Michel Molinier

Magneto-structural correlations in fluoromanganates(III)

Abstract A series of structurally closely related fluoromanganates(III) with chain structures [A 2 MnF 5 (H 2 O), etc.] and with layered structures (AMnF 4 ) have been synthesized and the crystal structures determined for most of them. In the trans-chain compounds, [MnF 6 ] octahedra, strongly elongated by the Jahn-Teller effect of the d 4 high-spin configuration, are arranged in a ferrodistortive manner, while similar octahedra are arranged in an antiferro-distortive manner in the quadratic layers of the AMnF 4 structures. Variation of A causes a wide variation in MnFMn bridge angles. Magnetic exchange energies determined for the 1d or 2d magnetic systems clearly correlate with the bridge angles in both systems. Suggestions regarding the type and relative strengths of the exchange interactions involved are made.

Structures of two polymorphs of MnF3·3H2O

Abstract From solutions of MnF 3 in hydrofluoric acid, MnF 3 ·3H 2 O is formed in two polymorphic forms whose crystal structures were determined by X-ray diffraction on single crystals. Form I : space group P 2 1 / c , Z 4; lattice constants a 8.413(1), b 9.618(1), c 6.370(1) A, β97.71(1)∘; R/wR O.0417/O.0295 for 710 reflections. Form II : space group P 2 1 / a , Z4; lattice constants a 8.481(7), b 9.473(6), c 6.473(4) A, β=100.51(6)∘; R/wR 0.0303/0.0290 for 650 reflections. In both polymorphs the structures consist of centrosymmetrical complex [Mn(H 2 O) 4 F 2 ] + cations and [Mn(H 2 O) 2 F 4 ] - anions. The cations exhibit very short MnF distances ( I , 1.795 A; II , 1.791 A) in octahedra weakly elongated by the Jahn-Teller effect (MnO: I , 2.074/2.022 A; II , 2.103/1.997 A). The anions are strongly elongated along the H 2 OMnOH 2 axis (MnO: I , 2.19 A; II , 2.15 A. MnF: I , 1.837/1.842 A; II , 1.837/1.856 A). Both forms of MnF 3 ·3H 2 O have approximately the same unit cell and very similar asymmetric units but they differ in the packing symmetry documented in the different space groups and, hence, in the networks of strong OH···F hydrogen bonds (O···F: 2.62 to 2.76 A). In I , pure cation and pure anion layers alternate along [100] direction; in II , the corresponding layers have ordered mixed cation/anion composition.

Die Kristallstrukturen der Tetrafluoromanganate(III) AMnF4 (A = K, Rb, Cs) / The Crystal Structures of the Tetrafluoromanganates(III) AMnF4 (A = K, Rb, Cs)

The crystal structures of AMnF4 compounds (A = K, Rb, Cs) were determined by X-ray diffraction on twinned single crystals: The K and Rb compound are isostructural: space group P21/a, Z = 4; KMnF4: lattice constants a = 769.9(2), b = 764.4(2), c = 576.9(1) pm, β = 90.54(3)°; R/wR = 0.029/0.025 for 481 reflections; pseudomerohedral (100) twin. RbMnF4: lattice constants a = 782.2(5), b = 777.7(3), c = 605.0(2) pm, β = 90.83(4)°; R/wR = 0.0665/0.0513 for 471 reflections. The structure of CsMnF4 has been refined anew in the space group P4/n (Z = 4) as a merohedral (110) twin (lattice constants a = 794.40(6), c = 633.76(9) pm; R/wR = 0.051/0.044 for 820 reflections). All three structures derive from the T1A1F4 type and show puckered [MnF4]- layers with an antiferrodistortive order of strongly Jahn-Teller distorted [MnF6] octahedra. The elongated axes (Mn-F for KMnF4: 215.8, RbMnF4 215.2, CsMnF4216.8 pm) alternate with shorter ones (188.1,188.3,185.4 pm) within the layer planes. The terminal Mn-F bonds are very short (180.3, 180.6,181.7 pm). The Mn-F-Mn bridging angles are 140.6° and 146.4° for the K, 148.3° and 152.1° for the Rb, and 161.9° for the Cs compound. The magnetostructural relations and the three puckering types are discussed for all the AMnF4 layered structures.

