Etude par rpe de Ni2+ dans RbMgF3 de structure 6H

The crystal structure of 6H-RbMgF₃ has been more precisely determined by using the crystal data of isostructural RbNiF₃. It crystallizes in the hexagonal P6₃/mmc space group with the parameters: $\text{a}\text{= 5.833}\text{A}\text{?}$ and $\text{c}\text{= 14.193}\text{A}\text{?}$.Both magnesium positions can be occupied by Ni²⁺ and detected by ESR. Analysis of the spectra reveals one site (octahedra sharing one face) corresponding to an important zero field splitting (D = 2,37 cm^?1) and another one of higher symmetry corresponding to the corner-sharing octahedra.     

Mössbauer effect and dielectric behavior of NixCu0.8−xZn0.2Fe2O4 compound

The ferrite system Ni_xCu_0.8−xZn_0.2Fe₂O₄ with 0.0 ≤ x ≤ 0.8 was synthesized. XRD measurement confirmed the presence of single-phase spinel structure. The area ratio of Fe³⁺ at the tetrahedral A- and octahedral B-sites was deduced from the spectral analysis of Mössbauer measurements. The results give evidence that Ni²⁺ replaces Cu at B-site in the present unit cell. The dielectric properties ?′, ?″, loss tangent tan(δ) and ac conductivity σ_ac have been studied for the prepared samples in the temperature range (300-600 K) and over the frequency range (10² to 10⁵ Hz). The electrical conductivity results revealed a semiconductor behavior with increasing nickel concentration with a change in the slope at the transition temperature T_c. The variation of the dielectric parameters (?′, ?″ and tan(δ)) with frequency and temperature displayed a strong dependence on nickel concentration. Dielectric anomaly at the transition temperature T_c was pronounced in the relations of ?′ and ?″ with temperature. The determined T_c was found to increase with increasing Ni content. The relation of tan(δ) with frequency at different temperatures showed two relaxation processes where the relaxation time and maximum frequency of the hopping conduction mechanism were determined. The results are explained in the light of cation-anion-cation and cation-cation interactions over the octahedral site in the spinel structure.     

Les composes Ag7AsS6 et Ag7AsSe6 etude des proprietes thermiques,cristallographiques et electriques

The preparation and the partial phase diagrams Ag₂X“As₂X₅” (X = S or Se) are described. Peritectic decompositions occur at 560°C for Ag₇AsS₆ and 360°C for Ag₇AsSe₆. Phase changes are observed at 250°C for Ag₇AsS₆ and 150°C for Ag₇AsSe₆. For each compound, the low temperature form is cubic P2₁3, and the high temperature form has the Ag₈GeTe₆ structural type, F$\text{4}$3m. The high temperature forms are not quenchable. Ionic and electronic conductivity have been measured in Ag₇AsS₆ and Ag₇AsSe₆. Ionic conductivity was measured using RbAg₄I₅ as a blocking electrode for electronic conduction. Electromotive force measurements confirm transference numbers. At room temperature, ionic conductivities were 1,5.10^?6 (Ω.cm)^?1 and 0,08 (Ω.cm)^?1 for Ag₇AsS₆ and Ag₇AsSe₆ respectively.     

New Li+ ion conductors in the system,Li4GeO4-Li3VO4

New Li⁺ ion conducting solid electrolytes have been found in the system Li₄GeO₄-Li₃VO₄. Of the compositions studied, Li_3.6Ge_0.6V_0.4O₄ has the highest conductivity with σ ～ 4 × 10^?5 ohm^?1cm^?1 at 18°C rising to ～ 10^?2 ohm^?1 cm^?1 at 190°C. The activation energy is ～0.44 eV. These conductivity values are among the highest yet found for Li⁺ ion conductors; the room temperature value is much higher than in LISICON, Li_3.5Zn_0.25GeO₄, or in Li_3.4Si_0.4P_0.6O₄ and is comparable to that in $\text{LiI/A}\text{l}{}_2\text{O}{}_3$ mixtures. These solid electrolytes are easy to synthesize, thermodynamically stable and insensitive to atmospheric attack. Structurally, they are solid solutions based on γ_II Li₃VO₄, a γ tetrahedral structure; high conductivity is due to the interstitial Li⁺ ions which are created during solid solution formation.     

