I n honour of Prof. Dr. Dr. h. c. P. GORLICH'S 70th birthday (CH,NH,),CdCI, is found to be isotructural a t room temperature with (CH,NH,),MnCl, having the orthorhombic space group Cmca. A high temperature phase transition t o a tetragonal phase could not be observed because of a decomposition slightly above 383 K. Two low temperature phase transitions are detected by means of DTA a t about 253 and 173 K. The first one leads to the space group P4,lncm. A microscopic model for this phase transition is proposed.Fur die Raumtemperaturphase von (CH,NH,),CdC14 wird die mit (CH37XH,),MnCI, isostrukturelle, orthorhombische Raumgruppe Cmca gefunden. Eine Hochtemperaturumwandlung in eine tetragonale Phase konnte wegen thermischer Zersetzung oberhalb yon 383 K nicht beobachtet werden. Mittels DTA werden zwei Tieftemperatur-Phasenumwandlungen bei ungeflhr 283 und 173 K gefunden. Die erstere fiihrt zu einer tetragorialen Modifikation mit der Raumgruppe P$/ncm. Ein mikroskopisches Modell dieser Phasenumwandlung wird vorgeschlagen.
1ntroduct.ion= 1, 2 , . . . crystallize in perovskite type layer structures exhibit,ing several interesting structural phase transitions in the paramagnetic phase. These phase transitions were studied in the past years especially for the simplest n = 1 compounds by means of differential thermal analysis (DTA), bircfringence measurements, X-ray diffraction, neutron diffraction [l to 31, group theoretical analysis 141 as well as by nuclear resonance techniques [5, 61. These studies revealed a strange sequence of phase transitions starting from a high temperature tetragonal phase I4/mmm leading to an orthorhombic phase Cmca (M = Mn, Fe) or Pbca (M = Cu). At lower temperatures the structure changes again t'o tetragonal and afterwards to monoclinic. The two latter phases are not known in detail.Since paramagnetic centres hinder NMR experiments considerably, especially in relaxation measurements, an examination of diamagnetic (CH,NH,),CdCI, is of particular interest. This substance was used already in a comparative specific heat study a t low temperatures [ 7 ] , but no structural data were given. The aim of the present study was to prove the isostructural nature of the Cd oompound by an X-ray structure determination a t room temperature, to determine its thermal stability and to scan by means of DTA and optical microscopy the occurrence of phase transitions. Since the scanning techniques yielded a firstorder phase transition a t 283 K the X-ray studies were extended also to this low-temperature phase.The cornpounds (C,H2R+1NH,),MC14 with M = Cu, Mn, Fe, arid 2 9 *
