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.     

Crystal structure of Fe2P2O7

Fe₂P₂O₇ crystallizes in the $\text{C}\text{1}$ space group with lattice parameters a = 6.649(2)Å, b = 8.484(2)Å, c = 4.488(1)Å, α = 90.04°, β = 103.89(3)°, γ = 92.82(3)°, and ?_cal = 3.86 g/cc. It is essentially isostructural with β-Zn₂P₂O₇. As in the Zn compound, the bridging oxygen atom in the P₂O₇ group shows a high anisotropic thermal motion. It appears that the P-O-P bond angle is linear as a result of extensive π bonding with the p orbitals on the bridging oxygen atom. The high thermal motion is vibration of the atom into cavities in the structure.     

The effects of titanium or chromium doping on the crystal structure of V2O3

The crystal structures of five samples of (Ti_xV_1?x)₂O₃ (0.011 ≤ x ≤ 0.077) and seven samples of (Cr_xV_1?x)₂O₃ (both metallic and insulating phases, 0 ≤ x ≤ 0.05) were determined from X-ray diffraction data collected from single crystals. These compounds are isomorphous with α-alumina. The cell dimensions change such that the a axes increase and the c axes decrease with increasing Ti or Cr. In the CrV₂O₃ system, from 0 to 1.25% Cr doping, changes in structure parallel those observed in the TiV₂O₃ system. These changes are consistent with a slight weakening of the bonding metal-metal interactions in the basal plane, leading to an increase in the metal-metal distances coupled with changes which maintain constant metal-oxygen distances. A discontinuity appears at about 1.25% Cr as the transition from metal to insulating behavior occurs with increasing Cr content. No change in crystal symmetry accompanies this transformation. It appears that the metal-metal bonding interactions are retained even in the insulating phase of Cr-doped V₂O₃. A comparison of the structural variation in the Cr- and Ti-doped systems suggests that the change from metallic to insulating behavior cannot be a structure effect. These changes are, however, consistent with the band model proposed by others for these systems.     

Some studies on the solid solutions in the systems Sr2Nb2O7Eu2Nb2O7 and Sr2Ta2O7Eu2Ta2O7

Preparation of new solid solutions containing divalent europium have been tried in the systems Eu₂Nb₂O₇Sr₂Nb₂O₇ and Eu₂Ta₂O₇Sr₂Ta₂O₇. These solid solutions described as Eu_2xSr_2(1?x)M₂O₇ (M = Nb and Ta) exist in a pure orthorhombic phase in a limited region of x from 0 to about 0.5. The compounds with compositions close to Eu₂M₂O₇ exist but techniques have not been found to prepare them in pure form.     

Subsolidus phase relations in the systems CeO2RE2O3 (RE2O3 = C-type rare earth sesquioxide)

The systems CeO₂RE₂O₃ (RE₂O₃ = C-type rare earth sesquioxide) were studied to: (1) investigate the claims of several workers for the existence of a complete solid solution series between CeO₂ and RE₂O₃ and (2) to characterize the weak C-type X-ray diffraction peaks reported by others from samples in the single-phase fluorite solid solution region. It is shown that a complete solid solution series does not exist, and an explanation for the observations of others reporting such is tendered. It is also shown that the observation of C-type reflections in the supposed single-phase fluorite field can be attributed to the partial reduction of Ce⁴⁺ to Ce³⁺ at the firing temperature, resulting in the movement of the bulk composition into a two-phase field of the CeO₂RE₂O₃Ce₂O₃ phase diagram, rather than the formation of a domain structure due to ordering.     

Chemistry in fuming nitric acids—I. NMR spectroscopic study of PF5, HPO2F2 and P4O10 in the solvent system 44 wt.% N2O4 in 100% HNO3

³¹P and ¹⁹F NMR spectroscopy has been used to elucidate the nature of the interaction of PF₅, HPO₂F₂ and P₄O₁₀ with the solvent system 44 wt.% N₂O₄ in 100% HNO₃ (“High Density Acid”, HDA). PF₅ generates the species PF₆^?, HPO₂F₂ and HF (with some H₂PO₃F present as a minor product). HPO₂F₂ gives rise to H₂PO₃F and HF (with smaller amounts of PF₆ also present). The ³¹P NMR spectrum of P₄O₁₀ in HDA exhibits four resonances assigned to P(OH)₄⁺, H₄P₂O₇, (HPO₃)₄ and a mixture of cyclic and branched phosphoric acids, respectively.     

A new monoclinic phase in the Fe2O3TiO2 system. I. Structure determination and Mössbauer spectroscopy

Single crystals with compositions within the Fe₂O₃TiO₂ system were grown from a flux containing various amounts of the basic oxides. Apart from the known pseudobrookite (Fe₂TiO₅) and rutile (TiO₂) structures, a new monoclinic polytype of Fe₂TiO₅ was found, which was isostructural with V₃O₅. The structure was determined by X-ray analysis and Mössbauer spectroscopy contributed data on hyperfine parameters and the magnetic ordering temperature.     

