Neutron diffraction study on the spinel Cu0.72Co2.28O4

A neutron powder diffraction refinement of the positional and thermal structure parameters of Cu_0.72Co_2.28O₄ spinel has been carried out. It has been found that the spinel is inverse, with inversion parameter approximately equal to one half of the copper content value.     

Kinetics of thermal decomposition of Co3O4 powder and single crystals

Kinetics of thermal decomposition in vacuum of Co₃O₄ powder as well as single crystals has been investigated. Discrepancies with the results of previous authors have been discussed. Decomposition of Co₃O₄ proceeds through formation of a compact layer of CoO and hence diffusion is the rate-limiting factor. The experimental curves α(t) be described for 0.05 < α < 0.85 using a modified Ginstling-Brounshtein equation: 1 ? 2α/3 ? (1 ? α)^2/3 = ktⁿ where the activation energy varies with the degree of decomposition.     

Neutron diffraction study of the electrochemical passivation of nickel powder

With the aid of the Yamaoka mechanism and a.c. polarographic observables, rate constants for the homogeneous reduction of Co(III)pentammine complexes by Eu(II) are measured. Where comparison is possible, rate parameters obtained by this electrochemical procedure are found to be in good agreement with previous measurements by stopped-flow and pulse radiolysis procedures, with one exception. The order of reactivity for the halopentammineCo(III) complexes is found to follow the sequence RF²⁺>RCl²⁺>RBr²⁺>RI²⁺, where R=Co(III)(NH₃)₅³⁺. This and a pH dependence noted for the RF²⁺ case are suggestive of a predominantly inner sphere reaction pathway.     

Solid state reactions to CdTeMoO6 and its structural characterization

CdTeMoO₆ has been obtained by solid state reactions of CdMoO₄ with orth. TeO₂ at 425°C, with tetr. TeO₂ at 470°C, and with H₆TeO₆ at 490°C. Its crystal structure belongs to the tetragonal system (space group $\text{P4}\text{n}$ or $\text{P4}\text{nmm}$ with unit cell dimensions a = 5.279(2) Å, c = 9.056(2) Å. The specificity of this compound in the allylic oxidation reactions should be strictly related to its structural features, among which the presence of cis MoO₂ groups could be very important.     

The crystal and magnetic structures of Ca2YRuO6 and the electronic properties of the series M2+2X3+Ru5+O6 (M = Ca,Sr, Ba; X = La,Y)

Profile analysis of constant-wavelength powder neutron diffraction data has been used to refine the crystal structure of the ordered perovskite Ca₂YRuO₆. The material is monoclinic (space group $\text{P2}{}_1\text{n}$) with a disordered arrangement of calcium and yttrium on the A site and one of the B sites, such that the formula is best written as Ca_1.43Y_0.57[(Ca_0.57Y_0.43)Ru]O₆. Low-temperature neutron diffraction experiments showed that the material is a Type I antiferromagnet at 2.5 K with an ordered magnetic moment of 1.2(1)μ_B per Ru⁵⁺. It is suggested that the dominant factor in determining the electronic properties of the series M²⁺₂X³⁺Ru⁵⁺O₆ (M = Ca, Sr, Ba; X = La, Y) is the Ru-Ru separation distance.     

The dehydration of CuSr2(HCOO)6 · 8H2O: A crystallographic study

The isothermal dehydration of CuSr₂(HCOO)₆ · 8H₂O single crystals as well as powdered material has been followed by several techniques, mainly X-ray diffraction and optical microscopy. Evidence was found of a two-stage process, with an early appearance of an amorphous state (attributed to internal dissolution) and further recrystallization of the stable phases. One of these (CuSr(HCOO)₄, monoclinic, P_c, Z = 2, a = 7.345(10) Å, b = 8.692(15) Å, c = 6.702(10)Å, β = 97.25(5)°) has not been reported so far in the literature. When dehydration takes place near room temperature, the remaining product (Sr(HCOO)₂) bears a topotactic relationship to the parent matrix ((hk0)//(hk0)′; 〈0k0〉//〈0k0〉′). A striking metric match between both (hk0) sections, as well as the absence of any common structural motive, suggest an inner epitactic growth as the most probable mechanism for the transformation.     

