Uncompensated magnetization in the layered molecular antiferromagnet {N(n-C5H11)4[MnFe(ox)3]}∞

Studies on the magnetic properties of the molecular antiferromagnetic material {N(n-C₅H₁₁)₄[Mn^IIFe^III(ox)₃]}_∞, carried out by various physical techniques (AC/DC magnetic susceptibility, magnetization, heat capacity measurements and Mössbauer spectroscopy) at low temperatures, have been presented. Different experimental observations complement each other and provide a clue for the observation of an uncompensated magnetization below the Néel temperature and short-range correlations persisting high above T_N. It is understood that the honeycomb layered structure of the compound contains non-equivalent magnetic sub-lattices, (Mn^II–ox–Fe^III_A–...) and (Mn^II–ox–Fe^III_B–...), where different responses of the Fe^III_A and Fe^III_B spin sites towards an external magnetic field might be responsible for the observation of the uncompensated magnetization in this compound at T < T_N. The present magnetic system is an S = 5/2 2-D Heisenberg antiferromagnet system with the intralayer exchange parameter J/k_B = −3.29 K. A very weak interlayer exchange interaction was anticipated from the spin wave modeling of the magnetic heat capacity for T < 0.5T_N. The positive sign of the coupling between the layers has been concluded from the Mössbauer spectrum in the applied magnetic field. Frustration in the magnetic interactions gives rise to the uncompensated magnetic moment in this compound at low temperatures.     

Structure and Magnetism of Pr1−xSrxFeO3−δ

Crystal structure, redox, and magnetic properties for the Pr_1−xSr_xFeO_3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO₃-type structure for 0≤x≤0.80, and probably in the Sr₈Fe₈O₂₃-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe³⁺) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe⁴⁺ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K.     

Non-stoechiométrie dans le système FeF3---ZrF4

The FeF₃---ZrF₄ system has been investigated using X-ray diffraction, micro-DTA, magnetic measurements, and Mössbauer spectroscopy. At 850°C two solid solutions with formulas Fe_1−xZr_xF_3+x have been obtained, which are based on the VF₃ type for 0 ≤ x < 0.10 and on the ReO₃ type for 0.10 ≤ x ≤ 0.23, respectively. A phase transition occurs in the VF₃-type domain: the transition temperature decreases with increasing x. All phases exhibit an antiferromagnetic behavior. The thermal variation of Mössbauer parameters has been studied for x = 0.05 and x = 0.23 as well as their variation vs composition at 300 K. The magnetic behavior is discussed on the basis of the structural data presently available.     

The nature of the product from the reaction of tetracyanoethylene with iron pentacarbonyl

The reaction of tetracyanoethylene with iron pentacarbonyl in mesitylene at 90° gave an insoluble product, Fe₂C₁₆H₁₂N₈O₇. The Mössbauer and IR spectra, the magnetic susceptibilities over the temperature range 95–298 K, and the thermal decomposition temperatures in a nitrogen atmosphere were measured. The solid is best described as an iron (III) ketoamine polymer with hydroxo bridges between iron atoms.     

Synthesis, crystal structure and characterization of iron pyroborate (Fe2B2O5) single crystals

Single crystals of iron(II) pyroborate, Fe₂B₂O₅, were prepared at 1000–1050 °C under an argon atmosphere. The crystals were transparent, yellowish in color and needle-like or columnar. The crystal structure of Fe₂B₂O₅ was analyzed by single-crystal X-ray diffraction. Refined triclinic unit cell parameters were a=3.2388(2), b=6.1684(5), c=9.3866(8) Å, α=104.613(3)°, β=90.799(2)° and γ=91.731(2)°. The final reliability factors of refinement were R1=0.020 and wR2=0.059 [I > 2σ(I)]. Transmittance over 50% in the visible light region from 500 to 750 nm was observed for a single crystal of Fe₂B₂O₅ with a thickness of about 0.3 mm. The light absorption edge estimated from a diffuse reflectance spectrum was at around 350 nm (3.6 eV). Magnetic susceptibility was measured for single crystals at 4–300 K. Fe₂B₂O₅ showed antiferromagnetic behavior below the Néel temperature, T_N≈70 K, and the Weiss temperature was T_W=36 K. The effective magnetic moment of Fe was 5.3μ_B.     

The Glaserite-like Structure of Double Sodium and Iron Phosphate Na3Fe(PO4)2

The double sodium and iron phosphate Na₃Fe(PO₄)₂ was synthesized and studied by the XRD method, the second harmonic generation technique, and Mössbauer and IR spectroscopy. The compound crystallizes into a monoclinic system (space group C2/c) with unit cell parameters a=9.0736(2) Å, b=5.0344(1) Å, c=13.8732(3) Å, β=91.435(2)° and is found to be related to the K₃Na(SO₄)₂ structure type. The crystal structure was determined by Rietveld analysis (R_wp=5.86, R_I=2.03). Iron cations occupy the M (Na) position while sodium cations occupy the X (K) and Y (K) positions of the glaserite-like structure. Mössbauer spectroscopy shows the presence of high-spin Fe³⁺ in octahedral coordination.     

