Calorimetric, dielectric, infrared spectra and thermal expansion studies of structural phase transitions in ((CH3)2CHNH3)2MX5 (M=Sb, Bi; X=Cl, Br) crystals

Abstract

Differential scanning calorimetric, thermal expansion, dielectric dispersion, infrared and preliminary x-ray diffraction studies on ((CH3)2CHNH3)2MX5 (M=Sb, Bi; X=Cl, Br) crystals are reported. All the studied salts are isomorphous, crystallizing in monoclinic symmetry. They undergo low-temperature structural phase transitions of first and second-order type: ((CH3)2CHNH3)2SbBr5 at 171 and 180 K, ((CH3)2CHNH3)2BiCl5 at 164 K, ((CH3)2CHNH3)2BiBr5 at 155 and 133 K. Dielectric dispersion studies in all isopropylammonium crystals between 100 Hz and 1 MHz reveal a low-frequency relaxation process described by the Cole-Cole formula with alpha approximately=0.15-0.20 in the phase transition temperature region. All phase transitions are likely to be due to the motion of the isopropylammonium cations.