Conformational calculation and phase transitions in NH3-(CH2)(4)-NH3CdCl4

Raman spectra of polycrystalline samples of NH3-(CH2)(4)-NH3CdCl4 were stud ied at temperatures ranging from 300 to 500 K). The two phase transitions p reviously detected at 338 K and 367 K were characterized. These transitions display a displacive and order-disorder mechanism involving mainly conform ational changes of the organic chain [NH3-(CH2)(4)-NH3](2+) coupled with di stortion of the CdCl6 octahedra. The existence of three conformations of th e chain at different temperatures has been evidenced in the spectral region [300-600 cm(-1)]. In order to identify unambigously these conformations we have calculated the geometry and the frequencies of different conformation al states, using the SCF semi empirical method PM3. The results are in good agreement with those established by previous X-ray diffraction studies.