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Jahn‐Teller Ordering in Manganese(III) Fluoride Sulfates. II. Phase Transition and Twinning of K2[MnF3(SO4)] and 1D Magnetism in Compounds A2[MnF3(SO4)] (A = K, NH4, Rb, Cs)According to single‐crystal X‐ray investigations, K2[MnF3(SO4)] crystallizes at low temperature, like the isostructural Rb, NH4, and Cs analogues in space group P21/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: wR2 = 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 2nd‐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 [MnF3(SO4)]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 A2[MnF3(SO4)] 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.

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RBsumB.-L'influence de variations du volume atomique sur les trois types d'interactions hyperfines (le deplacement isomerique, le couplage quadrupolaire et l'interaction magnktique dipolaire) est passee en revue. La variation d'une interaction hyperfine en fonction du volume est habituellement obtenue par application de pressions elevees ; ces experiences sont discutkes de f a~o n assez dktaillke. Caccent est mis sur l'interprktation des coefficients volumiques du dkplacement isomerique dans des mktaux et des composb. Le coefficient volumique d'une interaction hyperfine peut servir a interprkter d'autres phknomknes qui s'accompagnent de changements de volume. Un exemple caracteristique est la variation avec la tempkrature des interactions hyperfines dans les solides. Afin de donner une idke des problemes relib aces ktudes, une sklection &experiences de ce genre est discutke.