Synthesis and Phase Composition of NaxMxTi8 – xO16(0.67 ≤ x≤ 2.0; M = Al, Ga, In)

The phase composition of Na _x M _x Ti_{8 – x} O₁₆(M = Al, Ga, In) materials was studied by x-ray diffraction. In the Al system, a hollandite phase was obtained in the composition range 0.67 x 2.0. The results are interpreted in terms of Goldschmidt's tolerance factor. It is shown that A₂B₈O₁₆compounds with the hollandite or Na _x TiO₂-type structure are more difficult to prepare at smaller ionic radii of M. The assumption is made that the monoclinic Na _x TiO₂-type phase may, under certain conditions, have a channel structure.     

The Characteristics of the Superconducting and Magnetic Phases in the Polycrystalline Samples of Ruthenocuprates of Nominal Compositions RuSr2GdCu2O8, Ru0.98Sr2GdCu2O8 and Ru0.5Sr2GdCu2.5O8−δ

The temperature dependencies of the resistivity for the superconducting ruthenocuprates of nominal compositions RuSr₂GdCu₂O₈, Ru_0.98Sr₂GdCu₂O₈ and Ru_0.5Sr₂GdCu_2.5O_8?δ were examined for the magnetic field dependent characteristics of the superconducting transitions. The effect of the insignificant diminishing of the Ru/Cu ratio in parent RuSr₂GdCu₂O₈ was confirmed as relevant for the stabilisation of the superconducting phase. Noted differences in the compared characteristics are interpreted for possible inhomogeneous nucleation of the superconducting phase in the parent ruthenocuprate. The phase anisotropy in RuSr₂GdCu₂O₈ and Ru_0.98Sr₂GdCu₂O₈, in presence of the compounds Ru magnetism, appears to be a cause of a significant softening of the H _c2(T) phase line. An anomalous lowering of the magnetoresistivity was observed in the approx. 10 K range above the onset of the superconducting transition, which may suggest the presence of enhanced superconducting fluctuations in the samples. The positive magnetic field shift of the temperatures, which limit the magnetoresistivity and the specific heat signatures of the magnetic ordered state of the Ru sublattice, suggests probing the influence of the ferromagnetic Ru interactions in an effective metallic-like conduction channel present in the samples. Superconducting characteristics of the Ru_0.5Sr₂GdCu_2.5O_8?δ reveal a significant contribution of the Gd paramagnetic signal at low temperatures, interpreted for the presence of a significant anisotropy of the superconducting phase. It is concluded that the Ru–Cu substituted phases of ruthenocuprates may present an opportunity to investigate the effectively anisotropic superconducting phase despite its comparatively high T _c in the compounds related to the 123-type cuprate superconductor.     

The Interplay Among Electrical, Magnetic, and Structural Properties in the Non-superconducting Ru(Sr2?x Ba x )GdCu2O8±z System

The electrical, magnetic, and structural properties of the non-superconducting RuSr₂GdCu₂O_8±z system were studied through Ba substitution on the Sr sites. Samples of the non-superconducting Ru(Sr_2?x Ba _x )GdCu₂O_8±z system, with x=0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.50 were synthesized through the solid-state reaction method at ambient pressure, in air, at temperatures between 980?°C and 1025?°C. X-ray diffraction data show that substitution of Sr by Ba takes place iso-structurally into a tetragonal structure (space group P4/mmm, No.?123) with a solubility up to x=0.25. Rietlveld-refinement analysis indicates that the cell volume monotonically increases with barium content, while both Cu–O(1) and [1–Ru–O(1)] bond lengths, as well as the Cu–O(2)–Cu bond angles, all increase with increasing x with a maximum peak around x=0.15, and then all decrease for larger values of the barium content x, resulting into bell-type curves. The electrical resistance for the samples with 0 ≤x≤0.25 annealed in flowing oxygen at 960 °C for 2 h shows a semiconducting behavior. DC-magnetization measurements indicate that all samples exhibit ferromagnetic ordering with a magnetic transition temperature T ^Curie between 131 K and 141 K. Both the normalized resistance values (at a fixed temperature) and T ^Curie show each a similar bell-type dependence on x, a result that seems to be similar to that found for the bond lengths and angles. These five bell-type dependences on x suggest that a strong interplay among electrical, magnetic and structural properties are taken place in the non-superconducting Ru(Sr_2?x Ba _x )GdCu₂O_8±z system.     

