Etude cristallographique et par effet Mössbauer du fluorure ferrimagnétique Na5Fe3F14γ

The structure of ferrimagnetic γ-Na₅Fe₃F₁₄ has been determined by single-crystal X-ray diffraction. The cell is tetragonal with space group P4₂2₁2 and parameters a = 7.345 ± 0.007 Å and c = 10.400 ± 0.007 Å. The iron atoms occupy a twofold and a fourfold position in the lattice and are octahedrally surrounded by fluorines. These octahedra share corners and form two-dimensional layers of formula (Fe₃F₁₄)^5n?_n. The Mössbauer spectra were measured from 4.2 to 293°K and the results are discussed in terms of the position and environment of iron atoms in the lattice.     

Etude par diffraction de neutrons de la phase Sr0,5La1,5Li0,5Fe0,5O4

A neutron diffraction study has been carried out on Sr_0.5La_1.5Li_0.5Fe_0.5O₄ of K₂NiF₄-type derived structure and it has shown that iron in the tetravalent state has a high spin configuration (t³_2ge¹_g) and that the material has some stacking defects. At room temperature this compound shows an ordering between iron and lithium atoms leading to a nuclear cell a₀√2, a₀√2, c₀ (a₀ and c₀ are the parameters of the K₂NiF₄-type cell). At low temperature ($\text{T <}\text{T}{}_{\mathrm{<mtext>N</mtext>}}\text{2}$) the magnetic structure can be described as antiferromagnetic, corresponding likely to a colinear pattern with a propagation vector of 0.5 ($\text{2π}\text{a}{}_0$) along the [110] axis. At higher temperature ($\text{T}{}_{\mathrm{<mtext>N</mtext>}}\text{2}\text{< T < T}{}_{\mathrm{<mtext>N</mtext>}}$) helimagnetic structure is consistent with a propagation vector of 0.47 ($\text{2π}\text{a}{}_0$) [110].     

Substitution of Co in YBa2Fe3O8

The accommodation of Co in the oxygen-saturated solid-solution phase YBa₂(Fe_1−zCo_z)₃O_8+w has been investigated by powder X-ray and neutron diffraction techniques, as well as by Mössbauer spectroscopy. Of the nominal composition range 0.00?z?1.00 tested, the solid-solution limit under syntheses at 950°C in is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co³⁺ replaces Fe³⁺. Despite the nearly constant value of w, Mössbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a,b plane) decreases with z in accordance with the high-spin states of the majority Fe³⁺ and Co³⁺ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe³⁺ by the smaller Co³⁺ ion. A contrast is pointed out with the substitution of cobalt in YBa₂Cu₃O₇ where it is a larger Co²⁺ ion that replaces smaller Cu²⁺.     

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.     

Mild hydrothermal synthesis and ferrimagnetism of Pr3Fe5O12 and Nd3Fe5O12 garnets

Two pure light rare earth iron garnets Pr₃Fe₅O₁₂ and Nd₃Fe₅O₁₂ single crystals were synthesized under mild hydrothermal conditions and structurally characterized by single crystal and powder X-ray diffraction methods. Both compounds crystallize in cubic space group Ia3?d with lattice parameters a=12.670(2) Å for Pr₃Fe₅O₁₂ and a=12.633(2) Å for Nd₃Fe₅O₁₂, respectively. The synthesis of compounds was studied with regard to phase evolution and morphology development with hydrothermal conditions. We proposed the formation mechanisms and formulated a reasonable explanation for their growth habits. Ferrimagnetic Curie temperatures which have been inferred from thermo-magnetization curves were 580 K for Pr₃Fe₅O₁₂ and 565 K for Nd₃Fe₅O₁₂, and the transitions of long range order were also evidenced by differential scanning calorimetry method. The result of magnetic properties has shown that moments of the large radius Pr³⁺ and Nd³⁺ ions are parallelly coupled with net moments of iron ions.     

