Heat treatment effects on spinel phase of Zn x Ni1 − x Fe2O4 and MnFe2O4 nanoferrites

We report the effects of heat treatment on Zn _x Ni_1???x Fe₂O₄ (x?= 0, 0.5 and 1.0) and MnFe₂O₄ ferrite nanoparticles. The as-prepared compounds were sintered from 400°C to 1100°C. Pure ZnFe₂O₄ (x?= 1.0) and MnFe₂O₄ could be obtained under low reaction temperature of 200°C. NiFe₂O₄ (x?= 0) and Zn_0.5Ni_0.5Fe₂O₄ (x?= 0.5) nanoferrites crystallized with single phase cubic spinel structure after annealing at 600°C. The single phase cubic spinel structure of these compounds was destroyed after annealing at temperature above 700°C. The magnetization measurements indicate superparamagnetic behavior of the nanosized compounds produced.     

New compounds in the Mn-X-Bi system where X = Ni, Cu, RhorPd

A new class of magnetic compounds has been discovered in the temary system Mn-X-Bi where X is Ni, Cu, Rh or Pd. The approximate compositions of these compounds are Mn₅Ni₂Bi₄, Mn₃Cu₄Bi₄, Mn₅Rh₂Bi₄, and Mn₅Pd₂Bi₄. The Bravais lattice is face-centered cubic, and the lattice constants are 12.16 Å (X = Ni), 12.18 Å (X = Cu), 12.31 Å (X = Rh) and 12.44 Å (X = Pd). These compounds are probably ferromagnetic and have Curie temperatures in the range -7°C to 183°C. A crystal structure is proposed for these compounds which contains 88 atoms/unit cell.     

Surface properties of Fe76Mo8Cu1B15 alloy after annealing

NANOPERM-type alloy Fe₇₆Mo₈Cu₁B₁₅ is investigated in amorphous and in partially crystallized state. Samples were prepared by 1 h isothermal annealing in vacuum at temperatures ranging from 330°C up to 700°C. Bulk and surface microstructural characteristics were studied using transmission and conversion electron Mössbauer effect techniques, respectively. Surface features were checked by the help of atomic force microscopy. Presence of nanocrystalline bcc-Fe phase was detected during the first crystallization stage. The crystallization process starts at 450°C and it is more pronounced in surface regions than in the bulk. With progressing crystallization, hyperfine parameters especially of the amorphous residual phase are altered. Distinctions in surface morphology are revealed between wheel and air side of the ribbon-shaped samples.     

Structural and electrical characterization of MxNb3Se4 (M = Cu, H)

The compounds Cu_xNb₃Se₄ (0≤x≤0,45) and H_xNb₃Se₄ (0≤x≤2·10⁻³) were prepared by electrochemical titration from Nb₃Se₄. The samples were characterized by X-ray analysis and q-probe conductivity measurements as a function of temperature. The Cu-compound is isostructural with Nb₃Se₄ for 0≤x≤0.2 and shows new phases for 0,2≤x≤0,45. The H-compound shows an impurity controlled conductivity in the temperature range from 20 to 200 °C and an intrinsic type conductivity in the temperature range from 330 to 450 °C. The activation engines are 0 and 0.15 eV, respectively. Rapid proton conduction in H_xNb₃Se₄ makes it difficult to control the composition as demonstrated by exposure of the samples to different atmospheres. An increasing H-concentration decreases drastically the conductivity by several orders of magnitude.     

The thermal decomposition of mono-, di-, and tripotassium salts of trinitrophloroglucinol

The kinetics of the thermal decomposition of mono-, di-, and tripotassium salts of trinitrophloroglucinol (TNPG) was studied in the solid phase by the manometric method at m/V from 1 × 10^?4 to 15 × 10^?4 g/cm³ over the temperature ranges 125–150°C (K₁TNPG), 200–230°C (K₂ TNPG), and 200–250°C (K₃ TNPG). All the compounds decomposed according to the topochemical mechanism: there was an induction period, after which the rate of gas release was maximum. This rate then gradually decreased. The second decomposition stage was observed for K₁TNPG as the temperature increased to 200–250°C. The special features of changes in the rate of the process during transformation and the influence of the degree of vessel filling with a substance, particle size, and temperature on the kinetics of decomposition were studied. The kinetic results and composition of gaseous products and some condensed decomposition products lead to certain conclusions concerning the mechanism of the chemical transformations.     

