Raman spectroscopic study of structural phase transitions in the layer crystals (EtNH₃)₂MCl₄with M=Cd and Mn

The authors have studied the Raman spectra of the crystals (EtNH3)2MCl4 (M=Cd and Mn) as functions of temperature from 5 to 300K. The external vibrations have been assigned. The space group of the Cd crystal below 114K is found to be P22/b and the phase transition P21/b-Pbca is described as a displacive first-order transition. The order-disorder transitions Pbca-Abma in (EtNH3)2MCl4 for M=Cd and M=Mn are compared. Linewidth measurements performed on the carbon-carbon stretching mode at about 870 cm-1 suggest the possibility of a second-order transition in the Mn crystal, while the first-order behaviour of the transition in the Cd crystal is confirmed.