Stability field diagrams for <Emphasis Type="Italic">Ln</Emphasis>–O–Cl systems

Isothermal stability field diagrams for Ln?O?Cl systems (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) are developed by taking partial pressures of volatile components oxygen and chlorine as variables. Thermodynamic properties of all the oxides and trichlorides (LnCl₃) are available in the literature. However, data for oxychlorides (LnOCl) and dichlorides (LnCl₂) are limited. Based on systematic trends in stability of these compounds across the lanthanide series, missing data are estimated to construct the diagrams for 13 Ln?O?Cl systems at 1000 K. All the lanthanide elements form stable LnCl₃ and LnOCl. Dichlorides of Nd, Sm, Eu, Dy, Tm and Yb are stable. For systems in which dichlorides are unstable (Ln = La, Ce, Pr, Gd, Tb, Ho, Er), the LnOCl is in equilibrium with the metal (Ln) and the stability field of LnOCl is sandwiched between those of oxides and trichlorides. Stability field diagrams of lanthanide systems forming stable LnCl₂ are of two kinds: in the first kind (Ln = Nd, Dy) the stability fields of Ln and LnOCl are in contact and the stability field of LnOCl separates the fields of chlorides and oxides. In diagrams of the second kind (Ln = Sm, Eu, Tm, Yb) there is a direct equilibrium between the oxides and dichlorides at low partial pressures of oxygen and chlorine. There is no contact between the stability fields of Ln and LnOCl; the stability field of LnOCl intervenes between the oxide and chloride phases only at higher partial pressures.     

Effect of isovalent lanthanide cations compensation for volatilized A-site bismuth in Aurivillius ferroelectric bismuth titanate

Bismuth titanate ferroelectric ceramics (BiT) compensated(doped) by 3 mol% of Bi³⁺ or Ln³⁺ (BiT–Bi, BiT–Ln; Ln = La, Nd, Sm) were synthesized by conventional solid-state reaction, then the effects of compensation towards the lattice structure as well as the ferroelectric properties were studied. Rietveld refinement of the structure confirmed the pure Aurivillius phase with space group B2cb in all the constituents, however the trace amount of Bi₂O₃ and Ln₂O₃ could also act as sintering aids during sintering. The 6s ² lone pair electrons of Bi³⁺ in perovskites localize into a lobe shape and the lobes major axis prefer to align along the a-axis, resulting the Bi³⁺ in perovskites showing a smaller effective radius compared with Ln³⁺ and causing higher lattice distortion. In addition, the paraelectric–ferroelectric phase transition temperature (Curie temperature, T _c ), piezoelectric coefficient (d ₃₃), spontaneous polarization (P _s ), a.c. conductivity [σ(ω)] and activation energy of dielectric relaxation (E _a ) increase with decreasing A-site cation radius. Furthermore, ferroelectric polarization and electromechanical properties of the BiT–Bi were enhanced dramatically due to the oxygen vacancies suppression by the compensation, which was confirmed by electric modulus approach.     

Effect of Boron Substitution for Bi on Superconductivity of the Bi-2212 Phase in the Bi-Sr-Ca-Cu-O System

Possibility of boron substitution for Bi and the substitution effect on superconductivity is investigated for the Bi-2212 phase of Bi-Sr-Ca-Cu-O. From X-ray diffraction study, it is found that samples in the (Bi_2?x B _x )Sr₂CaCu₂O _z system are mainly of the single 2212 phase in a composition range of 0.0≤x≤0.6, and both of the lattice parameters a and c change with increasing x up to 0.6. From measurements of the magnetic susceptibility and the electrical resistivity, the superconducting transition temperature is found to increase up to 0.6 with increasing x. These results are considered to show that boron is substitutable for Bi up to x=0.6 in the (Bi_2?x B _x )Sr₂CaCu₂O _z system and that the boron substitution causes the number of hole-carriers to decrease in this system.     