Etude structurale de l'hydruro-oxyde LaHO par diffraction des rayons X et par diffraction des neutrons

The hydride-oxide LaHO crystallizes with a fluorite superstructure. The unit cell is tetragonal, space group $\text{P4}\text{nmm}\text{, a = 8,074(1)}$ Å, c = 5,739(1) Å, and Z = 8. The structure has been determined by two complementary techniques: X-ray and neutron diffraction. The La³⁺ ions are situated in the center of cubes at the corners of which four O^2? and four H^? are placed, there exist three different types of environment according to the distribution of the two anion species (the anions of the same type form either a tetrahedron or occupy a face or a diagonal plane of the cube). The LaO(H) distances observed are discussed by taking into account the relationship which exists between bond strength and bond length. The average value of d 〈OH〉 is 2,85 Å, which is rather unusual. It confirms that we are indeed dealing with a hydride-oxide simultaneously associating the O^2? and H^? ions in a ternary combination. Through energy considerations (electrostatic site potential and lattice potential), the structural arrangement of LaHO is compared to that encountered in the case of the homologous lanthanum oxifluorides.     

Structure de La2Fe2S5 et de La2Fe1.87S5

The compound La₂Fe₂S₅ is orthorhombic. Cell parameters are: a = 3.997(2)Å; b = 16.485(5)Å; c = 11.394(4)Å. Space group is Cmc2₁ (Z = 4. In the cell, chains of polyedra comprised of sulfur atoms tetrahedrally or octahedrally coordinating centrally located iron atoms give a monodimensional character to the structure. This one is refined to R = 0.037. To complete the study of these chains, in the La₂Fe_2?xS₅ system, vacancies are introduced on iron atom sites. The ordered compound, La₂Fe_1.87S₅, having such vacancies, is an orthorhombic superstructure of the stoechiometric compound. Cell parameters are: a = 3.9996(5)Å; b = 49.508(3)Å; c = 11.308(3)Å. Space group is Cmc2₁ and Z = 12. The structure is refined to R = 0.068. Only two iron atom sites have vacancies. One is tetrahedral, the other octahedral. In this last case the chain deformations are the more important. The chain becomes a sort of tunnel made of atoms of sulfur, with in its center the short iron-iron separation of 2.82 Å.     

Etude spectroscopique du role des liaisons PF et PO pontes dans les composes oxyfluores du phosphore V. Cas de P2O3F4 et de K2P2O5F2.

The compounds P₂O₃F₄ and K₂P₂O₅F₂ have been studied by vibrational spectroscopy. From the shifts of the symmetric vibration of the terminal PO bonds it appears that the molecular structure contracts as the number of PO (bridged) and/or PF bonds increases. The changes occur at the expense of PO terminal bonds. In these tetrahedral phosphorus (V) compounds the PO (bridged) and PF bonding have a similar role, and the same phenomenon occurs in sulfur VI oxide fluoride compounds.     

Determination des structures de deux ferrites mixtes nouveaux de formule BaLa2Fe2O7 et SrTb2Fe2O7

The structures of the two new ferrites BaLa₂Fe₂O₇ and SrTb₂Fe₂O₇ with tetragonal symmetry have been resolved by X-ray and neutron diffraction performed on powder samples. In both compounds the arrangement of atoms present a close resemblance with the idealized Sr₃Ti₂O₇ structure. It consists of a packing along the c axis of two different blocks. One is formed by the adjunction of two perovskite cells and the other one by a halved rocksalt Sr(Ba)O cell. The iron cation lattice is built by infinite double layers perpendicular to the c axis with the shortest Fe³⁺Fe³⁺ distances inside the double layers much shorter than between the layers. In BaLa₂Fe₂O₇, Ba²⁺ is located on a regular 12-coordinated site and La³⁺ in a regular 9-coordinated polyhedron. Fe³⁺ is surrounded by five oxygen neighbours at 1.98 Å, building a rather regular tetragonal pyramid, with a sixth oxygen at 2.25 Å. In SrTb₂Fe₂O₇, Sr has only eight close neighbours at ～2.80 Å and four more distant at 3.15 Å. Tb³⁺ has seven close neighbours, six building a distorted octahedron with the largest triangular face capped by the seventh oxygen. Fe³⁺ again has five neighbours, but due to the lowering of the symmetry, the square pyramid has become a distorted trigonal bipyramid.     

