Properties of MgB2 bulks after combined doping with Fe and C by adding Iron(II) lactate (C6H10FeO6)

Bulk MgB₂ doped with C and Fe was prepared by using the solid state sintering method with C₆H₁₀FeO₆ as dopant. The phase composition, microstructure, and superconducting properties were studied. X-ray diffraction (XRD) shows the presence of iron after the doping. The addition of C₆H₁₀FeO₆ increases the a- and c-axis parameters of MgB₂, as evidenced by the shifting of the (100) and (002) peaks to a lower angle on the XRD patterns. Fe atoms were distributed uniformly, as shown by the field emission scanning electron microscope images, while the magnetization of the sample was dominated by the signals from the MgB₂ superconductor, although the iron-containing materials also contributed a minor amount of magnetization. The residual resistivity ratio was decreased as the C₆H₁₀FeO₆ doping level increased. The critical temperature also decreased with increased doping level, as did the critical current density, J_c. The doping also caused decreases in the irreversibility field, H_irr, and the upper critical field, H_c2. The decrease in H_c2 and H_irr, together with the harmful effects from impurity phases such as MgO is the reason for the decrease in J_c.     

Sonochemical synthesis and luminescence properties of single-crystalline BaF2:Eu nanospheres

BaF₂ nanocrystals doped with 5.0 mol% Eu³⁺ has been successfully synthesized via a facile, quick and efficient ultrasonic solution route employing the reactions between Ba(NO₃)₂, Eu(NO₃)₃ and KBF₄ under ambient conditions. The product was characterized via X-ray powder diffraction (XRD), scanning electron micrographs (SEM), transmission electron microscopy (TEM), high-resolution transmission electron micrographs (HRTEM), selected area electron diffraction (SAED) and photoluminescence (PL) spectra. The ultrasonic irradiation has a strong effect on the morphology of the BaF₂:Eu³⁺ particles. The caddice-sphere-like particles with an average diameter of 250 nm could be obtained with ultrasonic irradiation, whereas only olive-like particles were produced without ultrasonic irradiation. The results of XRD indicate that the obtained BaF₂:Eu³⁺ nanospheres crystallized well with a cubic structure. The PL spectrum shows that the BaF₂:Eu³⁺ nanospheres has the characteristic emission of Eu^{3+ 5}D₀-⁷F_J (J=1-4) transitions, with the magnetic dipole ⁵D₀-⁷F₁ allowed transition (590 nm) being the most prominent emission line.     

Magnetic susceptibilities of UO2ZrO2 solid solutions

The magnetic susceptibility of cubic solid solutions of UO₂ and ZrO₂ with a fluorite structure has been measured from room temperature to 2.3 K. The magnetic moment and the Weiss constant have been determined in the temperature range where the Curie-Weiss law holds. These values decrease monotonically with increasing ZrO₂ concentration. The solid solutions of UO₂ diluted with 0 ～ 20 mole% ZrO₂ show an antiferromagnetic transition and have a linear dependence of Néel temperature on concentration, with a critical concentration of 78 mole% UO₂. The molecular field theory which includes next-nearest-neighbor interaction was applied to the results. The interaction between nearest-neighbor spins, J₁, decreases with increasing ZrO₂ concentration, whereas that between next-nearest-neighbor spins, J₂, increases. The behavior of J₁ against composition is thought to be caused from the direct effect of magnetic dilution with ZrO₂. Regarding the behavior of J₂, the effect of increasing magnetic interaction due to the smaller distance of uranium ions is considered to be stronger than the magnetic dilution effect.     

