Y. Calage

Hexagonal tungsten bronze-type FeIII fluoride: (H2O)0.33FeF3; crystal structure, magnetic properties, dehydration to a new form of iron trifluoride

(H2O)0.33FeF3, grown by hydrothermal synthesis, crystallizes in the orthorhombic system with cell dimensions a = 7.423(3) A, b = 12.730(4) A, c = 7.526(3)A, and space group Cmcm, Z = 12. The structure, derived from single crystal X-ray diffraction data (605 independent reflections) is refined to R = 0.019 (Rω = 0.021). The framework of the FeIIIF6 octahedra is related to that of hexagonal tungsten bronze (HTB) Rb0.29WO3. At 122°C, zeolithic water is evolved from hexagonal tunnels without any noticeable change of the fluorine skeleton. The related anhydrous compound represents a new form of iron trifluoride which is denoted HTBFeF3; at 525°C, it transforms into the cubic form of ReO3-type. (H2O)0.33FeF3 and HTBFeF3 are antiferromagnetic, with Neel temperatures of TN = 128°7 ± 0.5 K and TN = 97 ± 2 K, respectively.

Idle spin behavior of the shifted hexagonal tungsten bronze type compounds FeIIFeIII2F8(H2O)2 and MnFe2F8(H2O)2

Abstract FeIIFeIII2F8(H2O)2 and MnFe2F8(H2O)2, grown by hydrothermal synthesis ( P ⋍ 200 MPa , T = 450 or 380° C ), crystallize in the monoclinic system with cell dimensions (A): a = 7.609(5), b = 7.514(6), c = 7.453(4), β = 118.21(3)°; and a = 7.589(6), b = 7.503(8), c = 7.449(5), β = 118.06(3)°, and space group C2 m , Z = 2 . The structure is related to that of WO 3 · 1 3 H 2 O . It is described in terms of perovskite type layers of Fe3+ octahedra separated by Fe2+ or Mn2+ octahedra, or in terms of shifted hexagonal bronze type layers. Both compounds present a weak ferromagnetism below TN (157 and 156 K, respectively). Mossbauer spectroscopy points to an “idle spin” behavior for FeIIFeIII2F8(H2O)2: only Fe3+ spins order at TN, while the Fe2+ spins remain paramagnetic between 157 and 35 K. Below 35 K, the hyperfine magnetic field at the Fe2+ nuclei is very weak: Hhf = 47 kOe at T = 4.2 K. For MnFe2F8(H2O)2, Mn2+ spin disorder is expected at 4.2 K. This “idle spin” behavior is due to magnetic frustration.

Crystal structure and magnetic characterization of [C(NH2)3]3FeF6

Abstract The compound [C(NH 2 ) 3 ] 3 FeF 6 is prepared in closed bombs by digestion of FeF 3 and C(NH 2 ) 3 F in aqueous HF solutions. The material is cubic, space group Pa 3, a = 14.130(5) A, Z = 8. The crystal structure is determined from single-crystal diffraction intensity data refined to the conventional values of the indexes R = 0.0409 and R w = 0.0409 for the model containing the nonhydrogen atoms. All the hydrogen atoms are located by the final difference-Fourier map and lie near N atoms at distances in the range 1.02 to 1.09 A. The structure is characterized by two different sets of regular isolated (FeF 6 ) octahedra. Guanidinium cations are located on general 24-fold positions. The NH … F hydrogenbonding network is discussed. As expected, the compound is paramagnetic ( μ exp = 6.01 μ B , ϑ p = −5.6 K) and characterized by Mossbauer spectroscopy.

Crystal structure and magnetic properties of a new form of NH4MnFeF6

The hydrothermal synthesis at 380°C, 200 MPa of NH4MnFeF6, NH4MnCrF6, and RbMnFeF6 leads to a new AMIIMIIIF6 structural type of orthorhombic symmetry with Z = 8. Lattice constants are found to be, respectively, a = 7.844 (4), b = 12.819 (8), c = 10.582 (6); a = 7.808 (5), b = 12.755 (9), c = 10.501 (7); and a = 7.913 (5), b = 12.858 (9), c = 10.619 (5). The structure was solved for NH4MnFeF6 from 755 X-Ray reflections and refined to Rω = 0.029 in the space group Pb2nC62v. The network is built from edge-sharing MnFeF10 bioctahedra connected to each other by their vertices. RbMnFeF6 upon heating transforms irreversibly to the modified pyrochlore structure at 881 K. From magnetic and Mossbauer experiments, NH4MnFeF6 and NH4MnCrF6 are established to be antiferromagnetic with TN = 117.7 ± 0.5 K and < 6 K, respectively.

Ordered magnetic frustration: VII. Na2NiFeF7: Reexamination of its crystal structure in the true space group after corrections from renninger effect and refinement of its frustrated magnetic structure at 4.2 and 55 K

Abstract A new refinement of the crystal structure at 300 K and of the magnetic structure at 4.2 and 55 K of the ferrimagnetic weberite Na2NiFeF7 is undertaken in order to fully reveal both the true space group of this compound and its magnetically frustrated character. The reflections which previously obliged us to choose space groupImm2 are only due to Renninger effect. The true space group isImma (a = 7.2338(3)A,b = 10.3050(3)A,c = 7.4529(3)A,Z = 4) at 300 K. The structure was refined from 1148 independent reflections toR = 0.025 (Rw = 0.030). The ferrimagnetic behavior is confirmed (Tc = 88(2)K). Neutron powder diffraction shows that the nuclear and magnetic cells are identical and that there is an accident in the thermal evolution of the intensity of some magnetic peaks. Among the different modes given by the Bertauts macroscopic theory, the best fit is obtained for both temperatures with the modes−Fx andFx,Gz for Fe3+ and Ni2+ sublattices, respectively, instead of−Fx and+Fx in the solution previously proposed by Heger. The corresponding moments are 4.93(11) and 1.36(21) μB at 4.2 K (Rmag = 0.045) and 4.34(12) and 0.97(22) μB at 55 K (Rmag = 0.052). The slight anomaly in the thermal variation of the intensity of some magnetic reflections at 50 K is due to a significant change in the spin canting at this temperature, without any modification of the magnetic modes. A Mo¨ssbauer study confirms the anomaly from the thermal variation of the magnetic hyperfine field at the Fe nucleus.