A series of new compounds A3+Fe2 O4 (A = Ho,Er, Tm,Yb, and Lu)

A series of new compounds A Fe₂ O₄ (A = Ho, Er, Tm, Yb, and Lu) have been successfully synthesized under the lower oxygen partial pressures at 1200°C and their unit cell dimensions were determined.     

Preparation of dense La<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>Ga<Subscript>0.8</Subscript>Mg<Subscript>0.2</Subscript>O<Subscript>3-δ</Subscript> with high ionic conductivity by solid-state synthesis

The results of a systematic investigation on the effects of processing steps, via solid-state reactions, on structural phase characteristics and ionic conductivity of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ solid electrolyte are reported. The main purpose of this work is to establish an optimized route for obtaining good densification and high ionic conductivity of this solid electrolyte. Processing routes with three successive calcinations at 1250 °C followed by attrition milling (R1), and with two sequences of calcination at 1350 °C with intermediary attrition milling (R2) give rise to near full density at 1450 °C sintering temperature. The rate of grain growth is fast when the relative density reaches 95%. Elemental mapping reveals uniform distribution of the constituents in the matrix along with La₄Ga₂O₉, LaSrGa₃O₇ and sub-micrometer MgO grains at grain boundaries. The ionic conductivity of grains remains unchanged with the processing route and sintering profile. The blocking effect of charge carriers at grain boundaries decreases with increasing the dwell temperature.     

Ionic transport in the lithium nitride bromides,Li6NBr3 and Li1 3N4Br

The ionic and electronic conductivities of the lithium nitride bromides Li₆NBr₃ and Li_{1 3}N₄Br have been studied in the temperature range from 50 to 220°C and 120 to 450°C, respectively. Both compounds are practically pure lithium ion conductors with negligible electronic contribution. Li₆NBr₃ has an ionic conductivity Ω of 2 × 10^-6Ω^-1cm^-1 at 100°C and an activation enthalpy for σT of 0.46 eV. Li_{1 3}N₄Br shows a phase transition at about 230°C. The activation enthalpy for σT is 0.73 eV below and 0.47 eV above this temperature. The conductivities at 150 and 300°C were found to be 3.5 × 10^-6 Ω^-1cm^-1 and 1.4 × 10^-3Ω^-1cm^-1, respectively. The crystal structure is hexagonal at room temperature with $\text{a = 7.415 (1)}\text{A}\text{?}$ and $\text{c = 3.865 (1)}\text{A}\text{?}$.     

Structural and dielectric properties of Pb5(Ge,Si)O11

The ferroelectric lead germanate (Pb₅Ge₃O₁₁) and its isomorphous compounds are important because of their uses as pyroelectric and electro-optic devices. Comparison of inter-planar d-spacings of Pb₅Ge_3−x Si _x O₁₁ (x=0, 0.3, 0.7 and 1.00) suggests that there is no change in basic structure of Pb₅Ge_3−x Si _x O₁₁ when Si is substituted for Ge in small quantity (x<1). The dielectric properties of the Si-substituted compounds have been studied as a function of temperature (30 to 200°C). The ferroelectric-paraelectric phase transition has been observed at 185°C. The Si doping causes (a) Curie point to shift towards low temperature, (b) peak value of the dielectric constant to decrease and (c) phase transition diffuse. The fast increase in dielectric constant of pure Pb₅Ge₃O₁₁ with temperature (beyond transition temperature) may be attributed to the development of space charge polarization in the system.     