Low temperature specific heats and magnetic properties of LaRh2Si2 and HoRh2Si2

Specific heats, static magnetizations and a.c. susceptibilities have been measured on polycrystalline LaRh₂Si₂ and HoRh₂Si₂. Both compounds have been confirmed to have the ThCr₂Si₂-type crystal structure down to 4 K.LaRh₂Si₂ is not superconducting and shows no ordered magnetism above 1.3 K. It has a Debye temperature θ_D = 330 K, electronic specific heat coefficient γ = 6.4mJ mol⁻¹ K⁻², temperature-independent magnetic susceptibility χ_m = 1.6 × 10⁻⁴ e.m.u. mol⁻¹, and is considered to be an ordinary metal.HoRh₂Si₂ has two peaks in the specific heat vs. temperature curve, a broad peak at 11 K and a very sharp one at 27 K. The magnetization vs. temperature curve also shows two peaks at 10 K and 29 K. The magnetic part of the specific heat obtained by subtraction of the specific heat of LaRh₂Si₂ from that of HoRh₂Si₂, provides the total entropy ΔS = 24 J mol⁻¹ K⁻¹, which is close to Rln(2J + 1) for J = 8. This fact suggests that all the excess entropy is attributed to the ground state multiplet of Ho³⁺ ion (⁵I₈) and that the lower temperature peak observed in the specific heat (and also in the magnetization) does not originate in itinerant electrons nor in an impurity phase.     

Thermal analysis on C<Subscript>6</Subscript>H<Subscript>10</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>-doped MgB<Subscript>2</Subscript>

Additives to MgB₂ can improve the superconducting functional characteristics, such as critical current density (J _c) and irreversibility field (H _irr). Recently, we have shown that repagermanium (C₆H₁₀Ge₂O₇) is an effective additive, enhancing both J _c and H _irr. To look into details of the processes taking place during the reactive sintering, a thermal analysis study (0.167 K s^?1, in Ar) is reported. We used differential scanning calorimetry between 298 and 863 K and simultaneous thermogravimetric—differential thermal analysis between 298 and 1233 K. Samples were mixtures of powders with composition 97 mol% MgB₂ and 3 mol% C₆H₁₀Ge₂O₇. Up to 863 K, repagermanium decomposes by multiple steps and forms amorphous phases. A reaction with MgB₂ is not observed. Above this temperature, partial decomposition of MgB₂ occurs. Crystalline Ge and MgO are detected before formation of Mg₂Ge and MgB₄, when temperature approaches the melting point of Ge (1211 K). Carbon substitution for boron in the crystal lattice of MgB₂ is observed for samples heated above 863 K. The amount of substitutional C does not significantly change with temperature.     

Hydrurofluorures ioniques MF2−xHx (M = Sr,Ba): Synthèse et étude structurale par diffraction des neutrons

The study of the substitution of H^? for F^? ions in SrF₂ and BaF₂ has shown the existence of the cubic hydride fluorides MF_2?xH_x (M = Sr, Ba). The variation of the lattice parameter with hydrogen replacement is stronger than it is in the case of CaF_2?xH_x. The upper limit of substitution diminishes for the series calcium, strontium, barium. Neutron diffraction confirms the simple structural model, already proposed for calcium hydride fluoride: a fluorine cubic structure with a statistical distribution of H^? and F^? on 8(c) sites.     

Hydrothermal synthesis of fluorated single crystals: BaMnF4 and BaF2·HF

Investigations to eliminate defects in the incommensurate phase BaMnF₄, led to a method of preparation at a temperature lower than the melting point of BaMnF₄. Qualitative studies of the systems BaF₂ -H₂O -HF and MnF₂ -H₂O -HF showed that very pure single crystals of BaMnF₄ could be grown at a temperature lower than 300°C. Several new phases could be isolated as single crystals, in particular: BaF_2^′HFSingle crystal X-ray analysis revealed the compound to be monoclinic (Space Group P2₁ and Z = 2) and confirmed the formula found by chemical analysis. However, because of the difficulty to distinguish between fluorine and oxygen atoms, some problems remain, which will be solved by neutron diffraction studies. Infrared analysis indicate the presence of HF^?₂ and suggest the following representation: (Ba²⁺)₂(HF^?₂)₂(F^?)₂.It is possible to extend this method of synthesis to the preparation of other fluorides and related compounds, in particular to those of europium and strontium.     

