(NH4)Mg(HSO4)(SO4)(H2O)2 and NaSc(CrO4)2(H2O)2, two crystal structures comprising kröhnkite‐type chains, and the temperature‐induced phase transition (NH4)Mg(HSO4)(SO4)(H2O)2 (NH4)MgH(SO4)2(H2O)2. Issue 3 (22nd February 2021)

• Record Type: Journal Article
• Authors: Weil, Matthias; Kolitsch, Uwe
• Is Part Of: Acta crystallographica. Volume 77:Issue 3(2021)
• Page Start: 144
• Page End: 151
• Publication Date: 2021-02-22
• Subjects: ammonium magnesium sulfate -- strong hydrogen bond -- sodium scandium chromate -- tetrahedral‐octahedral chains -- kröhnkite‐type chains -- phase transition -- crystal structure -- IR spectroscopy; Crystallography -- Periodicals; Crystals -- Periodicals; 548.3
• DOI: 10.1107/S2053229621001650
• Languages: English
• ISSNs: 2053-2296

Abstract

Abstract : Both dihydrates (NH4 )Mg(HSO4 )(SO4 )(H2 O)2 and NaSc(CrO4 )2 (H2 O)2 comprise kröhnkite‐type tetrahedral–octahedral chains made up of {SO4 /SO3 (OH), [MgO4 (OH2 )2 ]} and {CrO4, [ScO4 (OH2 )2 ]} building units, respectively. (NH4 )Mg(HSO4 )(SO4 )(H2 O)2 shows a reversible phase transition between 100 and 296 K. Abstract : The crystal structure of the mineral kröhnkite, Na2 Cu(SO4 )2 (H2 O)2, contains infinite chains composed of [CuO4 (OH2 )2 ] octahedra corner‐linked with SO4 tetrahedra. Such or similar tetrahedral–octahedral `kröhnkite‐type' chains are present in the crystal structures of numerous compounds with the composition An M ( X O4 )2 (H2 O)2 . The title compounds, (NH4 )Mg(HSO4 )(SO4 )(H2 O)2, ammonium magnesium hydrogen sulfate sulfate dihydrate, and NaSc(CrO4 )2 (H2 O)2, sodium scandium bis(chromate) dihydrate, are members of the large family with such kröhnkite‐type chains. At 100 K, (NH4 )Mg(HSO4 )(SO4 )(H2 O)2 has an unprecedented triclinic crystal structure and contains [MgO4 (OH2 )2 ] octahedra linked by SO3 (OH) and SO4 tetrahedra into chains extending parallel to [ 10]. Adjacent chains are linked by very strong hydrogen bonds between SO3 (OH) and SO4 tetrahedra into layers parallel to (111). Ammonium cations and water molecules connect adjacent layers through hydrogen‐bonding interactions of medium‐to‐weak strength into a three‐dimensional network. (NH4 )Mg(HSO4 )(SO4 )(H2 O)2 shows a reversible phase transition and crystallizes at room temperature in structure type E in the classification scheme for structures with kröhnkite‐type chains, with half of the unit‐cell volume for the resulting triclinic cell, and with disordered H atoms of the ammonium tetrahedron and the H atom between two symmetry‐related sulfate groups. IR spectroscopic room‐temperature data for the latter phase are provided. Monoclinic NaSc(CrO4 )2 (H2 O)2 adopts structure type F1 in the classification scheme for structures with kröhnkite‐type chains. Here, [ScO4 (OH2 )2 ] octahedra (point group symmetry ) are linked by CrO4 tetrahedra into chains parallel to [010]. The Na + cations (site symmetry 2) have a [6 + 2] coordination and connect adjacent chains into a three‐dimensional framework that is consolidated by medium–strong hydrogen bonds involving the water molecules. Quantitative structural comparisons are made between NaSc(CrO4 )2 (H2 O)2 and its isotypic Na M (CrO4 )2 (H2 O)2 ( M = Al and Fe) analogues.