A new iron oxophosphate SrFe3(PO4)3O with chain-like structure

A new strontium iron oxophosphate SrFe₃(PO₄)₃O was synthesized by the solid state method and its structure was studied by single-crystal X-ray and electron diffraction, high-resolution electron microscopy, Mössbauer and IR spectroscopy. The compound crystallizes in a monoclinic system (space group P2₁/m) with unit-cell parameters: a = 7.5395(7), b = 6.3476(7) c = 10.3161(13) Å, β = 99.740(9)°. The structure of SrFe₃(PO₄)₃O represents a new structural type and is made up of isolated PO₄ tetrahedra and FeO_n polyhedra connected via common vertices and edges to form a 3D framework. Iron cations occupy three crystallographically independent sites with different oxygen environment: Fe1 and Fe2 occupy two octahedral sites, and Fe3 is five-coordinated. Two particularities of this structure are remarkably mentioned: the isolated {FeO₆}_n octahedral chains along the b direction and the five coordinated environment for the Fe3 position. Mössbauer spectroscopy confirmed the presence of only high-spin Fe³⁺ cations in two types of coordination environment. The IR-data show the presence of only PO₄³⁻ groups.     

EuTZn (T=Pd, Pt, Au) with TiNiSi-type structure—Magnetic properties and Eu Mössbauer spectroscopy

The europium compounds EuTZn (T=Pd, Pt, Au) were synthesized from the elements in sealed tantalum tubes in an induction furnace. These intermetallics crystallize with the orthorhombic TiNiSi-type structure, space group Pnma. The structures were investigated by X-ray diffraction on powders and single crystals: a=732.3(2), b=448.5(2), c=787.7(2) pm, R₁/wR₂=0.0400/0.0594, 565 F² values for EuPdZn, a=727.8(3), b=443.7(1), c=781.7(3) pm, R₁/wR₂=0.0605/0.0866, 573 F² values for EuPtZn, and a=747.4(2), b=465.8(2), c=789.1(4) pm, R₁/wR₂=0.0351/0.0590, 658 F² values for EuAuZn, with 20 variables per refinement. Together the T and zinc atoms build up three-dimensional [TZn] networks with short T–Zn distances. The EuTZn compounds show Curie–Weiss behavior in the temperature range from 75 to 300 K with μ_eff=7.97(1), 7.70(1), and 7.94(1) μ_B/Eu atom and θ_P=18.6(1), 34.9(1), and 55.5(1) K for T=Pd, Pt, and Au, respectively, indicating divalent europium. Antiferromagntic ordering was detected at 15.1(3) K for EuPdZn and canted ferromagnetic ordering at 21.2(3) and 51.1(3) K for EuPtZn and EuAuZn. ¹⁵¹Eu Mössbauer spectroscopic measurements confirm the divalent nature of the europium atoms by isomer shift values ranging from −8.22(8) (EuPtZn) to −9.23(2) mm/s (EuAuZn). At 4.2 K full magnetic hyperfine field splitting is observed in all three compounds due to magnetic ordering of the europium magnetic moments.     

Manganite charge and orbitally ordered and disordered states probed by Fe substitution into Mn site in , and (, Gd, Sm, Nd, Pr, La)

The layered manganese oxides LnBaMn_1.96Fe_0.04O_y (Ln=Y, Gd, Sm, Nd, Pr, La) have been synthesized for y=5,5.5 and 6. In the oxygen-saturated state (y=6) they exhibit the charge and orbital order at ambient temperature for Ln=Y, Gd, Sm, but unordered e_g-electronic system for Ln=La, Pr, Nd. Fourfold increase of quadrupole splitting was observed owing to the charge and orbital ordering. This is in agreement with the jumplike increase in distortion of the reduced perovskite-like cell for the charge and orbitally ordered manganites compared to the unordered ones. Substitution of 2% of Mn by Fe suppresses the temperatures of structural and magnetic transitions by 20–50 K. Parameters of the crystal lattices and the room-temperature Mössbauer spectra were studied on 40 samples whose structures were refined within five symmetry groups: P4/mmm, P4/nmm, Pm-3m, Icma and P2/m. Overwhelming majority of the Fe species are undifferentiated in the Mössbauer spectra for most of the samples. Such the single-component spectra in the two-site structures are explained by the preference of Fe towards the site of Mn(III) and by the segmentation of the charge and orbitally ordered domains.     