Synthesis and investigation of superconducting properties of heterosubstituted magnesium diborides

The properties of magnesium diborides with the composition Mg_{1 ? x} M _x B₂, where M = Al, Ga, In, and Tl, and Mg (B_{1 ? x} E _x ), where E = N, P, and Si (0.05 ≤ x ≤ 0.3), made by sintering of calculated amounts of boron, magnesium, and a substituent element or by a solid-phase exchange reaction of a corresponding halogenide of a metal with MgB₂ at 1070–1170 K are studied. Using RFA technique, it is shown that, from all employed heterosubstituents, only the atoms of aluminum, gallium, and silicon in the amount x ≤ 0.2 are incorporated into magnesium and boron sublatticies. Here, the temperature of the superconducting transition T _c = 39 ± 1K determined for pure magnesium diboride hardly changes.     

Synthesis of Zirconium Diboride Films and ZrB<Subscript>2</Subscript>/BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Heterostructures

Using mechanochemical synthesis, we have prepared zirconium borohydride, Zr(BH₄)₄, as a precursor for ZrB₂ film growth by chemical vapor deposition. We have carried out the thermodynamic modeling of phase formation processes in the Zr–B–(N)–H and Zr–B–(N)–H–O systems in a wide temperature range, from 100 to 2500°C, at various p_(H2)/p_(Zr(BH4)4) and p_(NH3)/p_(Zr(BH4)4) partial pressure ratios in the starting gas mixtures. A process has been proposed for the growth of zirconium diboride films by Zr(BH₄)₄ decomposition using two techniques: chemical vapor deposition and plasma-enhanced chemical vapor deposition. We also developed a process for the growth of multilayer ZrB₂-and BC _x N _y -based structures.     

Effect of Zr Addition on Bi<Subscript>1.8</Subscript>Pb<Subscript>0.4</Subscript>Sr<Subscript>2.0</Subscript>Ca<Subscript>1.1</Subscript>Cu<Subscript>2.1</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Superconductor

The polycrystalline Bi_1.8Pb_0.4Sr_2.0Ca_1.1Cu_2.1 M_xO _y , with M = Zr (x = 0.0, 0.02, 0.04), were synthesized by solid-state reaction method and studied by X-ray diffraction analysis (XRD), scanning electron microscopy equipped with energy dispersive of X-ray analysis (SEM/EDX) and resistivity versus temperature measurements. The influence of the Zr addition on the Tc and microstructure properties of the superconducting compounds has been studied. SEM observations show whiskers grains randomly distributed and microstructural change due to the addition of Zr. The ZrO₂ was incorporated into the crystalline structure of BSCCO system in all samples. The crystallographic structure remains in a tetragonal form where a= b≠c. Generally, all samples exhibit semiconductor behaviour above \(T_{\mathrm {c}}^{\text {onset}}\). The onset critical temperature \(T_{\mathrm {c}}^{\text {onset}}\) increases up to 86 with x = 0.02. There is an enhancement in the critical temperature for doped samples as compared with pure Bi_1.8Pb_0.4Sr_2.0Ca_1.1Cu_2.1O _y .Changes in superconducting properties of ZrO₂ nanoparticle added Bi-2212 system were discussed.     

Filamentary Superconductivity in Semiconducting Policrystalline ZrSe<Subscript>2</Subscript> Compound with Zr Vacancies

Z r S e ₂ is a band semiconductor studied long time ago. It has interesting electronic properties, and because its layer structure can be intercalated with different atoms to change some of the physical properties. In this investigation, we found that Zr deficiencies alter the semiconducting behavior and the compound can be turned into a superconductor. In this paper, we report our studies related to this discovery. The decreasing of the number of Zr atoms in small proportion according to the formula Zr _x Se₂, where x is varied from about 8.1 to 8.6 K, changing the semiconducting behavior to a superconductor with transition temperatures ranging between 7.8 and 8.5 K, is depending on the deficiencies. Outside of those ranges, the compound behaves as semiconducting with the properties already known. In our experiments, we found that this new superconductor has only a very small fraction of superconducting material determined by magnetic measurements with applied magnetic field of 10 Oe. Our conclusions is that superconductivity is filamentary. However, in one studied sample, the fraction was about 10.2 %, whereas in others is only about 1% or less. We determined the superconducting characteristics; the critical fieldsthat indicate a type 2 superonductor with Ginzburg-Landau κ parameter of the order about 2.7. The synthesis procedure is quite normal following the conventional solid state reaction. In this paper, included are the electronic characteristics, transition temperature, and evolution with temperature of the critical fields.     