Nitrogen implantation in GaAs1−xPx (x=0.4; 0.65)

Nitrogen ions were implanted in GaAs_1−xP_x (x=0.4; 0.65) at room temperature at various doses from 5×10¹² cm⁻² to 5×10¹⁵ cm⁻² and annealed at temperatures from 600°C up to 950°C using a sputtered SiO₂ encapsulation to investigate the possibility of creating isoelectronic traps by ion implantation. Photoluminescence and channeling measurements were performed to characterize implanted layers. The effects of damage induced by optically inactive neon ion implantation on photoluminescence spectrum were also investigated. By channeling measurements it was found that damage induced by nitrogen implantation is removed by annealing at 800°C. A nitrogen induced emission intensity comparable to the intensity of band gap emission for unimplanted material was observed for implanted GaAs_0.6P_0.4 after annealing at 850°C, while an enhancement of the emission intensity by a factor of 180 as compared with an unimplanted material was observed for implanted GaAs_0.35P_0.65 after annealing at 950°C. An anomalous diffusion of nitrogen atoms was found for implanted GaAs_0.6P_0.4 after annealing at and above 900°C.     

Onset of magnetism in concentrated ternary alloys II: Laves phase compounds A(Fe1-xBx)2 (A = Y,Zr, U; B = Mn,Co and Al)

The onset of magnetism as a function of concentration and the magnetic properties of nine ternary Laves phase systems A(Fe_1-xB_x)₂ (A=Y, Zr, U; B=Mn, Co and Al) are discussed in terms of homogeneous and heterogeneous models based on the SEW model and the Landau theory of phase transition of second order. A significant influence of the nomagnetic elements Y, Zr and U upon the Fe magnetic moment substituted by Mn, Co and Al is observed. Low temperature freezing phenomena affect the magnetization process around the critical concentration x_c in Y and Zr compounds while they seem to be of minor importance for the U systems. Comparing the magnetic properties of these nine systems implies that the magnetization process becomes more homogeneous in the sequence of Y-, Zr- and U(Fe, B)₂ compounds. This - together with volume considerations and the different magnetovolume effects observed as well as susceptibility measurements lead to the suggestion that the Fe moment tends to become more delocalized as one proceeds from Y to U compounds as a consequence of the growing extent to which Y, Zr, U d-electrons are hybridized with the 3d electrons. Furthermore, U can be regarded as tetravalent in these Laves phase compounds, except UAl₂, in which presumably an admixture of U³⁺ and U⁴⁺ occurs.     

Structural,dielectric, electrical and piezoelectric properties of Ba<Subscript>4</Subscript>SrRTi<Subscript>3</Subscript>V<Subscript>7</Subscript>O<Subscript>30</Subscript> (R=Sm,Dy) ceramics

Polycrystalline samples of Ba₄SrRTi₃V₇O₃₀ (R=Sm and Dy), members of the tungsten-bronze family, were prepared using a high-temperature, solid-state reaction technique and studied their electrical properties (using complex impedance spectroscopy) in a wide range of temperature (31–500°C) and frequency (1 kHz-1 MHz). Preliminary structural (XRD) analyses of these compounds show the formation of single-phase, orthorhombic structures at room temperature. The scanning electron micrographs (SEM) provided information on the quality of the samples and uniform distribution of grains over the entire surface of the samples. Detailed studies of the dielectric properties suggest that they have undergone ferroelectric-paraelectric phase transition well above the room temperatures (i.e., 432 and 355°C for R= Sm and Dy, respectively, at frequency 100 kHz). Measurements of electrical conductivity (ac and dc) as a function of temperature suggest that the compounds have semiconducting properties much above the room temperature, with negative temperature coefficient of resistance (NTCR) behavior. The existence of ferroelectricity in these compounds was confirmed from a polarization study.      