Phases in the Ba3Fe1+xS5 series: The structure of β-Ba9Fe4S15 and its low-temperature α polymorph

The crystal structure of β-Ba₉Fe₄S₁₅ shows that it is a phase in the infinitely adaptive series of compounds Ba₃Fe_1+xS₅, 0 ? x ? 1. The material is synthesized by reacting a slightly sulfur-rich mixture at 900°C in a sealed quartz ampoule. Lattice constants are a = 25.212(3), Å, b = 9.594(1), Å, c = 12.575(1), Å, Pnma, z = 4. Three thousand thirty-three structure amplitudes were refined to R = 0.049. BaS₆ trigonal prisms share triangular faces to form infinite columns; the columns in turn share edges and create nearly hexagonal enclosures. Within these rings are additional Ba and S and tetrahedral interstices are created which can be occupied by Fe. The variation of the Fe occupancy from ring to ring gives rise to phases in which one dimension is an integral multiple of the 8.5-Å repeat observed in one end member of the series, Ba₃FeS₅. The other end member is Ba₃Fe₂S₅. At temperatures below 900°C a polymorphic phase is formed. Its lattice constants are a = b = 9.634(1), Å, c = 34.311(3)Å, $\text{I4}{}_1\text{a}\text{, z = 4}$. One thousand five hundred eighty-three structure amplitudes were refined to R = 0.0483. Trigonal prisms and bisdisphenoids articulate to form a complex three-dimensional structure. Two of the S atoms in the structure have statistical site occupancies.     

Mechanism of formation of (η-C5H5)2NbH3 from the reaction of (η-C5H5)2NbCl2 with hydridoaluminate reducing agents

The mechanism of the transformation of (η⁵-C₅H₅)₂NbCl₂ to (η⁵-C₅H₅)₂NbH₃ by hydridoaluminate reducing agents has been investigated. Results suggest disproportionation of a niobium(IV) hydrite, leading to the trihydride product and a niobium(III) hydridoaluminate, (η⁵-C₅H₅)₂NbH₂AlR₂, which in turn is converted to the trihydride on hydrolysis. (η⁵-C₅H₅)₂NbH₂AlH₂ has been isolated; deuterium labelling shows that hydrogens exchange between ring and metal-bridging positions in this molecule.     

Phase analysis in the Fe2O3NiO mixed oxides prepared by co-precipitation

Iron-nickel mixed oxides containing up to 10 mole% of NiO have been prepared by hydroxide coprecipitation technique. The oxide samples have been heated to different temperatures ranging from 300 to 1300°C and studied by infrared spectroscopy, magnetic susceptibility, and X-ray diffraction measurements. The maximum solubility of NiO in Fe₂O₃ under the present experimental conditions is found to be 2 mole% when the samples are heated to 550°C. The solubility decreases with increase of sintering temperature and also as the total NiO content of the sample increases. Formation of nickel ferrite phase at concentrations higher than 2 mole% of NiO is clearly indicated.     

Direct synthesis and NMR spectra of [(C6H5)2PCH2]2CuB5H8: Comments on the solution structure of 2,3-μ-metallopentaboranes

The species [(C₆H₅)₂PCH₂]₂CuB₅H₈ has been prepared directly from [(C₆H₅)₂PCH₂]₂CuI and K[B₅H₈]. NMR spectra unequivocally indicate that the species has a static structure in solution and an argument is presented that all 2,3-,μ-metallopentaboranes have similarly static solution structures.     

The crystal structure of vanadium ditelluride,V1+xTe2

Vanadium ditelluride, V_1.04Te₂, has a Cd(OH)₂-type structure with unit cell dimensions a_h = 3.638 Å and c_h = 6.582 Å above the transition temperature T_t of 482 K. Below T_t the structure is monoclinic, space group $\text{C2}\text{m}$, with cell dimensions a_m = 18.984 Å(≈3a_h√3), b_m = 3.5947 Å (≈a_h), c_m = 9.069 Å (≈√(3a²_h + c²_h)), β = 134.62°. This low-temperature form is isostructural with NbTe₂ and TaTe₂ (which do not show a phase transition); the vanadium atoms form double zigzag chains with VV distances of 3.316 Å, which distort the Te lattice. Complex diffraction patterns were observed due to the simultaneous occurrence of the distortion of the Cd(OH)₂-type structure of vanadium ditelluride in three equivalent directions. Similar patterns were found for the Nb and Ta ditellurides.     

The reported ion Fe3S2(NO)5: A re-investigation

Salts reported to contain the iron-sulphur cluster nitrosyl anion Fe₃S₂(NO)₅⁻ are shown to be identical with those containing the well-known anion Fe₄S₃(NO)₇⁻.     

