DIFFERENTIAL SCANNING CALORIMETRY STUDIES ON STRUCTURAL PHASE-TRANSITIONS IN (CNH2N-5, X=BR OR CL)(1NH3)(2)ZNX(4) CRYSTALS (N=1)

The structural phase transitions in (CnH2n+1NH3)(2)ZnX(4) (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 sho wed at least one structural phase transition over the temperature rang e investigated. The results for the Cl compounds agreed well with the reported data obtained by DSC or adiabatic calorimetry measurements. T he transition temperatures of the Br compounds are in good agreement w ith 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 l ike 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 tetrachlo rozincate ions are estimated.