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.     

Etude magnetothermoélectrique de la transition métal-semiconducteur dans les composés Cr2+εSe3 (0 ≤ ε ≤ 0,04): I. Dispositif de mesure de l'effet Nernst longitudinal

Experimental equipment is described which permits the measurement of the thermoelectric power (TEP) between 5 and 300 K in a magnetic field (longitudinal Nernst effect) up to 10 T. It is to be used to study thoroughly the metal-semiconductor transition in Cr_2+εSe₃ which encompasses n-type semiconductors and p-type metallic compounds. The delicate problem of temperature measurements below 20 K, in a magnetic field, has been solved by the use of “Carbon-Glass” resistance sensors which have exhibited high sensitivity and reproducibility in high magnetic fields. At 5 K, the very small temperature difference (about 0.5 K) between the sides of the sample is measured with an accuracy better than 3%. The TEP in a magnetic field is obtained with an accuracy of approximately 5%.     

Calculation of dissociation energies in interatomic pair potentials and review of the relationship between coefficients of thermal expansion and heat capacities of simple metals

Dissociation energies for a two-term inverse power type of interatomic pair potential have been calculated for several metals by considering all interatomic interactions in the solid and using the experimental values of the cohesive energies. The results are then employed to calculate the ratio of thermal expansion coefficients and heat capacities of metals. It is found that the calculated values are in much better agreement with the experimental observations compared to an earlier work.     

Pressure dependence of the Grüneisen parameter of silver bromide

For the first time, the Grüneisen parameter of silver bromide is calculated as a function of pressure by using the Callaway integral technique and pressure dependence of the thermal conductivity. The values are reported up to 2.0 GPa at 130 and 292 K. Good agreement between theory and experiment is obtained in the low-pressure region where the latter data exist.     

High-temperature defect structure and electrical conduction in Ni1−xMgxO

The ionic transference number, the electrical conductivity, and Seebeck coefficient of Ni_1?xMg_xO (0.1 ≤ x ≤ 0.9) were measured as functions of temperature (900–1400°C) and oxygen partial pressure (10²–10⁵ Pa). The contribution of ionic conduction to the total conductivity of Ni_0.9Mg_0.1O was of the order of 10^?3?10^?2 at 900–1300°C, which led us to assume that the electronic conduction was predominating in Ni_1?xMg_xO (x ≤ 0.9). The electrical conductivities of both undoped and Al-doped Ni_1?xMg_xO depended on the $\text{1}\text{4}$ power of P_O2, which indicated a significant impurity effect on the defect equilibria and was interpreted as showing that doubly ionized cation vacancies were the dominant point defects at high temperatures. Analyses of the difference in the temperature dependences of conductivity and Seebeck coefficient showed that band-like conduction took place in the NiO-rich composition range (x ≤ 0.1), while thermally activated hopping of small polarons occurred in Ni_1?xMg_xO with x ≥ 0.3. The calculated drift mobility abruptly decreased in the composition region where the conduction mechanism changed.     

Gibbs energy of formation of cobalt divanadium tetroxide

The Gibbs energy of formation of V₂O₃-saturated spinel CoV₂O₄ has been measured in the temperature range 900–1700 K using a solid state galvanic cell, which can be represented as $\text{Pt, Co + CoV}{}_2\text{O}{}_4\text{+}\text{V}{}_2\text{O}{}_3\text{(CaO)}\text{ZrO}{}_2\text{Co}\text{+}\text{CoO, Pt}$. The standard free energy of formation of cobalt vanadite from component oxides can be represented as CoO (rs) + V₂O₃ (cor) → CoV₂O₄ (sp), ΔG° = ?30,125 ? 5.06T (± 150) J mole^?1. Cation mixing on crystallographically nonequivalent sites of the spinel is responsible for the decrease in free energy with increasing temperature. A correlation between “second law” entropies of formation of cubic 2–3 spinels from component oxides with rock salt and corundum structures and cation distribution is presented. Based on the information obtained in this study and trends in the stability of aluminate and chromite spinels, it can be deduced that copper vanadite is unstable.     