A study of the microstructure of La1 − x Ca x MnO3 (x = 0.5, 0.8) solid solutions

HREM and XPD methods are used to study the microstructure of La_{1 − x} Ca _x MnO₃ perovskite-like oxides synthesized by pyrolysis of polymer-salt compositions. The XPD data show that the samples studied are single-phase solid solutions. Morphological transitions are observed in series of samples depending on the substitution parameter x. At 0 ≤ x ≤ 0.4 the samples can be characterized by rhombic symmetry (space group Pnmb); at 0.5 ≤ x ≤ 0.8 the symmetry increases to the tetragonal space group I4/mmm; at 0.9 ≤ x ≤ 1 the symmetry lowers to the monoclinic space group P1121. Heating of samples with x = 0.5 and 0.8 up to 1200°C in air does not lead to noticeable changes in the sample structure, which indicates their high thermal stability. When the La_0.2Ca_0.8MnO₃ sample is heated to 1100°C in vacuum nanocrystalline states with particle disintegration into microphases of different structures form because of a partial decomposition of the solid solution. As a result, a system of Mn₃O₄ nanoparticles appears, which is coherently bound with the perovskite phase of a defective structure.     

Structural and electrical characterization of the NASICON-type Li2FeZr(PO4)3 and Li2FeTi(PO4)3 compounds

In order to develop new electrolytes for all-solid-state rocking chair lithium batteries, the NASICON-type compounds Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ were investigated by powder X-ray diffraction technique and impedance spectroscopy. Li₂FeZr(PO₄)₃ is orthorhombic Pbna (a=8.706(3), b=8.786(2), c=12.220(5) Å) and Li₂FeTi(PO₄)₃ is orthorhombic Pbca (a=8.557(3), b=8.624(3), c=23.919(6) Å). They show no phase transitions from RT to 800 °C. In the same temperature range logσT vs. 1/T show no slope variations. The activation energies for the ionic conductivity were 0.62 and 0.64 eV for Li₂FeTi(PO₄)₃ and Li₂FeTi(PO₄)₃, respectively. In order to better evaluate the present results they were compared with those of α and β-LiZr₂(PO₄)₃ phases, which were also prepared and characterised. A change of activation energy from 0.47 eV to 1.03 eV was observed in the case of β phase, at about 300 °C; attributed to the β (orthorhombic) ? β′ (monoclinic) phase transition. In the α phase the activation energy 0.47 eV in the temperature range 150 – 850 °C. The Li₂FeZr(PO₄)₃ and Li₂FeTi(PO₄)₃ compounds can be interesting for applications as solid electrolytes in high temperature (>300 °C) lithium batteries.     

Study on structural, thermal, sintering and conductivity of Cu-Co doubly substituted Bi4V2O11

Doubly substitution of vanadium by Cu and Co in the limit of 10% in Bi₄V₂O₁₁, has led to the formation of the Bi₄V_1.8Cu_0.2−xCo_xO_10.7 solid solution. X-ray diffraction shows that all the compositions present a tetragonal symmetry. The thermal analysis has revealed that the polymorph γ' phase, which is formed by a partial ordering of oxygen ions in the γ high temperature form, is stabilized at room temperature. The influence of sintering temperature on the microstructure of the samples was investigated by the scanning electron microscopy (SEM). The ceramics sintered at 820 °C for more than 3 hours present micro-craks. The evolution of the electrical conductivity with temperature and the degree of substitution has been investigated by impedance spectroscopy. The sample with x=0.1 presents the highest value of the conductivity ≈4.6×10⁻² S·cm⁻¹ at 600 °C.     