The standard free energy of formation of YbFe2O4, Yb2Fe3O7, YbFeO3, and Yb3Fe5O12 at 1200°C

The standard free energy of formation of YbFe₂O₄, Yb₂Fe₃O₇, YbFeO₃, and Yb₃Fe₅O₁₂ from metallic iron, Yb₂O₃, and oxygen was determined to be ?100.38, ?158.38, ?58.17, and ?283.40 kcal/mole, respectively, at 1200°C on the basis of the phase equilibria in the FeFe₂O₃Yb₂O₃ system. The FeFe₂O₃-Lanthanoid sesquioxide systems were classified into four types with respect to the assemblage of the ternary compounds in stable existence at 1200°C, and the standard free energy of formation of YbFeO₃ was compared with those of the other lanthanoid-iron perovskites.     

Etude des phases du système FeVO4VO2, obtenues par synthèse hydrothermale à 70 kbar et 1000°C

The solid solutions, Fe³⁺_xV⁵⁺_xV⁴⁺_2(1?x)O₄, have been synthesized at 70 kbar and 1000°C under hydrothermal conditions. Their structure is rutile-type when x < 0.66, whereas it is α-PbO₂-type when x > 0.66. In the latter zone, four different phases have been obtained. For x > 0.9, these compounds have an orthorhombic symmetry (O) if the cations are disordered, while the symmetry lowers to monoclinic (Mβ) if the cations are ordered. When 0.66 < x < 0.75) only one monoclinic phase (Mγ) has been obtained. The transition from Mβ to Mγ or from O to Mγ occurs through a triclinic phase T which is stable for 0.75 < x < 0.9. A single-crystal structural refinement of this phase has been carried out and seems to indicate that this phase is also disordered.     

Etude par RMN de la mobilité du lithium dans trois oxydes à structure pseudo-bidimensionnelle: Li8SnO6, Li7NbO6 et Li6In2O6

The lithium ion mobility in three solid electrolytes (Li₈SnO₆, Li₇NbO₆, and Li₆In₂O₆) has been studied by NMR at several resonance frequencies from 170 to 500°K. The ⁷Li quadrupolar lineshape evolution shows the predominant influence on the conductivity mechanism of the vacancies in the octahedral sites of the oxygen close packing. In Li₈SnO₆, which has no vacancies, the lithium ions situated in the tetrahedral sites have the highest mobility. Spin-lattice relaxation times are in good agreement with the hypothesis of a Li₇NbO₆ 2D conductivity. The values of the activation energy, increasing from Li₇NbO₆ to Li₆In₂O₆ and to Li₈SnO₆, are found to be three times lower than those obtained from conductivity measurements.     

Etude structurale de Li2TeO4. Coordination du tellure VI et du lithium par les atomes d'oxygéne

A single crystal of Li₂TeO₄ has been prepared by hydrothermal synthesis and its structure has been determined from three-dimensional X-ray analysis. The crystals are tetragonal, space group P4₁22 with a = b = 6.045(3) Å, c = 8.290(2) Å, and Z = 4. The structure is a distorted inverse spinel with helicoidal chains of Te(VI) octahedra parallel to the [001] axis which can be formulated as [TeO₄]^2n?_n.     

Etudes par diffraction neutronique et RMN de ZnMn3C et GaMn3C0.935

Above a temperature Θ_T the perovskite type compounds ZnMn₃C and GaMn₃C_0.935 are ferromagnetic as GaMn₃C studied previously. Below Θ_T the magnetic structures of ZnMn₃C and GaMn₃C_0.935 are tetragonal noncollinear, with a ferromagnetic component along the tetragonal Oz axis and an antiferromagnetic component in the Oxy plane while GaMn₃C is a collinear antiferromagnet below Θ_T. The two compounds ZnMn₃C and GaMn₃C_0.935 show with respect to GaMn₃C the common feature of a reduction of the number of electrons, but obtained in substantially different ways, by the substitution Ga → Zn in ZnMn₃C, by the production of vacancies in GaMn₃C_0.935. By NMR we have observed resonances on ⁶⁹Ga and ⁷¹Ga which can be attributed to neighborhoods of zero, one and two C-vacancies in GaMn₃C_0.935 resonances on ⁶⁷Zn in ZnMn₃C and resonances on ⁵⁵Mn.There is a good agreement between the symmetry changes observed by neutron diffraction and NMR, between the thermal variation of the ferromagnetic components, determined by the same techniques and by magnetic measurements.     