New members of a family of layered bismuth compounds

Two new compounds, Bi₃Ti₂O₈F and PbBi₃Ti₃O₁₁F, were prepared and identified by X-ray diffraction analysis. These compounds are members of the family called layered bismuth compounds. Thermal properties of the new compounds were also studied. Besides the preparation and identification of these new compounds, a new method for preparing already known members, Bi₂NbO₅F and Bi₂TiO₄F₂, was reported. Moreover, the possibility of the existence of other new members belonging to the family was discussed.     

Thermodynamic study of niobium oxides with ONb ratios from 2.47 to 2.50 using a high-temperature galvanic cell

The partial molar free energy, enthalpy, and entropy of oxygen in niobium oxides with $\text{O}\text{Nb}$ ratios from 2.47 to 2.50 were measured with a galvanic cell in the temperature range from 1084 to 1325 K. The partial molar enthalpies of oxygen of the Nb₂O_5?x and V phases were observed to be nearly independent of composition, indicating the presence of only weak interactions between defects. The value of the slope for the plots of log x in Nb₂O_5?x against log P_O2 was observed to be $\text{?1}\text{5.2}$ which is interpreted in terms of a defect structure involving both singly ionized and doubly ionized oxygen vacancies. The previously proposed phase diagram in the vicinity of Nb₂O_5?x was confirmed by the present emf measurements.     

Some aspects of room-temperature phosphorescence in liquid solutions

Experimental requirements for room-temperature phosphorescence measurements in liquids (RTPL) are discussed. Phosphorescence quantum yields and triplet lifetimes of some brominated naphthalenes and halogenated biphenyls at 77 K in 2-methyltetrahydrofuran and at room temperature in hexane are reported and compared. Surprisingly the naphthalenes show only little loss in quantum yields in going from 77 K to room temperature. Sensitized phosphorescence is discussed as a means of expanding the analytical potential of RTPL. Results with a model system of benzophenone as a donor (analyte) and 1,4-dibromonaphthalene as an acceptor are presented.     

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.     

Thermal and X-ray diffraction investigations of some lanthanum(III) and neodymium(III) oxide-alkali persulfate binary systems

Different molar ratios of La₂O₃ or Nd₂O₃:Na₂/K₂S₂O₈ have been prepared, and the results of their TG and DTA investigations, under an atmosphere of static air, are reported. The effects of either La₂O₃ or Nd₂O₃ on the thermal decomposition of the persulfates from ambient to 1050°C, using a derivatograph, have been studied. It has been found that La₂O₃ lowers the initial decomposition temperatures of these alkali persulfates through catalytic activity. Nd₂O₃ shows little or no catalytic effect and therefore it acts as an insulator. Intermediate and final products are identified by X-ray diffraction analysis. The stoichiometric molar ratios of the solid state reactions are 1:3::R₂O₃:M₂S₂O₈. (R = La or Nd. M = Na or K), which give double salts of formulae: NaLa(So₄)₂, KLa(SO₄)₂, NaNd(SO₄)₂, and KNd(SO₄)₂. No sulfates or oxysulfates of lanthanum or neodymium have been identified.     

Mise en evidence et localisation des protons dans les bioxydes de plomb PbO2α et β chimiques et électrochimiques

Chemical and electrochemical samples of lead dioxides PbO₂α (orthorhombic) and PbO₂β (tetragonal) are studied by quasi-elastic neutron scattering and by X-ray diffraction between 77 and 550 K. The presence of OH^?-like proton species is established: the electrochemical species are characterized by two types of local proton motions ($\text{ΔE}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 300 μ}\text{eV}\text{, ΔE}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 40 μ}\text{eV}$). The thermal expansion of the chemical and electrochemical sample shows: (1) the loss of OH^?-like charged species after heating to 550 K, (2) an anisotropy of the OH ··· O bonds in the [PbO₆] octahedra chains which are oriented differently in the α and β structures. An interpretation of the role of protons is proposed.