Raman Spectra of Lattice Vibrations and the Order-Disorder Phase Transition in Ammonium Halides

Ammonium chloride and bromide undergo an order-disorder phase transition at Tc = 243°K and 235°K[1, 2] respectively. For the corresponding deuterated crystals the critical temperatures are only slightly shifted. Above this transition (Phase II) the structure of both is tetrahedral (point group Td) but disordered: the NH4 tetrahedra are randomly distributed between the two inverted configurations. At temperatures sufficiently below Tc such that complete order is attained, (Phase III) NE4Cl becomes rigorously tetrahedral with all NH4 ions similarly oriented forming domains[3]. Phase III of NH4Br is tetragonal (point group D4h), with a pair of inverted molecules per unit cell[2]. A macroscopic crystal consists of tetragonal domains. The bromide undergoes yet another transition at about 108°K, but with little information available about its nature. We do have evidence in this work that such a transition indeed occurs.