Etude par diffraction X et par spectrometrie de vibration de composes de coordination des halogenures d'aluminium avec l,acetonitrile: analyse vibrationnelle des cations AlCl(CH3CN)2+5 et AlBr(CH3CN)52+

The compounds AlX₃-2CH₃CN and AlX₃-1.66CH₃CN (X = Cl, Br) are prepared in order to study the vibrational properties of AlX(CH₃CN)²⁺₅. The different chemical phases obtained are characterised by X-ray diffraction. It is shown that AlCl₃-2CH₃CN and AlBr₃-2CH₃CN are isomorphic and unit cell parameters are determined. The spectra of the cations AlX(CH₃CN)²⁺₅ in the 600-50cm^?1 range are interpreted by comparison with those of Al(CH₃CN)³⁺₆ and from the data obtained from H-D substitution. Analysis of the spectra of the complexes AlX₃-1,66CH₃CN enables the formula [2AlX^?₄: AlX(CH₃CN)|²⁺₅] to be postulated.     

Structure et mecanisme de cristallisation des produits obtenus par hyper-trempe dans les systemes BaO·Fe2O3 et SrO·Fe2O3

Splat cooling applications to the BaO·Fe₂O₃ and SrO·Fe₂O₃ systems are described. The complete study of structural and crystallographic properties is given for several compositions in these systems. The mechanism of the new cristallization after vitrification is explained for these phases. New compounds are formed, so the interest of the method is proved for the study of binary diagrams. Application of these compounds may be possible on the basis of their magnetic properties.     

Thermodynamic studies on SrFe12O19(s), SrFe2O4(s), Sr2Fe2O5(s) and Sr3Fe2O6(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₂O₄(s), Sr₂Fe₂O₅(s) and Sr₃Fe₂O₆(s) powders and the compounds were characterized by X-ray diffraction analysis. Different solid-state electrochemical cells were used for the measurement of emf as a function of temperature from 970 to 1151 K. The standard molar Gibbs energies of formation of these ternary oxides were calculated as a function of temperature from the emf data and are represented as (SrFe₂O₄, s, T)/kJ mol⁻¹ (±1.7)=−1494.8+0.3754 (T/K) (970?T/K?1151). (Sr₂Fe₂O₅, s, T)/kJ mol⁻¹ (±3.0)=−2119.3+0.4461 (T/K) (970?T/K?1149). (Sr₃Fe₂O₆, s, T)/kJ mol⁻¹ (±7.3)=−2719.8+0.4974 (T/K) (969?T/K?1150).Standard molar heat capacities of these ternary oxides were determined from 310 to 820 K using a heat flux type differential scanning calorimeter (DSC). Based on second law analysis and using the thermodynamic database FactSage software, thermodynamic functions such as Δ_fH°(298.15 K), S°(298.15 K) S°(T), C_p°(T), H°(T), {H°(T)-H°(298.15 K)}, G°(T), free energy function (fef), Δ_fH°(T) and Δ_fG°(T) for these ternary oxides were also calculated from 298 to 1000 K.     

Etude par spectrométrie Mössbauer des carbures de Fer Fe3C et Fe5C2

Mössbauer spectra of Fe₃C and Fe₅C₂ have been obtained from 4°K up to the Curie temperature of each carbide. The asymmetry parameter of the Fe_I atom of Fe₃C is close to 1. The splitting of site II of Fe₅C₂ is observed. A comparison of the quadrupole splittings of Fe₃C and Fe₅C₂, in the paramagnetic state, leads us to attribute sites I and II of Fe₅C₂ to Fe_I and Fe_II, respectively. In the case where there is only a spin direction for Fe_I, this direction is located in the planes or near the planes (a, c) or (a, b).