Structure and properties of Ca9 − x Pb x R(PO4)7 (R = Sc, Cr, Fe, Ga, In) whitlockite-like solid solutions

We have prepared solid solutions based on whitlockite-structure ferroelectrics, Ca_{9 ? x} Pb _x R(PO₄)₇ (R = Sc, Cr, Fe, Ga, In), through Pb substitution for Ca. Single-phase Ca_{9 ? x} Pb _x R(PO₄)₇ materials with a polar whitlockite-like structure (sp. gr. R3c) exist in the range 0 ≤ x ≤ 1.5 for all of the R metals studied. X-ray powder diffraction profile analysis results for R = In (x = 0.5, 0.8, and 1.0) demonstrate that the trivalent cations reside on the octahedral site M5 of the whitlockite structure, the calcium cation occupy the M1–M3 sites, and the lead cations are located predominantly on the M3 site. Differential scanning calorimetry, dielectric permittivity, and second-harmonic generation data attest to a ferroelectric phase transition, whose temperature is 580–610°C in Ca₉R(PO₄)₇ and decreases monotonically to 480–520°C as x increases to 1.5.     

The effect of thermodynamic properties of Me–Ti (Me = In,Sn, Ga,Au, and Ge) melts on the wetting of the CaF<Subscript>2</Subscript> substrate

The effect of Ti additions on the wetting behavior of CaF₂ by non-reactive liquid metals (In, Sn, Ga, Au, Ge) was investigated. Pure metals do not wet CaF₂ while minor additions of Ti improve wetting. Small changes of the contact angle were observed in the CaF₂/Au–Ti and CaF₂/Ge–Ti systems, which are characterized by strong Me–Ti interaction in the melt, while considerable decrease of contact angle was obtained in the CaF₂/In–Ti, CaF₂/Sn–Ti and CaF₂/Ga–Ti systems, which display a relatively weak Me–Ti interaction. According to a thermodynamic analysis and experiential observations, Ti does not react with the substrate to form condensed phases at the metal/CaF₂ interface. Therefore, it was assumed that the mechanism of the wetting improvement is attributed to the Ti segregation at the interface. The results of the XPS analysis confirm a Ti enrichment of the region close to the interface, moreover, according to the high resolution XPS spectrum, obtained from this region, the position of the In4d peak has a chemical shift, which is typical for In–Ti intermetallic compounds. The XPS analysis does not provide sufficient evidence for the formation of the intermetallic interfacial layer at elevated temperature. Thus, further investigations have to be designed and conducted in order to clarify this issue.     

Electrical conductivity and chemical stability of BaCe0·8 − x A x Gd0·2O3 − δ (A = In, Zr, Ta; x = 0, 0·1) ceramics

BaCe_0·8???x A _x Gd_0·2O_3???δ (A = In, Zr, Ta; x?=?0, 0·1) ceramics were synthesized by solid-state reaction method. Microstructure and electrical properties of BaCe_0·8???x A _x Gd_0·2O_3???δ ceramics were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance analysis at intermediate temperatures of 573–1073 K in different atmospheres. BaCe_0·8Gd_0·2O_3???δ , BaCe_0·7In_0·1Gd_0·2O_3???δ and BaCe_0·7Zr_0·1Gd_0·2O_3???δ ceramics exhibit a single cubic perovskite structure. BaCe_0·7In_0·1Gd_0·2O_3???δ ceramic has the highest conductivity of 4·6 × 10^???2 S·cm^???1 in air at 1073 K. BaCe_0·7In_0·1Gd_0·2O_3???δ and BaCe_0·7Zr_0·1Gd_0·2O_3???δ ceramics exhibit an excellent chemical stability against boiling water. Indium is a suitable doping element to promote the sintering densification and to enhance both electrical conductivity and chemical stability of Gd-doped BaCeO₃ at operating temperatures.     

Composition dependence of UV–visible and MID-FTIR properties of Se98−x Zn2In x (x = 0, 2, 4, 6 and 10) chalcogenide glasses

The UV–VIS absorption spectra of thin film of Se_98?x Zn₂In _x (x = 0, 2, 4, 6 and 10) chalcogenide glasses were measured in the wavelength range of 200–1100 nm by using spectrophotometer. It is observed from UV–VIS absorption measurements that the optical energy gap (E _g ) decreases with In content and it found a minimum for the Se₉₂Zn₂In₆ system. The refractive index (n), extinction coefficient (k), real dielectric constant (?′) imaginary dielectric constant (?″), absorption coefficient (α) are evaluated maximum for the Se₉₂Zn₂In₆ composition. However, FTIR spectra were recorded in the wavenumber range 4000–400 cm^?1. The FTIR recorded spectrum was shown to be a broad spectrum with increasing transmission in the MID (6–25 m) infrared region. The MID-IR spectrum transmittance percentage was also found to be considerably high for the Se₉₂Zn₂In₆ system.     

