Infrared and Raman spectroscopic investigations of methylammonium haloantimonates(III), $[N(CH_3)_{4-n}H_n]_3Sb_2X_9$ (n = 0-3, X = Cl or Br), through their phase transitions

Infrared and Raman spectra of methylammonium chloroantimonates(III) of the general formula $[N(CH_3)_{4-n}H_n]_3Sb_2{Cl}_9$ with n=O, 1, 2 and 3 and of the methylammonium bromoantimonate(III), $(CH_3NH_3)_3Sb_2Br_9$, have been investigated in detail across their phase transitions. Vibration modes due to $CH_3$ 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 $Sb_2{Cl}_9$ anion. The anion modes differ with the number of $CH_3$ 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 $(CH_3NH_3)_3Sb_2Br_9$ and $(CH_3NH_3)_3Bi_2Br_9$ are similar and in the high-temperature phase of these compounds the ${(CH_3NH_3)}^+$ cations are oriented randomly