Electrical conductivity investigation of diluted potassium chloride solutions in binary mixture triethylamine-water near its consolute point

The binary liquid mixture of triethylamine + water (TEA-W) has a lower consolute point at a critical composition of 32.27 mass % triethylamine. Starting at a temperature within the one-phase region, the electrical conductivity of a sample of this mixture with addition of (K⁺, Cl⁻) ions was measured and found to be accurately described by the Vogel-Fulcher-Tammann (VFT) law. Before that, for the pure system, in a temperature range ΔT = T_c − T < 2 °C where T_c is the critical temperature, the electrical conductivity (σ) exhibits a monotonous deviation from the VFT behaviour. This anomaly is finite at T_c. The asymptotic behaviour of the electrical conductivity anomaly is described by a power law t^1−α, where t is the reduced temperature |(T−Tc)/Tc| and α is the critical exponent of the specific heat anomaly at constant pressure. For the electrolyte mixtures, by combining the viscosity and the electrical conductivity data, the value of the computed Walden product has been determined and the salt dissociation degrees as well as the Debye screening length have been estimated.     

Spin dimer analysis of the three-dimensional antiferromagnetic ordering in the quaternary manganese sulfides BaLn2MnS5 (Ln=La, Ce, Pr)

The quaternary manganese sulfides BaLn₂MnS₅ (Ln=La, Ce, Pr) consist of (MnS₄)⁶⁻ anions separated with short S?S distances slightly longer than the van der Waals distance. Nevertheless, these sulfides are known to undergo a three-dimensional (3D) antiferromagnetic ordering at a reasonably high temperature (i.e., T_N=58.5, 62.0 and 64.5 K for Ln=La, Ce and Pr, respectively). The origin of this observation was probed by studying the Mn-S?S-Mn super-superexchange interactions of BaLn₂MnS₅ on the basis of spin dimer analysis. The non-bonding S?S contacts in the vicinity of the van der Waals distance are found essential in determining the strengths of the Mn-S?S-Mn super-superexchange interactions. The antiferromagnetic spin exchange between adjacent (MnS₄)⁶⁻ anions along the c-direction (J₂) is calculated to be stronger than that in the ab-plane (J₁) by a factor of ∼10, so that the strongly interacting spin units of BaLn₂MnS₅ (Ln=La, Ce, Pr) are 1D chains made up of the exchange paths J₂. The relative strengths of the spin exchange interactions for the J₁ and J₂ paths are consistent with the finding that the Néel temperatures of BaLn₂MnS₅ are reasonably high, and they increase in the order BaLa₂MnS₅<BaCe₂MnS₅< BaPr₂MnS₅.     

High temperature structural investigation of the delafossite type compound CuAlO2

Crystal structure parameters were determined for the delafossite type compound CuAlO₂ at 295, 450, 600, 750, 900, and 1200 K with single crystal high temperature X-ray diffraction technique. Anisotropic refinements result in conventional R values of 0.021, 0.027, 0.029, 0.030, 0.032, and 0.036 at respective temperatures. Crystals of CuAlO₂ have the rhombohedral space group $\text{R}\text{3}\text{m}$ with a = 2.8584(7), c = 16.958(3) Å and Z = 3 at 295 K. The mean thermal expansion coefficient for the dimension a is 11.0 × 10^?6 K^?1 about three times larger than 4.1 × 10^?6 K^?1 for c. In the structure, the AlO₆ octahedra are linked by their OO edges and form AlO₂ layers perpendicular to the c axis with the thickness corresponding to the height of an octahedron. With increasing temperature, the AlO₆ octahedra expand along the directions of the basal plane, while expansion scarcely occurs along the c axis. The Cu atom lying between the AlO₂ layers shows a large anisotropic behavior in the thermal vibration. The temperature factor for Cu atom in the basal plane becomes very large (0.044 Ų) at 1200 K, but the ratio of the temperature factor perpendicular to c to that parallel to c does not change appreciably with increasing temperature.     