Crystal and Magnetic Structures of NaMnF4

An X-ray single crystal structure determination (monoclinic, space group P21/c, a = 573.6(2), b = 489.2(1), c = 574.8(2) pm, β = 108.07(2), Z = 2; wR = 0.038 for 380 reflections) shows that NaMnF4 crystallizes in the same layered structure type as LiMnF4. In the quadratic layers the Mn-F-Mn bridges are strongly asymmetric, due to the Jahn-Teller effect, leading to an antiferrodistortive order of elongated octahedra. The bridging angle is 138.4°. The Na+ ions are 6-coordinated as well. The common structural arrangement of both [MnF6] and [NaF6] octahedra shows topological relation to the rutile structure. In the magnetic measurements performed on powder samples NaMnF4 behaves as an antiferromagnet with a weak ferromagnetic component below 13 K. By neutron diffraction on powder (4-70 K) a magnetic cell doubled along the α-axis is found and below a Neel temperature of TN = 13 K the magnetic structure shows colinear antiferromagnetic arrangement of the spins pointing slightly (16) out of the layer plane. The resulting magnetic moment is 3.52 μΒ.

Magnetism of Alkalitetrafluoromanganates (III) AMnF4 (A = K, Rb, Cs): Neutron Diffraction, Mössbauer and Magnetization Investigations

Abstract The magnetic properties of the d4 Jahn-Teller systems AIMnIIIF4 with layered structures were investigated. Neutron diffraction on powders of KMnF4 and RbMnF4 revealed different antiferro-magnetic spin arrangements below TN = 4.5 K and 2.3 K, respectively: for KMnF4 canted antiparallel along a and b, for RbMnF4 parallel along a and antiparallel along b, in both cases parallel along c, the stacking direction of layers. Mössbauer investigations on 57Fe doped KMnF4 confirmed a spin orientation approximately within the layer plane. A discussion is given of the contributions to the magnetic hyperfine field and the Mössbauer linewidth in quasi-two-dimensional antiferromagnets with Ising anisotropy due to thermal excitation of domain wall dynamics (solitons). The experimental data seem to confirm the predicted exponential temperature dependence of the linewidth. From magnetization measurements on powders and a single crystal of KMnF4 the 2-d exchange energy and the out-of-plane and in-plane anisotropies could be extracted. In addition, from susceptibility measurements the exchange energies of NaMnF4 , RbMnF4 and CsMnF4 were calculated. A linear dependence of these exchange energies (positive for ferromagnetic CsMnF4 , negative for the other AMnF4 compounds) on the cos2 of the Mn-F-Mn bridge angle is observed and compared with the behaviour of the AFeF4 compounds which is also linear but with reverse sign of the slope. The specific superexchange mechanisms active in Jahn-Teller systems with antiferrodistortively ordered layers are suggested to be responsible for these findings.

Structures and magnetism of alkali tetrafluoromanganates(III)

Abstract The AMnF 4 compounds (A = alkali atom) have Jahn-Teller-distorted layered structures deriving from the TIAIF 4 type. Thus, the compounds are interesting models for studies of 2D magnetic properties and of the concurring ferromagnetic σ- and antiferromagnetic π-superexchange interactions dependent on the bridge angles within the layers. Although most of these compounds are known since several years (1,2) the detailed crystal structures were determined only for the LiMnF 4 compound recently (3). In our single crystal investigations we found that the different size of the alkali counter cations gives rise to three different structure variants: 1. NaMnF 4 , isotypic with LiMnF 4 (3), space group P2 1 /c, a = 573,6(2), b = 489,2(1), c = 574,8(2) pm, β = 108.07(2)° (see fig. below). 2. KMnF 4 , space group P2 1 /a, a = 769.9, b = 764.4, c = 576.9 pro, β = 90.54°. RbMnF 4 , isotypic, a = 780.6, b = 776.6, c = 604.6 pm, β = 90.69°. 3. CsMnF 4 , space group P4/n, a = 794.4, c = 633.8 pm, merohedral (110) twins, seeming P4/nmm space group (2). Other variants are found for the corresponding TI (4) and NH 4 (5) compounds. The structural classes differ in the Mn-F-Mn bridge angles and in the puckering type of the [MnF 4 ] - layers. The correlations between these geometric features and the magnetic properties, investigated by magnetic measurements and neutron diffraction, are discussed.