Far-infrared and Raman spectra of crystalline ammonium and alkali-metal trichloromercurate(II) salts and some related compounds

Abstract

A complete vibrational assignment has been made for the tetragonal (α) form of the salt NH₄[HgCl₃] in terms of the known apex-shared octahedral polymeric sheet structure. The assignments have been aided by study of isomorphous ND₄⁺ and Rb⁺ analogues. An order–disorder phase transition in the NH₄⁺ and ND₄⁺ salts, similar to those in ammonium halides, has been found from observation of a cation librational mode at low temperatures. Another (β) form of the salt NH₄[HgCl₃] has been characterized as having a double-rutile chain structure; vibrational assignments have been made by comparison with spectra of the isomorphous salts β-ND₄[HgCl₃] and M[CdCl₃](M = NH₄, ND₄, K, or Rb). The previously described ‘Na[HgCl₃]’ has been shown not to exist; dehydration of the compounds NaHgCl₃,2H₂O or NaHgCl₃,2D₂O gives a physical mixture NaCl–HgCl₂. Vibrational spectra of the salts NH₄[HgBr₃], K[HgCl₃], K₂[HgCl₄],H₂O, and K₂[HgCl₄],D₂O have also been studied.