Rare-earth-doped PbTiO<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions

(Pb_{1 ? x} Ln _x )(Zr_0.53Ti_0.47)O₃ and (Pb_{1 ? x} Ln _x )(Zr_0.65Ti_0.35)O₃ (x = 0.02, 0.06; Ln = La, Pr, Gd, Yb) solid solutions have been prepared by modified solid-state synthesis using organic-ligand precursors. The solid solutions have been characterized by thermal analysis, IR spectroscopy, x-ray powder diffraction, and atomic force microscopy. All of them have a rhombohedrally distorted perovskite structure (sp. gr. R3c).     

Synthesis and Characterization of Ni<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>3</Subscript> Superconductor

In this paper, we characterized the microstructure and superconducting properties of Cu-doped NiBi₃ samples. The polycrystalline Ni_1?x Cu _x Bi₃ (0 ≤ x ≤ 0.10) samples were prepared using a solid-state reaction method. The crystal structure and unit cell parameters were determined by Rietveld refinement of powder X-ray diffraction. The data showed that the main phase present corresponded to NiBi₃ without dependence on the Cu concentration, but with small quantities of Ni and Bi. The SEM and AFM measurements revealed that the main phase was inhomogeneous at microscopic level, with Bi richer regions in comparison to other regions. However, Raman spectroscopy results did not show significant changes in the spectra with Cu doping and in different regions of the samples. Another finding was that regardless of Cu doping, the superconducting transition temperature was 4.05–4.06 K.     

Crystal structure and phase transitions of Sr<Subscript>2</Subscript>Ni<Subscript>1–<Emphasis Type="Italic">y</Emphasis></Subscript>Mg<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>MoO<Subscript>6</Subscript> solid solutions

Sr₂Ni_1–yMg _y MoO₆ mixed oxides with a double perovskite structure have been synthesized via the pyrolysis of glycerol–salt mixtures. The phase composition and structure of the samples have been determined using X-ray diffraction and Raman spectroscopy. The temperatures of phase transitions in Sr₂Ni_1–yMg _y MoO₆ with y = 0.25, 0.5, and 0.75 have been determined by high-temperature X-ray diffraction and dilatometry.     

Synthesis,Structure, and Superconductivity of (La1.5Pb0.5−<Emphasis Type="Italic">x</Emphasis>Sr<Emphasis Type="Italic">x</Emphasis>)CuO<Emphasis Type="Italic">z</Emphasis>

The substitution of strontium for lead in the material (La_1.5Pb_0.5?xSr _x )CuO _z , x = 0–0.15 has been carried out. A stable and reproducible single phased superconducting materials can be obtained inside an evacuated quartz tube. The X-ray diffraction pattern shows that the superconducting phase can be indexed on the basis of an orthorhombic symmetry (F_mmm) for x = 0 and on the basis of tetragonal symmetry (I_4/mmm) for x > 0. The transition temperature T _c increases as the strontium substitution parameter x increases. We observed the maximal T _c around x = 0.15 with 38 K with fairly large Meissner volume fraction of 38% (FC).     

Magnetoresistance and Irreversibility Fields of Bismuth-Based 1G Tape

The magnetoresistance and irreversibility fields of commercial bismuth-based 1G tape were studied on the basis of the temperature dependencies of the magnetoresistance at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape are the following: T _{c50 %} = 110.3 K and T _c0 = 109.9 K, and the width of superconducting transition is ΔT = 0.5 K. The widths of the transition to the superconducting state versus applied magnetic fields were derived for both orientations. The experimental data were fitted using the formula ΔT = C H ^m + ΔT ₀. The irreversibility field values were obtained and successfully fitted as a function of temperature. At 77 K, they were found to amount to H _{i r r} = 72.8 kOe and H _{i r r} = 5.5 kOe for the parallel and perpendicular directions, respectively.     