The study of phase transition of KHF2 doped with sodium ions with Raman intensities of the lattice modes as the order parameter

The phase transition of doped K_1-xNa_xHF₂ is studied with the K⁺ translatory mode and the HF⁻₂ librational mode, which are Raman active in the low temperature phase while inactive in the high temperature phase, as the order parameter. Particular attention is paid to the elucidation of the critical exponent β which is defined as the parameter relating the lattice Raman intensity to the temperature near T_c through (T_c - T)^β. β values for various dopings are interpreted to understand the dynamics of the phase transition in the critical region. The analysis shows that a very small amount of doping results in a drastically large effect in the critical region. Further doping does not induce further a critical effect in a proportional way. This enables one to define the undoped and doped systems with a very small amount of dopant as two distinct states.     

Magnetic susceptibilities of UO2ThO2 solid solutions

The magnetic susceptibility of UO₂ThO₂ solid solutions has been measured from room temperature to 2.0 K. The magnetic moment and the Weiss constant have been determined in the temperature range in which the Curie-Weiss law holds. For the solid solutions showing antiferromagnetic transition, the Néel temperature has been also determined. These values decrease monotonically with increasing ThO₂ concentration. The results were analyzed using the molecular field theory which includes the interaction between next-nearest neighbor spins. The interactions between nearest neighbor spins, J₁, and those between next-nearest neighbor spins, J₂, both decrease with increasing ThO₂ concentration. The change of J₁ with composition is larger than that of J₂. The effect of magnetic dilution with ThO₂ is considered to be stronger on the interaction between nearest neighbor uranium ions.     

Investigation of the Mn2+-F− distance for Mn2+ in fluoride lattices with hexahedral coordination

The Mn²⁺-F^? distance for Mn²⁺-doped CaF₂, CdF₂, SrF₂ and BaF₂ is derived from the experimental isotropic superhyperfine constant A_s. The results obtained indicate the existence of inwards relaxation processes in all these cases, as well as values of the Mn²⁺-F^? distance which are ≈0.1 A larger than those found for systems with [MnF₆]^4? units. This allows one to explain the low 10D_q values recently found by Alonso an Alcalá in CdF₂: Mn²⁺.     

Ion-beam irradiation of lanthanum compounds in the systems La2O3-Al2O3 and La2O3-TiO2

Thin crystals of La₂O₃, LaAlO₃, La_2/3TiO₃, La₂TiO₅, and La₂Ti₂O₇ have been irradiated in situ using 1 MeV Kr²⁺ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La₂O₃ remained crystalline to a fluence greater than 3.1×10¹⁶ ions cm⁻² at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (T_c) of 647 K for LaAlO₃, 840 K for La₂Ti₂O₇, 865 K for La_2/3TiO₃, and 1027 K for La₂TiO₅. The T_c values observed in this study, together with previous data for Al₂O₃ and TiO₂, are discussed with reference to the melting points for the La₂O₃-Al₂O₃ and La₂O₃-TiO₂ systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between T_c and melting temperature (T_m) in the two systems. More complex relationships exist between T_c and crystal structure, with the stoichiometric perovskite LaAlO₃ being the most resistant to amorphisation.     

Nonlinear dielectric effect in a critical solution

This paper presents measurements of Δ/E² versus T for the critical solution nitroethane in cyclohexane. The Δ/E² for the temperature near the critical temperature T_c has a positive value and satisfies the following dependence: Δ/E² ∝ 1/(T ? T_c)^λ where λ = 0.34.     

