Infrared and Raman spectroscopic investigations of methylammonium haloantimonates(III), [N(CH₃)_4−nH_n]₃Sb₂X₉ (n=0-3, X=Cl or Br), through their phase transitions

Abstract

Infrared and Raman spectra of methylammonium chloroantimonates(III) of the general formula [N(CH₃)_4−nH_n]₃ Sb₂X₉ with n=0, 1, 2 and 3 and of the methylammonium bromoantimonate(III), (CH₃NH₃)₃Sb₂Br₉, have been investigated in detail across their phase transitions. Vibration modes due to CH₃ deformation and those involving the NH_n (n=1, 2 and 3) groups show significant changes across the phase transitions of the chloroantimonates, besides those due to the Sb₂Cl₉ anion. The anion modes differ with the number of CH₃ groups since the nature of the anion itself changes, the free anion being present only in the quarternary compound. The high-temperature phases of the chloroantimonates seem to involve random orientations of the methylamino groups; a plastic-type state clearly manifests itself in the trimethyl and the tetramethylamino derivatives. A phase transition is predicted in all the chloroantimonates below 150 K, based on the changes in the anion bands at low temperatures. Changes in the IR and Raman spectra accompanying the phase transitions of (CH₃NH₃)₃Sb₂Br₉ and (CH₃NH₃)₃Bi₂Br₉ are similar and in the high-temperature phase of these compounds the (CH₃NH₃)⁺ cations ard oriented randomly.