Mössbauer study of Fe2+/Fe3+ order–disorder and electron delocalization in K3Fe5F15 at the 490‐K phase transition

The prediction made in J. Appl. Phys. 65, 3987 (1989) that the ferroelectric–ferroelastic phase transition at Tc =490 K in K3Fe5F15 will be accompanied by a change from order to disorder among the Fe2+/Fe3+ ions has been confirmed by Mossbauer spectroscopy. The thermal dependence of both the Fe3+ quadrupolar splitting and the Fe3+ isomer shift exhibits a smooth decrease with increasing temperature to Tc, each with a sharp and reproducible change in slope above Tc. The average Fe2+ quadrupolar splitting (QS) similarly undergoes a major decrease with increasing temperature, with a clear change in slope above Tc. Furthermore, the thermal dependence of the Fe2+ QS distribution shows that the Fe2+/Fe3+ order required crystallographically below Tc is not conserved at higher temperatures. The proportion of Fe2+ present below Tc is close to the stoichiometric 60% value; the proportion decreases above Tc rapidly with temperature and becomes less than 50% by 543 K, consistent with increasing delocalization of an Fe...

Mössbauer study of the new pyrochlore form of FeF3

Abstract Mossbauer spectrometry has been undertaken as a function of temperature on a new form of FeF 3 with the modified pyrochlore structure, recently synthesized by topotactic oxidation. The Mossbauer data (in zero field and in external magnetic field) lead to a noncollinear magnetic structure, in agreement with previous neutron diffraction results. The low value of T N is discussed in terms of magnetic frustration.

New A-deficient perovskites in the series LixLa2/3Ti1-xFexO3 (0.12 ≤ x ≤ 0.33) and La(2+x)/3Ti1-xFexO3 (0.5 ≤ x ≤ 1.0)

Two new series of A-deficient perovskites have been synthesized and structurally characterized from Rietveld treatment of their X-ray diffraction powder patterns. The first one, LixLa2/3Ti1-xFexO3, for 0.12 ≤ x ≤ 0.33, results from the substitution mechanism Ti4+ → Fe3+ + Li+. The structure is closely related to that of La2/3-xLi3xTiO3 (a ≈ ap, b ≈ ap and c ≈ 2 ap) with a symmetry evolution leading to the existence of two domains: for 0.12 ≤ x ≤ 0.19 the symmetry is orthorhombic (Pmmm), while for 0.20 ≤ x ≤ 0.33 a tetragonal symmetry is obtained (P4/mmm). In both cases, the population of La3+ ions, unequally distributed on two adjacent sites for smaller x values, is directly affected by the increase of the Li content and tends to be equalized with larger x values. The second series is found to result from another substitution mechanism Ti4+ → Fe3+ + 1/3La3+, leading to the formula La(2+x)/3Ti1-xFexO3 in the domain 0.5 ≤ x ≤ 1.0. The structure is then closely related to that of LaFeO3, where La3+ ions occupy now only one crystallographic site in the Pnma space group. Transmission Electron Microscopy and Mossbauer spectrometry confirm the above models.

Crystal structure and magnetic characterization of (NH4)2FeF5·H2O

Abstract The compound (NH 4 ) 2 FeF 5 ·H 2 O is prepared by hydrothermal synthesis in HF solutions. The material is orthorhombic, space group Pbcn , a = 10.491(4) A˚, b = 8.090(3) A˚, c = 7.997(3) A˚, Z = 4 . The crystal structure—isotypic with that of (NH 4 ) 2 AlF 5 ·H 2 O—is determined from single-crystal diffraction intensity data and refined to the conventional values of the indexes R = 0.0272 and R w = 0.0262 . The structure is characterized by isolated [FeF 5 (H 2 O)] 2− octahedra linked together by O H … F strong hydrogen bonds and forming zigzag chains running along 001. NH + 4 ions connect these chains by N H … F hydrogen bonds. As expected, the compound exhibits a paramagnetic behavior in the range 80–300 K and is not magnetically ordered at 4.2 K as shown by Mo¨ssbauer spectroscopy.

The first ferroelectric fluoride with a tungsten bronze-type structure

Abstract K3Fe5F15 has been predicted to be both ferroelectric and ferroelastic, with a phase transition at 535 K, on the basis of the atomic coordinates. Subsequently, the dielectric permittivity has been found to reach a maximum at 495 (10) K as the dielectric loss undergoes a change in slope, characteristic of ferroelectric behavior. Furthermore, the heat capacity exhibits a Λ-type anomaly at 490 (10) K, with a corresponding entropy change of Δ S = 5.5 (2) J mol−1 K−1. Ferroelastic domains present at room temperature disappear sharply on heating above 490 (10) K, as K3 Fe5 F15 transforms from orthorhombic to tetragonal symmetry, and reappear on cooling below 480 (10) K. The phase transition was also predicted to be accompanied by a change from order to disorder among the Fe2+, Fe3+ ions on heating above Tc, and the prediction is confirmed by the thermal dependence of the Mossbauer effect in which lines due to Fe2+ broaden as an anomaly appears in the hyperfine structure at Tc.