Structure and Magnetic Properties of Ce-Substituted Yttrium Iron Garnet Prepared by Conventional Sintering Techniques

Y_3?xCe _x Fe₅ O ₁₂ (CeYIG) ceramics, with x = 0, 0.15, 0.25, 0.35, 0.45, and 0.5, were fabricated by a conventional ceramic sintering technique. We studied the structures and magnetic fields of a series of CeYIG ceramics using X-ray powder diffraction, a scanning electron microscope, and a superconducting quantum interference device magnetometer. Findings showed that the substitution limit of the concentration of Ce³⁺ ions in the yttrium iron garnet structure was approximately x = 0.25. An extra CeO₂ phase was detected in the ceramic when the addition of CeO₂ content overtook the limit. The lattice constants and relative densities increased by increasing the Ce³⁺ contents in the ceramics. First, the saturation magnetization increased gradually with increases in the substitute concentration of Ce³⁺ ions and then decreased gradually when x = 0.35, 0.45, and 0.5. Overall, this study showed that the Y_3?xCe _x Fe₅ O ₁₂ material with x ≤ 0.15 exhibited excellent magnetic properties. Hence, the material show promise for magneto-optical and microwave communication applications.     

Properties of fluorine-doped PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> ceramics

We report the first fluorine doping of lead magnesium niobate in the PbMg _{(1 + x)/3}Nb_{(2 ? x)/3}O_{3 ? x} F _x system in a wide composition range, x = 0.025 to 0.625. The fluorine content of the samples is shown to be substantially lower than the intended one because of the fluorine volatilization in the form of HF during synthesis and sintering in air. The ceramics consist of magnesium and lead oxides undetectable by x-ray diffraction, and a perovskite phase whose composition can be represented by the formula PbMg_{(1 + m)/3}Nb_{(2 ? m)/3}O_{3 ? m} F _m , where the fluorine content after sintering is m ≤ 0.12. The PbO and MgO contents of the ceramics depend on the starting mixture composition (x) and heat-treatment conditions (hydrogen fluoride and lead oxide volatilization). As a result of the low fluorine content, the diffraction patterns of the samples show no superlattice reflections, and their lattice parameter varies insignificantly with x. Data are presented on the temperaturedependent dielectric permittivity of ceramic samples sintered and annealed under different conditions.     

Effect of CdTe Addition on the Electrical Properties and AC Susceptibility of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor

The effect of CdTe addition on YBa₂Cu₃O_7?δ (CdTe) _x (x = 0–0.10) has been studied. XRD patterns showed the presence of impurities including CdTe for x ≥ 0.05 wt%. The resistance versus temperature curves showed metallic behavior for all samples. The onset temperature T _{c onset} for all samples is between 90 and 92 K. The superconducting transition width, ΔT _c = 4 K for all samples except for x = 0.1 wt% where ΔT _c = 6 K. The superconducting transition determined by AC susceptibility measurement also showed little change in \(T_{\mathrm {c}\chi ^{\prime }}\) (between 89 and 92 K). However, the peak temperature T _p of the imaginary part of the susceptibility χ″, showed a drastic decrease for samples with x > 0.05 wt%. This indicated that the superconducting grains were strongly decoupled for x > 0.05 wt% due to the impurities as observed in the XRD pattern. The intergrain critical current density, J _c at T _p for the x = 0 sample is J _c (82 K) = 17 A cm^?2. These results can be useful in the fabrication of semiconductor/YBCO superconductor hybrid systems.     

Band Modification Effect on the Normal and Superconducting State Properties of Bulk MgB<Subscript>2</Subscript>

Modification of σ and π bands was studied in MgB₂ by doping 3, 6 and 9 wt% of C and Fe, respectively. The samples synthesized by a solid-state route were characterized by XRD, and magnetization (M) and resistivity (ρ) measurements were in the temperature range (T) 4.2–300 K and magnetic field range (B) 0–12 T, respectively. The decrease (increase) of the lattice parameter a with C (Fe) doping, consistent with B (Mg) site substitution, confirms the expected changes in σ (π) bands. This is supported by the fact that normal-state ρ(T) of all the samples can be fitted by a two-band model and the scattering rates in both the bands are found to be dependent on the dopant. The influence of C and Fe doping on various superconducting properties of the host MgB₂ is also found to be significantly different. For instance, in the presence of magnetic field, Fe doping shows a much larger broadening of the superconducting transition when compared to C doping. The critical current density (J _C(B)) at 4.2 K vanishes for C (Fe) doping at around T～12 (～3). It is shown that the band modification and the superconducting properties are correlated.     