The unusual variations of photoluminescence and afterglow properties in monoclinic ZrO2 by annealing

The raw ZrO₂ is annealed at 600–1550 °C for 6 h. It is found that the emission at 492 nm increases greatly when the annealing temperature is higher than 1200 °C and its afterglow shows a small improvement at 1200–1450 °C and a large enhancement after annealing at 1550 °C. The results that are obtained indicate that the impurity Ti⁴⁺ in ZrO₂ is efficiently reduced to Ti³⁺ when the temperature is higher than 1200 °C, and the increase of Ti³⁺ centers contributes to the large improvement of emission at 492 nm. The thermoluminescence shows that at least two types of traps with different depths (0.65 eV and 1.46 eV) corresponding to oxygen vacancies exist in monoclinic ZrO₂. After annealing at 1200–1450 °C, some new trap clusters related to oxygen vacancies and Ti³⁺ form and causes the small improvement of afterglow at 1200–1450 °C. The large improvement of afterglow after annealing at 1550 °C originates from the sharp increase of proper shallow traps (0.65 eV) in ZrO₂. Accordingly, we present the feasible interpretations and luminescence mechanisms of monoclinic ZrO₂ for our observations.     

Properties of the ceramic manganite (La0.65Ca0.35)1 − x Mn1 + x O3 ± Δ (x = 0.2) sintered at temperatures of 1000–1450°C

It has been shown that the ceramics (La_0.65Ca_0.35)_{1 ? x} Mn_{1 + x} O_{3 ± Δ} (x = 0.2) sintered at temperatures up to 1450°C is formed as a composite material consisting of manganite and manganese oxide grains. It has been found that, at a sintering temperature of 1450°C, the manganite grain size abruptly increases, which is accompanied by the formation of a nanometer-sized layered structure. It has been revealed for the first time that the temperature dependence of the magnetoresistance of the ceramic manganite with this structure is characteristic of single crystals.     

Ionic conductivity of double sodium-scandium and cesium-zirconium molybdates

The electrical properties of the Na₃Sc(MoO₄)₃ and Cs₂Zr(MoO₄)₃ compounds are investigated using impedance spectroscopy (1–10⁶ Hz) in the temperature range 100–650°C. Double molybdates in the form of a fine-crystalline powder are obtained by solid-phase synthesis in air at 450–600°C for 20–50 h. It is found that the temperature dependence of the ionic conductivity of ceramic samples exhibits anomalies at temperatures of 605 ± 5°C for Na₃Sc(MoO₄)₃ and 425 ± 15°C for Cs₂Zr(MoO₄)₃ due to the phase transitions, which are confirmed by the data of thermal analysis. Above the superionic transitions, the ionic conductivity reaches 0.084 S/cm (650°C) for Na₃Sc(MoO₄)₃ and 0.002 S/cm (462°C) for Cs₂Zr(MoO₄)₃.     

Synthesis and characterization of the new high pressure phases ACu3 V4O12(A=Gd,Tb, Er)

New ACu₃V₄O₁₂ (A=Gd, Tb, Er) phases have been prepared at high pressure and high-temperature conditions (P～8–9 GPa, T～1000°C) in a toroid-type high pressure cell. These compounds crystallize in the cubic symmetry with a perovskite-like structure. At ambient pressure, they are paramagnetic and have activation-type conductivity. The effect of high pressure (10–50 GPa) on the electrical properties of the materials was analyzed in the temperature range from 78 to 300 K. Pressure ranges of the transition from activation type to metallic conductivity have been determined. The crystal structure of ACu₃V₄O₁₂ (A=Gd, Tb, Er) was found to be stable up to 50 GPa.     

Investigation of phase transformations in Cu1.8S-Cu2S thin films by kinematic electron diffraction

The phase transformations of the compounds Cu_1.8S-Cu₂S have been investigated. All the changes in the sample due to a thermal treatment up to 300°C were continuously photographed by the kinematic electron diffraction method. It was shown that, depending on experiment conditions, the low-temperature ordered phase of digenite Cu_Z-XS transforms completely or partly into the hexagonal modification of chalcocite Cu₂S, which becomes ordered in its low-temperature orthorhombic modification. At temperatures from room to 100°C the orthorhombic chalcocite coexists with the low-temperature digenite, in a range from 100°C to 300°C the high-temperature hexagonal chalcocite coexists with the low-temperature digenite and at 300°C the sample contains a mixture of the hexagonal chalcocite and the f.c.c.-phase of digenite. Djurleite and other phases with intermediate compositions are not observed in these experiments.