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Jahn‐Teller‐Ordnung in Mangan(III)‐fluoridsulfaten. II. Phasenübergang und Verzwillingung bei K 2 [MnF 3 (SO 4 )] und 1D Magnetismus in Verbindungen A 2 [MnF 3 (SO 4 )] (A = K, NH 4 , Rb, Cs)
Author(s) -
Klein Ute,
Hahn Friedemann,
Massa Werner,
Pebler Jürgen
Publication year - 2005
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.200500217
Subject(s) - isostructural , crystallography , monoclinic crystal system , orthorhombic crystal system , crystal twinning , chemistry , crystal structure , jahn–teller effect , manganese , phase transition , magnetism , condensed matter physics , ion , physics , microstructure , organic chemistry
Jahn‐Teller Ordering in Manganese(III) Fluoride Sulfates. II. Phase Transition and Twinning of K 2 [MnF 3 (SO 4 )] and 1D Magnetism in Compounds A 2 [MnF 3 (SO 4 )] (A = K, NH 4 , Rb, Cs) According to single‐crystal X‐ray investigations, K 2 [MnF 3 (SO 4 )] crystallizes at low temperature, like the isostructural Rb, NH 4 , and Cs analogues in space group P2 1 /c, Z = 4, e.g. at 100 K with a = 7.197, b = 10.704, c = 8.427Å, β = 91.84°. Below about 300 K, the crystals are found to be [001] axis twins. Using a new integration method for area detector records, nearly complete intensity data could be gained allowing for structure refinements of similar quality as for untwinned crystals (e.g. at 100 K: wR 2 = 0.050, R = 0.020 for all reflections). With rising temperature, the monoclinic angle approaches continuously 90°. For an ordering parameter Δβ = β−90° a 2 nd ‐order phase transition is observed with an exponent λ = 0.17. At the transition temperature of 280 K resulting from the fit, the monoclinic structure changes – with delay – to orthorhombic with the minimum super‐group Pnca, a = 7.243, b = 10.763, c = 8.457Å, R = 0.024, as found in an early structure determination at room temperature by Edwards 1971. In the chain‐like [MnF 3 (SO 4 )] 2− anions, manganese(III) is octahedrally coordinated by two trans ‐terminal and two trans ‐bridging fluorine ligands as well as by the O atoms of two trans ‐bridging sulfate ligands. At low temperature, the octahedral elongation by the Jahn‐Teller effect alternates between a F–Mn–F and an O–Mn–O axis (antiferrodistortive ordering). All bridges are asymmetric. From about 320 K on they become symmetric. Due to 2D dynamical Jahn‐Teller effect all octahedra appear compressed. All compounds A 2 [MnF 3 (SO 4 )] show 1D antiferromagnetism. The antiferrodistortive Jahn‐Teller order at low temperatures and the small bridge angles explain the much lower magnetic exchange energies and their inverse relation to the bridge angles as compared with other fluoromanganate(III) chain compounds with the usual ferrodistortive ordering.