Rapid H+ conductivity in hydrogen uranyl phosphate-A solid H+ electrolyte

We have found that the layered hydrate HUO₂PO₄. 4H₂O is a rapid proton conductor. The room temperature conductivity of 4 × 10^?3ohm^?1cm^?1 is higher than that of Na⁺ in β alumina. The activation energy is 30± 3 kJ mol^?1. The material is insoluble, and presses into transluscent discs suitable for solid electrolyte applications.     

Lithium insertion into Li2Ti3O7

Li₂Ti₃O₇ with the ramsdellite-type structure undergoes lithium insertion reactions with n-BuLi. Li_2+xTi₃O₇ phases form with x = 0.5 and 1.0 at room temperature and at 50°C, respectively. The ESR spectrum of Li₃Ti₃O₇ confirms the partial reduction of Ti⁴⁺ ions to Ti³⁺. The electrical conductivity of the fully lithiated phase is several orders of magnitude higher than that of the host compound, suggesting charge hopping in the mixed valent lithiated compound.     

Single crystals of In2Te5

Single crystals of semiconducting compound In₂Te₅ were grown by chemical transport employing iodine as a transport agent. The crystals had a plate-like habit with the [100] direction perpendicular to the flat surface of the platelets. Nominal dimensions are 10 × 1 × 0.05 mm. In₂Te₅ has a monoclinic structure with dimensions of the base centered cell: $\text{a}\text{= 13.47}\text{A}\text{?}$, $\text{b}\text{= 16.51}\text{A}\text{?}$, $\text{c}\text{= 4.365}\text{A}\text{?}$, β = 92°5′. The space group is $\text{C}{}_{\mathrm{<mtext>2</mtext><mtext>c</mtext>}}$. Pycnometric density is 5.96 g/cm³. The single crystals were all p-type. The conductivity, thermoelectric power and hardness were about 10^?5Ω^?1cm^?1, 650 mkV/°C, and 30 kg/mm², respectively. The minimum energy gap is 1.26 eV.     

Preparation and structure of EuII2EuIIITaO6

Europium tantalate, Eu^II₂Eu^IIITaO₆, has been prepared by the reaction of Eu₂O₃ and Ta₂O₅ with Ta metal at 1250–1350°C in argon or vacuum. The crystal structure of this tantalate is found to be the (NH₄)₃FeF₆ type with the space group, O⁵_h-Fm3m. Four molecules constitute the unit cell with a lattice constant of 8.309Å for Eu^II₂Eu^IIITaO₆. The pycnometric density of 8.45 g.cc_?1 for this compound is in good agreement with the X-ray density of 8.48 g.cc^?1. The magnetic susceptibility as measured for this compound is also in good agreement with a theoretical value (78.7 × 10^?6 emu/g). This compound is oxidized to Eu^III₃TaO₇ at 250–600°C in air.     

Etude magnetique et par spectroscopie Mössbauer de NH4FeF4

The NH₄FeF₄ lattice is constituted by sheets of (FeF₆^3? octahedra alternating with NH₄⁺ layers and may be described therefore as a two-dimensional material. At about 200 K the x^?1 vs. T curve shows actually a flat minimum typical of two-dimensional interactions. A calculation of the exchange integral has been performed using a high temperature series expansion technique and leads to $\text{J}\text{k}\text{= ? 26}\text{K}$. A Mössbauer spectroscopy investigation of the variation of the hyperfine field vs. T gives a Néel temperature T_N = 135 K and a critical exponent β = 0, 26. This last value is close to those obtained for RbFeF₄ and CsFeF₄ which have similar crystal and magnetic structures.     

Role of chromium ion valence states in ZnO-As2O3-Sb2O3 glass system by means of spectroscopic and dielectric studies

ZnO-Sb₂O₃-As₂O₃ transparent glasses containing small concentrations of chromium ions (introduced as Cr₂O₃) ranging from 0 to 0.2 mol% is prepared. A number of studies viz., XRD, SEM, DTA, optical absorption, FT-IR, Raman, ESR spectra, magnetic susceptibility and dielectric properties (constant ?′, loss tan δ, ac. conductivity σ_ac over a wide range of frequency and temperature as well as dielectric breakdown strength at room temperature) of these glasses have been carried out as a function of chromium ion concentration. The results have been analysed in the light of different oxidation states of chromium ions. The analyses indicates that when the concentration of chromium ions is low, these ions mostly exist in Cr⁶⁺ and Cr⁵⁺ states, occupy network forming positions with CrO₄²⁻ and CrO₄³⁻ structural units respectively and increase the rigidity of the glass network. When the concentration of chromium ions is gradually increased, these ions seem to be existing mostly in Cr³⁺ state.     