Generation of a heterobimetallic FeMoS system containing intermediate spin [FeIIN4S2] functionality: Synthesis,characterisation and redox chemistry of [(N–N)2FeS2MoS2] (N–N=bipy or phen)

Tetrathiomolybdate reacts with iron (II) in the presence of bidentate ligands to form neutral binuclear complexes [Fe(N–N)₂MoS₄] [N–N=2-2bipyridine(bipy) and 1,10-phenanthroline(phen)] showing intermediate spin character for Fe^II. The spin state of the complexes has been examined by variable temperature magnetic moment (VTM) measurements and by variable temperature Mössbauer spectroscopy. The Mössbauer spectra indicate the presence of two iron sites: one of intermediate spin and the other of low spin at room temperature. The low spin site predominates over the intermediate spin as the temperature is lowered. The structural features of the complexes are supported by i.r., Raman, electronic and FAB mass spectra and by X-ray powder diffraction data. Substitution of one bipy/phen ligand in [Fe(N–N)₃]²⁺ by MS₄ ^2– ligands does not impart any major effect towards the size of the redox potentials of the tris-bipy/phen complexes, although the reversible nature of their cyclic voltammetric response is affected.     

Neutron diffraction study of the β′ and γ phases of LiFeO2

The β, β′, γ and α phases of LiFeO₂, synthesized as powders, were annealed at different temperatures and characterized by X-ray measurements. The β′ and γ modifications were also studied by time-of-flight neutron diffraction (ISIS Facility, UK). The structure of the β′ phase was refined in the monoclinic C2/c space group (a=8.566(1), b=11.574(2), c=5.1970(5) Å, β=146.064(5)°) to wR_p=0.071–0.080 (data from four counter banks). Fe and Li atoms are ordered over two of the four independent sites, and partially disordered over the other two. The ordered Li has a distorted tetrahedral coordination. The γ structure was refined at RT (a=4.047(1), c=8.746(2) Å) and at 570 °C (a=4.082(3), c=8.822(6) Å) in the I4₁/amd symmetry, showing full order with Li in octahedral coordination at RT, and in a split-atom configuration at high temperature. On annealing, the β′ polymorph was found to transform to γ at 550 °C, thus suggesting that it is a metastable phase. Electrostatics is discussed as the driving force for the α→β′→γ ordering process of LiFeO₂.     

Antiferromagnetism of perovskite EuZrO3

Polycrystalline EuZrO₃ has been synthesized by the solid-state reaction between EuO and ZrO₂, and its structural and magnetic properties have been investigated. Rietveld analysis of the X-ray diffraction pattern indicates that EuZrO₃ crystallizes in an orthorhombic perovskite structure. ¹⁵¹Eu Mössbauer effect measurement reveals that almost all the europium ions are present as the divalent state and occupy distorted sites with non-axial electric field gradients, in agreement with the orthorhombic structure. In contrast to previous reports, an antiferromagnetic transition was observed around 4.1 K. The magnetic structure below the Néel temperature has been discussed.     

The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

The possibility to synthesize layered oxycarbonates, with nominal composition Sr₄Fe_2−xMn_xO₆CO₃ involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr₄FeMnO₆CO₃ using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO₃ groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr₄Fe₂O₆CO₃.     

Synthesis and characterization of Fe2O3/SiO2 nanocomposites

Fe₂O₃/SiO₂ nanocomposites based on fumed silica A-300 (S_BET = 337 m²/g) with iron oxide deposits at different content were synthesized using Fe(III) acetylacetonate (Fe(acac)₃) dissolved in isopropyl alcohol or carbon tetrachloride for impregnation of the nanosilica powder at different amounts of Fe(acac)₃ then oxidized in air at 400–900 °C. Samples with Fe(acac)₃ adsorbed onto nanosilica and samples with Fe₂O₃/SiO₂ including 6–17 wt% of Fe₂O₃ were investigated using XRD, XPS, TG/DTA, TPD MS, FTIR, AFM, nitrogen adsorption, Mössbauer spectroscopy, and quantum chemistry methods. The structural characteristics and phase composition of Fe₂O₃ deposits depend on reaction conditions, solvent type, content of grafted iron oxide, and post-reaction treatments. The iron oxide deposits on A-300 (impregnated by the Fe(acac)₃ solution in isopropanol) treated at 500–600 °C include several phases characterized by different nanoparticle size distributions; however, in the case of impregnation of A-300 by the Fe(acac)₃ solution in carbon tetrachloride only α-Fe₂O₃ phase is formed in addition to amorphous Fe₂O₃. The Fe₂O₃/SiO₂ materials remain loose (similar to the A-300 matrix) at the bulk density of 0.12–0.15 g/cm³ and S_BET = 265–310 m²/g.     