Enhanced Superconductivity in Double-Doping Cu<Subscript>0.15</Subscript>TaSe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">S</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Superconducting Cu _x TaSe₂(x=0.05, 0.15) and Cu_0.15TaSe_2?x S _x (x=0, 0.5, 1, 1.5) single crystals have been systematically fabricated by a chemical vapor transport method. It is found that the double doping in TaSe₂, i.e., the simultaneous intercalation of Cu and substitution of Se by S, can substantially enhance the superconducting transition temperature. Transport property measurements give evidence of the coexistence and competition of charge density wave state and superconductivity in Cu _x TaSe₂ which provide meaningful information to understand the complex electronic states in this system. The parallel shift and the fan-shape broadening behaviors are observed in the superconducting transition curves under magnetic fields of Cu_0.15TaSeS and TaSeS, respectively, indicating an increase of coherence length and suppression of superconducting fluctuation induced by copper intercalation.     

Metallization of molecular condensates and superconductivity

The possibility of explaining the absorption of light during pressure-induced xenon metallization in terms of a transition to the superconducting state is studied. The energy structure of the van der Waals condensates is considered as an alternative to the band structure. It is suggested that, as the condensate is compressed and the interatomic distance decreases to a certain a _B value such that a _F <a _B <a _W (where aF and a _W are the equilibrium distances in the Fermi and van der Waals dielectrics), the condensate occurs in a region of the superconducting state (until the onset of metallization and BCS superconductivity). This region corresponds to binding energies in the range from 40 to 60 kJ/mol. Many of the molecular condensates and metals are close to this region of instability. This state can be accessed not only by compressing a condensate or using chemical bonds: the role of compression can be performed by adsorption forces operative, for example, in the sorption (inclusion) compounds.     

Synthesis and Superconductivity of Electron-Doped <Emphasis Type="Italic">β</Emphasis>-ZrNCl with Partial Substitution of Ti on Zr Site

Highly crystallized α-TiNCl and ZrNCl with orthogonal network of [M ₂ N ₂] (M: Ti and Zr) were used to synthesize the solid-solution phase α-Ti _y Zr_1?y NCl, which was irreversibly transformed into its β-form polymorph with honeycomb network upon a heat treatment. The intercalation of lithium (Li) into the obtained powder samples was carried out to realize the electron doping in β-form compound. The measurements of magnetic susceptibility and electrical resistivity suggest that the intercalated compound β-Li_0.43Ti_0.09Zr_0.91NCl shows superconductivity with transition temperature (T _c ) of 15.0 K. Upon the cointercalation of solvent molecule of tetrahydrofuran (THF), two transitions were observed at 14.1 and ～21 K, corresponding to the main phase of β-Li _x (THF) _y ZrNCl and a THF-cointercalated phase of β-Ti _y Zr_1?y NCl, respectively. We consider that such a tiny substitution of Ti on Zr site in β-form structure play an important role in the increase of T _c .     

Crystal structures of Ln<Subscript>2</Subscript>TiO<Subscript>5</Subscript> (Ln = Gd,Dy) polymorphs

Single crystals of four Ln₂TiO₅ polymorphs have been grown, and their structures have been determined: orthorhombic (Gd₂TiO₅, a = 10.460(5), b = 11.317(6), c = 3.750(3) Å, Pnam, Z = 4), hexagonal (Gd_1.8Lu_0.2TiO₅, a = 3.663(3), c = 11.98(1) Å, P6₃/mmc, Z = 1.2), cubic (Dy₂TiO₅, a = 10.28(1) Å, Fd3m, Z = 10.4), and monoclinic (Dy₂TiO₅, a = 10.33(1), b = 3.653(5), c = 7.306(6) Å, β = 90.00(7)°, B2/m, Z = 2.4). The last polymorph has been identified for the first time.