Spectroscopy of Ammonium Occupying Symmetry-Inappropriate Positions in Crystal Structures of Salts

The vibrational spectra of the natural ammonium sulfates tschermigite NH₄Al(SO₄)₂(H₂O)₁₂, lonecreekite NH₄Fe(SO₄)₂(H₂O)₁₂, ammoniovoltaite (NH₄)₂Fe²⁺₅Fe³⁺₃Al(SO₄)₁₂(H₂O)₁₈, sabieite NH₄Fe(SO₄)₂, ammonioalunite NH₄Al₃(SO₄)₂(OH)₆, and ammoniojarosite NH₄Fe₃(SO₄)₂(OH)₆ were studied to determine the behavior of ammonium in symmetry-inappropriate positions. Disorder of the ammonium cation in salt crystals caused by the need to adjust the tetrahedral cation to the symmetry of the position to preserve the crystal symmetry was revealed. If the symmetry group of the position was not a subgroup of the tetrahedral symmetry group, the NH₄ tetrahedron was distorted by the subgroup Hʹ common for T_d and H, where H is the symmetry group of the position. Then, a polyhedron corresponding to the symmetry of the position was constructed from N = |H|/|Hʹ| (|H| and |Hʹ| are the orders of these groups) distorted tetrahedra. The disordered ammonium cation had several orientations (N), the superposition of which formally gave a polyhedron corresponding to the local symmetry. The maximum common subgroups for the given salts were C_3v and C₃. Distortion of ammonium led to activation of ν₁ and ν₂ [NH₄⁺ ] vibrations in the IR spectrum and splitting of ν₃ and ν₄ [NH₄⁺ ]. However, the effect was barely noticeable for ammonium in a centrosymmetric position, as in ammonioalunite and ammoniojarosite. The ν₄ [NH₄⁺] band was noticeably split and ν₁ and ν₂ [NH₄⁺ ] vibrations were clearly visible in spectra of the ammonium alums, ammoniovoltaite, and sabieite.