The synthesis, structural characterization, magnetochemistry and Mössbauer spectroscopy of [Fe3LnO2(CCl3COO)8H2O(THF)3] (Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Lu and Y)

The new tetranuclear complexes [Fe₃Ln(μ₃-O)₂(CCl₃COO)₈(H₂O)(THF)₃]·THF (Ln = Ce^III (1), Pr^III (2), Nd^III (3)) and [Fe₃Ln(μ₃-O)₂(CCl₃COO)₈(H₂O)(THF)₃]·THF·C₇H₁₆ (Ln = Sm^III (4), Eu^III (5), Gd^III (6), Tb^III (7), Dy^III (8), Ho^III (9), Lu^III (10) and Y^III (11)) have been prepared. All compounds were prepared by the reaction between [Fe₂BaO(CCl₃COO)₆(THF)₆] and the corresponding Ln^III nitrate salt. The crystal structures of 1–4, 8 and 9 have been determined; these isostructural molecules have a non-planar {Fe₃Ln(μ₃-O)₂} “butterfly” core. Magnetic susceptibility measurements show dominant intramolecular antiferromagnetic exchange interactions for all the complexes. ⁵⁷Fe Mössbauer spectroscopy shows three different environments for the Fe^III metal ions, all in their high-spin state S = 5/2 (confirming that no electron transfer from Ce^III to Fe^III occurs in 1). At the time scale of the Mössbauer spectroscopy (about 10⁻⁷ s), evidence of magnetization blocking, i.e. slow relaxation of the magnetization, is observed below 3 K for 7, which was confirmed by ac susceptibility measurements.     

Thermodynamique de composes azotes III. Etude Thermochimique de la glycine et de la l-α-alanine

The enthalpies of formation of glycine and L-α-alanine are determined by two complementary techniques: combustion calorimetry and change of state (sublimation) calorimetry.

Full-size image (2K)

Full-size image (2K)

The values of the experimental and theoretical energy of conjugation of these two molecules (partially conjugated) and the energetical value of the C---N bond in Laidler's scheme, derived from the experimental enthalpy of atomization, are also given.

Full-size image (3K)

Full-size image (3K)

Résumé

Les enthalpies de combustion à létat condensé de la glycine et de la L-α-alanine ont été déterminées à d'un calorìmetre isopéribolique à bombe rotative. Leurs valeurs à l'état standard à 298,15 K sont: δH°_c = −(972,98±0,19) kJ mol⁻¹ pour la glycine et ΔHδ_c = −(1621,45 − 0,48) kJ mol^-1 pour la L-α-alanine. Les enthalpies de sublimation à l'état standard à 298,15 K ont été obtenues par calori-     

Comparison effects of different doping on spin-Peierls transition in CuGeO(3)

The comparison between non-magnetic spin-Peierls (SP) and magnetic Néel ground states have been investigated in CuGeO₃ doped with Zn²⁺, Ni²⁺ and Mn²⁺ ions by using the electron spin resonance (ESR) techniques in the temperature range of 3–300 K. It was concluded that the one-dimensional (1D) antiferromagnetic (AF) spin chain formed of spin-1/2 (Cu²⁺) ions is broken by spin-0 (Zn²⁺), spin-1 (Ni²⁺), and spin-5/2 (Mn²⁺) ions, giving uncoupled spins at the end of the chains that give extra contribution to the spectra at lower temperature. An almost linearly dependence of frequency of resonance field has been showed for X-, K- and Q-band spectra. By the analysis of resonance field–frequency relations, the effects of the internal field is refined and thus the spectroscopic g-factor and internal field were calculated to be g = 1.9386 and H_i = 148 G, respectively.     

Magnetoelectric compounds with two sets of magnetic sublattices: UCrO4 and NdCrTiO5

Using magnetic and magnetoelectric (ME) powder susceptibility measurements, the low temperature magnetic properties of antiferromagnetic UCrO₄ and NdCrTiO₅ have been studied. Their Néel temperatures T_N are 44.5 and 20.5°K, respectively, the Cr³⁺ spin systems of both materials ordering cooperatively at T_N. Below T_N, the U⁵⁺ and Nd³⁺ moments are polarized due to their exchange interaction with the ordered Cr³⁺ spins. It is argued that, for both compounds, each of the two spin systems contributes to the ME susceptibilities. They are thus the first known ME materials possessing two distinct magnetic sublattices. The effective magnetic moments calculated from the magnetic susceptibilities are in good agreement with those previously reported by neutron diffraction studies.     

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

The SrMn_1−xFe_xO_3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^(4+δ)+ and Fe⁽³⁺⁾⁺. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e_g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe⁽³⁺⁾+/Fe^(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.