Electrical properties of sintered EuZrO3  EuNbO3 and EuNbO3

Electrical resistivity of two-phase products [yEuZrO₃ + (1 ? y) EuNbO₃] increased continuously with y, and a transition from a metallic to semiconducting characteristic occured at y = 0.14. The resistivity varied almost linearly with temperaure in the range y = 0 to y = 0.24, and thermal coefficients of resistivity at 300 K for the products decreased from +5.9 × 10^?4 K^?1 to ?7.4 × 10^?4 K^?1 according to the value of y. At y = 0.14, the thermal coefficient was almost zero. Thermal coefficients of electrical resistivity for the niobates with various oxygen contents were all positive in the range $\text{2.55 <}\text{O}\text{Nb}\text{< 3.24}$ and exhibited a sharp minimum at $\text{O}\text{Nb}\text{= 2.92}$. In all these niobates, Eu_xNbO₃ was a major phase and Eu₃NbO₆ or EuNbO₂ was detected as a second phase in the range $\text{O}\text{Nb}\text{> 3}$ or $\text{O}\text{Nb}\text{< 3}$ respectively. Peaks in the resistivity curves were correlated with a magnetic ordering temperature for samples with an overall ratio $\text{O}\text{Nb}\text{> 3}$.     

Domaine vitreux,structure et conductivite electrique des verres du systeme LiCl /1b Li2O /1b P2O5

The glass forming region in the LiCl /1b Li₂O /1b P₂O₅ system was determined. Glass transition temperature was obtained from differential thermal analysis studies. The determination of the electrical conductivity as a function of the temperature and the composition in Li₂O and LiCl was carried out. The conductivity reaches a maximum value of the order of 5·10^?7 (ohm^?1·cm^?1) at 25° C. Raman spectra are examined in order to show the influence of the composition on the vitreous structure.     

Un nouveau fluorure de platine : PtIIPtIVF6

From a temperature dependent study of the platinum-flourine system, a new fluoride Pt^IIPt^IVF₆ has been isolated, which contains platinum in an oxidation state lower than that found in the already known platinum fluorides PtF₄, PtF₅ and PtF₆. The material has been characterized by various analytical methods. The fluoride Pt^IIPt^IVF₆ crystallizes in the rhombohedral LiSbF₆ - type structure (R3) with the lattice parameters $\text{a}\text{= 5.56}{}_5\text{A}\text{?}$, α = 53.85°. Magnetic studies on polycristalline samples have shown that Pt^IIPt^IVF₆ orders ferromagnetically below 16 K. This is the first example of platinum (+II) in a fluorine environment and with a high spin structure in an octahedral surrounding.     

A mixed valence compound in the two dimensional MPS3 family: V0.78PS3 structure and physical properties

The non stoichiometric compound V_0.78PS₃ has been obtained as single crystals from a preparation corresponding to the atomic ratio V/P/S = 1/1/3. It cristallizes with monoclinic symmetry, space group C2/m, with the unit cell parameters $\text{a}\text{= 5.867(1)}\text{A}\text{?}\text{,}\text{b}\text{= 10.160(2)}\text{A}\text{?}\text{,}\text{c}\text{= 6.657(1)}\text{A}\text{?}\text{, β = 107.08(2)°,}\text{V}\text{= 379.3(1)}\text{A}\text{?}{}^3$ and Z =4. The structure refinement was made down to a reliability factor value R = 3.3% from 445 reflexions (I > 3 σ (I)) and 31 variables. The material has same layer structure as FePS₃ with the occurrence of the thiophosphate anion (P₂S₆)^4?-including a P₂ pair. In V_0.78PS₃, the charge equilibrium implies the following developped formula : V_0.34^II V_0.44^III □_0.22 P^IV S₃^?II. The phase is a semi-conductor with a small activation energy of 0.24 eV, in accord with a vanadium mixed valence, and it presents, at low temperature, an antiferromagnetic order (T_N = 62 K).     

