of orientational and positional orders in the alkylammonium and tetrachlorozincate ions are estimated.