PHASE-TRANSITIONS IN THE PEROVSKITE-TYPE LAYER COMPOUNDS (C10H21NH3)(2)MCL(4) (M=HG CD)

In the series of cadmium (Cd) compounds with n > 3, (C10H21NH3)CdCl4 ( short notation C10Cd) is the only one that has been the subject of a d iversified study [L. Ricard, M. Rey-Lafon, and C. Biran, J. Phys. Chem . 88, 5614 (1984); N.B. Chanh, P. Haget, c. Hauw, A. Meresse, L. Ricar d, and M. Rey-Lafon, J. Phys. Chem. Solids 44, 589 (1983); N.B. Chanh, C. Hauw, A. Meresse, M. Rey-Lafon, and L,. Ricard, J. Phys. Chem. Sol ids 46, 1413 (1985)]. X-ray diffraction, calorimetry, and Raman spectr oscopy show the existence of three crystalline phases: low temperature (LT), intermediate temperature (IMT), and high temperature (HT) as fo llows: [GRAPHICS] The transitions from LT to IMT and from IMT to HT oc cur, respectively, at T-1 = 308 K and T-2 = 312 K. The aims of the pre sent work are (I) to identify the symmetry of the intermediate phase o f the compound (C10H21NH3)(2)HgCl4 by Landau's theory, and (2) to pres ent the powder diffraction diagrams of the homologous compound (C10H21 NH3)(2)CdCl4 registered from 223 K to 423 R. This study also shows thr ee crystalline phases at, respectively, T-1 = 338 K and T-2 = 342 K. T he substitution of Cd by Hg (mercury) seems deplaced the transition's temperature.