Probing the electronic properties of ternary AnM3n−1B2n (n = 1: A = Ca,Sr; M = Rh,Ir and n = 3: A = Ca,Sr; M = Rh) phases: observation of superconductivity

Abstract

We follow the evolution of the electronic properties of the titled homologous series when n as well as the atomic type of A and M are varied where for n = 1, A = Ca, Sr and M = Rh, Ir while for n = 3, A = Ca, Sr and M = Rh. The crystal structure of n = 1 members is known to be CaRh₂B₂-type (Fddd), while that of n = 3 is Ca₃Rh₈B₆-type (Fmmm); the latter can be visualized as a stacking of structural fragments from AM₃B₂ (P6/mmm) and AM₂B₂. The metallic properties of the n = 1 and 3 members are distinctly different: on the one hand, the n = 1 members are characterized by a linear coefficient of the electronic specific heat γ ≈ 3 mJ mol^?1 K^?2, a Debye temperature θ_D ≈ 300 K, a normal conductivity down to 2 K and a relatively strong linear magnetoresistivity for fields up to 150 kOe. The n = 3 family, on the other hand, exhibits γ ≈ 18 mJ mol^?1 K^?2, θ_D ≈ 330 K, a weak linear magnetoresistivity and an onset of superconductivity (for Ca₃Rh₈B₆, T_c = 4.0 K and H_c2 = 14.5 kOe, while for Sr₃Rh₈ B₆, T_c = 3.4 K and H_c2 ≈ 4.0 kOe). These remarkable differences are consistent with the findings of the electronic band structures and density of state (DOS) calculations. In particular, satisfactory agreement between the measured and calculated γ was obtained. Furthermore, the Fermi level, E_F, of Ca₃Rh₈B₆ lies at almost the top of a pronounced local DOS peak, while that of CaRh₂B₂ lies at a local valley: this is the main reason behind the differences between the, e.g., superconducting properties. Finally, although all atoms contribute to the DOS at E_F, the contribution of the Rh atoms is the strongest.     

Independent Control of Low-Energy Resonant States and Polaron States by the Zn-Doping and the Structural Transition in La2−x Sr x CuO4 and La2−x Ba x CuO4 (x=0.11)

The superconductivity depression mechanisms at x≈1/8 in La_2−x Ba _x CuO₄ and Zn substitution in La_2−x Sr _x CuO₄ were investigated by Raman scattering. About 80% of low-energy electronic states are two-dimensional at x≈1/8 and form the Fermi arc around (π/2,π/2). The low-energy states are composed of the resonant peak relating to the insulator–metal transition and the polaron states of the B _3u phonons. Zn substitution depresses the resonant peak, while the LTT structure depresses the polaron states. The superconductivity is suppressed if one of them is reduced.     

Effect of the ratio of Sr to Ba at A2 sites on the dielectric and ferroelectric properties of tetragonal tungsten bronze Ca0.4SrxBa0.6−xNb2O6 ceramics

Journal of Materials Science: Materials in Electronics - Tetragonal tungsten bronze Ca0.4SrxBa0.6?xNb2O6 (x?=?0, 0.1, 0.2, 0.3, 0.4, 0.5) ceramics were prepared by a conventional...     

μSR Study of Magnetic Order in LTO and LTT Phase of La2−x M xCu1−y Zn y O4 (M=Sr, Ba)

In order to investigate the origin of so-called 1/8 anomaly in La(Sr,Ba)-214, we have performed SR experiments for the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structure phases in La_2–x Sr _x Cu_1–y Zn _y O₄ (LSCO) and La_2–x Ba _x Cu_1–y Zn _y O₄ (LBCO), and decided the magnetic order temperature T _m where -spin coherent rotation starts. 1% Zn-substitution depresses magnetic order in the LTT phase of LBCO while it induces or enhances magnetic order in the LTO phase of LSCO. With doping Tm rises first and keeps constant around x=1/8 and decreases in both Zn-substituted LSCO and LBCO. This implies that doped holes are never uniformly distributed on the CuO₂ plane, suggesting the segregation of spin and hole around x=1/8.     