Critical temperatures,pressures, and densities for the mixtures CO2–C3H8, CO2–nC4H10, C2H6–C3H8, and C3H8–nC4H10

A simple method is developed to estimate mixture critical temperatures (T_c), pressures (P_c), and densities (ρ_c) as a function of overall composition (X) from near critical region experimental coexistence data. This three-step method is applied to four mixtures, CO₂–C₃H₈, CO₂–nC₄H₁₀, C₂H₆–C₃H₈, and C₃H₈–nC₄H₁₀. Isothermal liquid–vapor coexistence data, which includes temperature, vapor pressure, coexisting densities (ρ^ℓ and ρ^v), and coexisting compositions for the more volatile component (x₁^v and x₁^ℓ) are used. In the first step, the difference of the saturated liquid and vapor densities (ρ^ℓ−ρ^v) is fitted to an empirical function in ((P_c−P)/P_c) to obtain P_c. Then P/P_c and ((ρ^ℓ+ρ^v)/2ρ_c) are simultaneously fitted to functions of a polynomial in (X₁−(x₁^v+x₁^ℓ)/2) yielding estimates of ρ_c and X₁. Finally, the discrete estimated critical data points are fitted with an equation to provide a continuous representation of the critical lines. The method is successfully tested for the mixtures, CO₂–C₃H₈ and CO₂–nC₄H₁₀, for which there is a reasonable amount of isothermal data. The procedure is then applied to the mixtures, C₂H₆–C₃H₈ and C₃H₈–nC₄H₁₀, for which there are sparse data. For all four mixtures, the critical temperature line, T_c vs. X₁, matches literature values within ±0.5%. The critical pressure line, P_c vs. X₁, and critical density line, ρ_c vs. X₁, match literature values, in general, within ±2%.     

Density and refractive index measurements of critical mixture 1,4-dioxane + water + saturated KCl in homogenous phase region

The ternary critical mixture of 1,4-dioxane (1) + water (2) + saturated KCl (3) has a lower critical point. The density ρ and refractive index n of this system have been measured as function of temperature for nine critical mixtures along the coexistence curve below the temperature of phase-transition. The water mole fraction in free basis x₂ in the mixtures extends from (0.550 to 0.880) and the molality m of KCl from 0.47 to 2.039 mol kg⁻¹. With increase of temperature, water mole fraction and KCl molality, the obtained density decreased, while the refractive index decreases with increase in temperature, water mole fraction and molality of salt. Both represented anomalies near the critical temperature T_c. The molar fraction of critical mixture, increase less than 1%, with temperature and decrease by 10%, with water mass fraction and molality of salt. The critical density and the critical refractive index vary linearly with water mass fraction w₂$w_2$ with molality m of KCl as a third degree polynomial.     

Static and dynamic light scattering studies of the critical behaviour of polymer mixtures

Critical fluctuations were studied in polymer mixtures of poly(dimethylsiloxane) and poly(ethylmethylsiloxane), which exhibit an upper critical temperature at T_c ⋍ 57 °C. The measurements were performed in a broad temperature range at three compositions in the miscible region close to the coexistence line. The temperature dependence of the static structure factor S(q=0) can be described by a mean field behaviour except for temperatures in the range of 6 K above T_c. There, a turnover to an Ising behaviour is observed according to a modified Landau-Ginzburg criterion. The mean field spinodal temperature T_s was determined by extrapolation of S(0)⁻¹ to zero. From the Ornstein-Zernike representation of the angular dependence of S(q), the correlation length ζ of the concentration fluctuations can be determined and leads to a critical amplitude ζ₀ = lim ζ(T→∞) = 20.5 Å. The interdiffusion dynamics described by the mutual diffusion constant D has been measured by quasielastic light scattering. It shows for the critical composition ϕ_c a critical slowing down as T approaches the critical temperature T_c. Furthermore, the q² scaling of the relaxation rate of the interdiffusion dynamics changes to q³ behaviour close to T_c according to the mode coupling theory by Kawasaki.     

Reactive and non-reactive quenching of O(1D2) by COF2

Quenching of O(¹D₂) by COF₂ has been investigated by time-resolved resonance fluorescence monitoring of the product O(³P_J) following 248 nm pulsed laser photolysis of O₃. The rate constant for total removal of O(¹D₂) by COF₂ is (7.4 ± 1.2) × 10^?11 cm³ molecule^?1 s^?1. 71 ± 7% of the quenching interactions result in formation of O(³P^J).