The synthesis and decarbonylation of (η5-C5H5)Mo(Co)2(COCH3)E(C6H5)3, where E is arsenic or antimony

The synthesis of the title compounds by reaction of (η⁵-C₅H₅)Mo(CO)₃CH₃ with excess As(C₆H₅)₃ or Sb(C₆H₅)₃ in CH₃CN is described. Thermal decarbonylation results in the preferential ejection of As(C₆H₅)₃ or Sb(C₆H₅)₃ from the new acetyl complexes, which accounts for the failure of previous attempts to synthesise the acetyl complexes. Photolytic decarbonylations lead to new-alkyl complexes (η⁵-C₅H₅)Mo(CO)₂(CH₃)E(C₆H₅)₃. IR and NMR data for the new complexes are tabulated.     

A study of TiO system between Ti3O5 and TiO2 at high temperature by means of electrical resistivity

The TiO system between the compositions TiO₂ and Ti₃O₅ has been studied in following the electrical conductivity against the oxygen partial pressure. Several features are discussed : the dependence of electrical conductivity versus oxygen pressure, the kinetics of approach to equilibrium, and reversibility during oxidation and reduction paths. The results suggest an important contribution of point defects for small departures from the composition TiO₂ at high temperature.     

Microdomain texture and oxygen excess in the calcium-lanthanum ferrite: Ca2LaFe3O8

The analysis by TEM and electron diffraction of the anion-deficient perovskite Ca₂LaFe₃O₈ confirms the model previously proposed by J. C. Grenier et al. (Mater. Res. Bull.11, 1219 (1976)) with a structure intermediate between perovskite and brownmillerite. The unit cell parameters are ～√2a_c, 3a_c, √2a_c (where a_c is the cubic perovskite unit cell parameter). However, the unit cell is sometimes doubled along the b axis. When the sample is treated in air at temperatures around 1400°C, an oxidation process is observed and the unit cell becomes cubic (a_c = 3.848(3) Å). Nevertheless, electron diffraction investigations suggest the existence of a much more complex situation in which three-dimensional microdomains intergrow within one crystal. Each of these microdomains appears to have a structure clearly related to the low-temperature sample, but the superstructure is randomly found along each of the three cubic subcell directions (i.e., the unit cell √2a_c, √2a_c, 3a_c alternates randomly with 3a_c, √2a_c, √2a, and with √2a_c, 3a_c, √2a_c). High-resolution electron microscopy allows one to ascertain this microdomain texture of the real crystal.     

The crystal and molecular structure of Rh2(CH3CO2)4[HCON(CH3)]2—effect of ligands on metalmetal bonding

The structure of Rh₂(CH₃CO₂)₄(DMF)₂ {DMF = HCON(CH₃)₂} has been determined by single crystal X-ray methods. The compound crystallizes with eight formula units in a cell of dimensions: a = 29.438(7) Å, b = 7.978(2) Å, c = 20.279(5) Å, β = 113.20(4)°, V = 4377.5 ų, space group C2/c. The structure has been refined by full-matrix least-squares method to a final R = 0.030 for the 4156 observed data. Two Rh(II) atoms are linked by four acetate groups forming a dimeric unit, where the RhRh distance is 2.383(1) Å. The coordination sphere about each Rh atom is completed by a DMF molecule; the average RhO(DMF) distance is 2.296(3) Å.     

35Cl NQR in N3P3Cl4Ph2 and N3P3Cl4(NMe2)2

³⁵Cl NQR has been investigated in two cyclotriphosphazene derivatives N₃P₃Cl₄Ph₂ and N₃P₃Cl₄(NMe₂)₂. The observed frequencies are assigned to the various chlorines and the temperature variation of the NQR frequencies studied in the range from 77 K to 300 K. The results are analysed using the Bayer-Kushida-Brown approach. Torsional (librational) frequencies are found to fall in the range 10–25 cm^?1 and are found to be only slightly temperature dependent.     

Structure types and phase transformations in KMnCl3 and TlMnCl3

KMnCl₃ and TlMnCl₃ are known to crystallize in tetragonal and cubic perovskite structures, respectively. Room temperature X-ray diffraction data obtained in our laboratory proved that the perovskite structure of KmnCl₃ is orthorhombic. The space group is Pnma and Z = 4. Unit cell parameters are a = 7.08(1), b = 9.97(1), and c = 6.98(1) Å. Experimental data showed that the perovskite structures of KMnCl₃ and TlMnCl₃ are not stable, and that both materials transform slowly into another orthorhombic, nonperovskite KCdCl₃ structure with space group Pnma and Z = 4. Cell parameters of these structures are a = 8.769(7), b = 3.883(9), and c = 14.42(1) Å for KMnCl₃ and a = 8.926(8), b = 3.839(9), and c = 14.77(1) Å for TlMnCl₃. The nonperovskite structures of KMnCl₃ and TlMnCl₃ transform on heating to the perovskite structures and these phase transitions are not immediately reversed. No correlation could be found between the KCdCl₃ structure and water incorporation in the crystal lattice as has been previously suggested. An analysis of the factors that cause the K structure to be exhibited in chloride and to be absent in the fluoride compounds is also presented.