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

Abstract

The vibrational spectra of ammonium sulfates found in nature are studied: 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, to determine the behavior of ammonium in positions with inappropriate symmetry. The disordering of the ammonium cation in salt crystals was revealed, caused by the need to adjust the tetrahedral cation to the symmetry of the position in order to preserve the symmetry of the crystal. If the symmetry group of the position is not a subgroup of the symmetry group of the tetrahedron, the NH₄⁺ tetrahedron is 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 is constructed from N = |H|/|Hʹ| (|H|, |Hʹ| – the orders of these groups) distorted tetrahedra. The ammonium cation under disordering has several orientations (N), the superposition of which formally gives a polyhedron corresponding to local symmetry. For the given salts, the maximum common subgroups are C_3ν and C₃. The distortion of ammonium leads to activation of ν₁, ν₂ [NH₄⁺] vibrations in the infrared spectrum and splitting of ν₃, ν₄ [NH⁴⁺] but in the case of finding the ammonium in a centrallysymmetrical position, as in ammonioalunite and ammonioarosite, the effect is hardly noticeable. On the contrary, in ammonium alum, ammoniovoltaite, sabeyite the band ν₄ [NH₄⁺] is noticeably split, and vibrations of ν₁, ν₂ [NH₄⁺] are clearly seen in the spectra.