Phase transition and molecular motions in [Cd(NH3)6](NO3)2

One phase transition was detected by DSC in [Cd(NH3)6](NO3)2 between 95 and 297 K. The phase transition temperatures, at heating  = 212.6 K and at cooling  = 205.3 K, were calculated by extrapolating to zero scanning rate the and values obtained at different scanning rates. The presence of ca 7.3 K hysteresis suggests that the detected phase transition is of the first-order type and the associated entropy change ΔS = 39.4 J mol−1 K−1 indicates its configurational character. Characteristic changes in temperature dependences of the FWHM of the bands connected with ρ r(NH3)F 1u and δ as(HNH)F 1u modes suggest the existence of a new phase transition at ca 80 K. Thus the examined compound has at 20–300 K range three solid phases, in which NH3 ligands perform fast (picoseconds), uniaxial reorientation around the Cd–N bond, with effective activation energies equal to: E aI = 6.1 kJ mol−1, E aII = 2.1 kJ mol−1 and E aIII = 1.3 kJ mol−1. Very low values of the activation energies for the NH3 reorientation (especially in phase III) suggest that this reorientation occurs through the translation--rotation coupling, which makes the barrier to the rotation of the ammonia ligands fluctuate. The reorientational motion of NH3 is only slightly disturbed at the phase transitions due to the dynamical orientational order--disorder process of anions. †Dedicated to Prof. J. A. Janik on the occasion of his 80th birthday.

[1]  G. Bator,et al.  Vibrational study of the structural phase transitions in the (CH3ND3)3Sb2Br9 (d-MABA) crystals by infrared spectroscopy , 2001 .

[2]  S. Wróbel,et al.  DSC Investigations of the Phase Transitions of [M(NH3 )6](ClO4)2 and [M(NH3)6](BF4)2 , where M = Co and Cd , 1999 .

[3]  P. Becker,et al.  Order-disorder and structural phase transitions in solid-state materials by Raman scattering analysis , 1997 .

[4]  W. Paulus,et al.  Quantum mechanics of the rotation-translation coupling of NH3-groups in metal hexaammine salts , 1997 .

[5]  W. Prandl,et al.  Single particle dynamics in an anharmonic potential with translation-rotation coupling: crossover from weak localisation via chaos to free rotation , 1997 .

[6]  E. Mikuli,et al.  Connection between Reorientational Motions of NH_{3} Groups and Phase Transitions in [Ni(NH_{3})_{6}](NO_{3})_{2} and [Mg(NH_{3})_{6}](NO_{3})_{2} Compounds , 1995 .

[7]  J. Janik,et al.  Neutron quasielastic scattering results for Me(NH3)6(XY4)2, Me(NH3)6(XY3)2 and Me(NH3)6X2 compounds, compared with the calorimetric and Raman line width data - a new analysis , 1991 .

[8]  G. Kearley,et al.  NH3 reorientation in phases I, II, and III of Ni(NH3)6(NO3)2 , 1988 .

[9]  R. Pick,et al.  Raman study of molecular motions in relation to phase transitions in [Ni(NH3)6](NO3)2 , 1987 .

[10]  I. Natkaniec,et al.  Low temperature measurements of incoherent inelastic and quasielastic neutron scattering in [Ni(NH3)6] (NO3)2, with a simultaneous control of phase , 1985 .

[11]  J. Janik,et al.  Comparison of calorimetry and neutron scattering results concerning phase transitions in /Ni(NH3)6/(NO3)2 with the raman band profile study , 1984 .

[12]  B. Fimland,et al.  Calometry, proton magnetic resonance and quasielastic neutron scattering studies of [Mg(NH3)6] (NO3)2 , 1983 .

[13]  I. Natkaniec,et al.  Incoherent inelastic and quasielastic neutron scattering with simultaneous neutron diffraction control of phase transitions in [Mg(NH3)6](NO3)2 , 1983 .

[14]  J. Janik,et al.  Incoherent quasielastic neutron scattering study of NH3 reorientations in various phases of [Ni(NH36] (NO3)2 , 1981 .

[15]  T. Jenkins,et al.  A Raman study of the orientational phase transitions of hexammine nickel (II) nitrate , 1978 .

[16]  K. Nakamoto.  Infrared and Raman Spectra of Inorganic and Coordination Compounds , 1978 .

[17]  J. Naudts,et al.  Anomalous thermoelastic behavior and molecular reorientations in crystals , 1977 .

[18]  A. Müller,et al.  Vibrational spectra and force constants of pure ammine complexes , 1976 .

[19]  S. Porto,et al.  On linewidth of phonons associated to a disorder mechanism , 1973 .

[20]  R. Katiyar,et al.  Analysis of the relationship between temperature dependence of the libration mode and dielectric relaxation in NaNO2 , 1973 .