Electrical conductivity of solid solutions in the system CeO2La6WO12

The total electrical conductivities were studied for the solid solutions in the pseudo-binary system CeO₂La₆WO₁₂. Maximum conductivities were observed at about 90 mol.% CeO₂, 1.1 ohm^?1 cm^?1 at 1500°C and 4.4 × 10^?3 ohm^?1 at 600°C. A minimum point was observed in conductivity isotherms at about 10 mol.% CeO₂. A pyrochlore like stoichiometric compound La₂$\text{(}\text{LaCe}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{W}\text{1}\text{2}\text{)}\text{O}{}_7$ was assumed. The crystal structure of La₆WO₁₂ is discussed. Ionic conductions were observed at low temperatures.     

Ionic conductivity of Ag+ exchanged α-zirconium phosphate

The conductivity of silver ion in silver exchanged α-zirconium phosphate, Zr(AgPO₄)₂, was measured in the temperature range 80 to 330°C. The data were found to conform to an Arrehnius type behavior in this temperature range with an activation energy of 0.53eV and ⁰300°C = 1.94 × 10^?4 ohm^?1 cm^?1. The conductivity values for Ag⁺ are higher than the corresponding values obtained for Na⁺ and K⁺ conduction in their respective exchanged forms. This enhancement is attributed to the greater polarizibility of the silver ion.     

Microstructure and Thermoelectric Properties of Bi- and Cu-Substituted Ca3Co4O9 Oxides

Bi- and Cu-substituted Ca₃Co₄O₉ samples were prepared by conventional solid-state reaction method and the e?ect of element substitution on the microstructures and thermoelectric properties was investigated. Partial substitution of Cu for Co leads to an increase in electrical conductivity and a decrease in Seebeck coe±cient due to the rise of hole concentration. The microstructure of Cu-substituted sample is almost unchanged compared with undoped Ca₃Co₄O₉. On the other hand, partial substitution of Bi for Ca gives rise to a significant increase in the grain size, and c-axis-oriented structure can be formed in Ca_2.7Bi_0.3Co₄O₉, resulting in an obvious increase in electrical conductivity. Cu and Bi co-substitution further increases the grain growth and the electrical conductivity of Ca_2.7Bi_0.3Co_3.7Cu_0.3O₉. Thus, Cu and Bi co-substitution samples possess the optimal thermoelectric performance at high temperature and the highest value of power factor can reach 3.1×10^-4 Wm^-1?K^-2 at 1000 K.     

Observations of electron-beam-induced thermal effects in amorphous Se80As20 and Se64As16Au20 thin films

Transmission and scanning electron microscopy have been employed to study the effect of localized 100 kV electron-beam heating on amorphous Se₈₀As₂₀ and Se₆₄As₁₆Au₂₀ glass films supported on carbon substrates. Crystallization, melting, vaporization and grain growth have been observed and related to the input electron-beam power density and estimated film temperature rise. These studies show that thin films of the former system are highly resistant to devitrification prior to their melting and vaporization. Films of the latter system, however, are more easily vaporized and undergo an apparent crystallization upon exposure at J ≈ 1.8 A cm^?2 to form α-AuSe and hexagonal arsenic. Exposure at current densities in excess of 2.4 A cm^?2 results in rapid vaporization of selenium and arsenic and the formation of an annulus of irregularly shaped twinned gold crystals at the hole periphery. Slow or prolonged heating of such peripheral zones was observed to result in the growth of crystallographically shaped nuclei consisting of assemblies of tetrahedra (hexagons, pentagons) together with trigonal platelets that exhibited non-integer (111) diffraction spots of type $\text{1}\text{3}\text{(422)}$. Exposures at J>3 A cm^?2 resulted in very rapid vaporization of Se and As and induced the growth of dendritic single-crystal gold films also exhibiting the anomalous (111) structure.     

Mössbauer studies of thiospinels VI. The system FexZn1−xCr2S4

The spinel system Fe_xZn_1?xCr₂S₄ with 0 ≦ x ≦ 1 has been prepared in polycristalline form. All samples are p type semiconductors. I.R. spectra have been measured between 200 cm^?1 and 600 cm^?1. Room temperature ⁵⁷Fe-Mössbauer spectra show the typical behaviour of tetrahedral site Fe(II) surrounded by different octahedral site neighbours. In comparison with Mössbauer spectra of the spinel system Fe_1+xCr_2?xS₄ the distribution, 0.01 < y < 0.02, is derived.