The relationship between the mean dopant-ion radii and conductivity of co-doped ZnO systems, Zn1−x−y M x M′ y O (M, M′ = Al, In, Ga, Y)

The conductivities of the Zn_1–x–y M _x M _y O (M, M = Al, In, Ga, Y) and Zn_1–x M _x O (M = Al, In, Ga) systems were measured from room temperature to 1173 K in order to elucidate a dominant parameter of the conducting mechanism. The conductivity at 873 K first increased with the dopant content. However, it showed a maximum value at a given dopant content, and then gradually decreased. For the samples with the same dopant content, their conductivity at 873 K was strongly dependent on the mean dopant-ion radii, and reached a maximum value at around 0.51 Å of the mean dopant-ion radii. The results suggested that the conductivity of the system would be influenced not only by the dopant content, but also by the mean dopant-ion radii. It was found that the co-doped ZnO system of Zn_0.995Al_0.003In_0.003O had a conductivity higher than that of the other usual mono-doped system.     

A review of the linear term in the low-temperature specific heat of La2CuO4 and La2?xMxCuO4 (M = Ba, Ca, Sr)

In this report I will summarize the results of an extensive review of the low-temperature specific heat of the La-based high-temperature superconductors, withT _c between 30 and 40 K, and the related nonsuperconducting compound La₂CuO₄. There have been two previous studies by Fisheret al. [1] and Fischeret al. [2] which also reviewed the low-temperature specific heat of these materials. This work was undertaken to extend a similar study of the 90 K superconductor YBa₂Cu₃O_7– which we recently completed [3]. This review will focus exclusively on the low-temperature specific heat and will update the previous studies by including recent results.     

Dielectric properties and crystal structure of Ba6−3x(Nd, M)8+2xTi18O54 (M = La, Bi, Y) microwave ceramics

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa₂O₃4TiO₂ and Ba_6-3x (Nd, M)d_8+2x Ti₁₈O₅₄ (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd_1-xBi_x)₂O₃4TiO₂ were _r = 89–92, Qf = 5855–6091 GHz, and _f = –7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd_1-xY_x)₂O₃4TiO₂ reduces the dielectric constant _r. Both the Y and Bi substitutions make _r positive. The relative dielectric constant _r and temperature coefficient _r are 109.5 and –180 ppm/°C in BaOLa₂O₃4TiO₂, 76 and +40 ppm/°C in BaO(Nd_0.77Y_0.23)₂O₃4TiO₂, respectively. The crystal structures were refined by Rietveld method using x-ray and neutron diffraction data. The most reliable results were obtained by refining the cation positions using the x-ray data and the oxygens from the neutron with a superlattice structure model Pnam(c-axis 7.6 Å). The refined structures show that the a/c ratios are related to the apical oxygen displacements of the Ti–O octahedra. The substitution of the small radius atom, Y, produced a structure of severely tilted and distorted Ti–O octahedra and large a/c ratio, while the large radius atom, La, small a/c ratio. Differential scanning calorimetry analysis showed heat anomaly indicating suspected phase transition in these materials. The relation between _r and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.     

Hydrothermal Crystallization in the Systems LiOH–MO2–GeO2–H2O (M = Ti, Sn) at 500°C and 0.1 GPa: Li2Ti[5]Ge[4]O5 and Li2Ge[6]Ge2 [4]O6 Phase Relations

Crystallization in the LiH–Ti₂–Ge₂–H₂O and LiOH–SnO₂–GeO₂–H₂O hydrothermal systems was studied at 500°C and 0.1 GPa. The phases crystallizing in the Ti system are Li₂Ti^[5]Ge^[4]O₅, Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂, Li₃HGe₄ ^[6]Ge₃ ^[4]O₁₆ · 4H₂O, and Li₂Ge^[4]O₃ (Ge in four- or sixfold oxygen coordination, and Ti in fivefold oxygen coordination). Over a wide range of TiO₂ : GeO₂ molar ratios (6 : 1 to 1 : 1), the dominant phase is Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂. At higher GeO₂ contents (TiO₂ : GeO₂ = 1 : 2 to 1 : 6), both Li₂Ti^[5]Ge^[4]O₅ and Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂ crystallize. The crystallization fields of Ge-containing phases at TiO₂ : GeO₂ = 1 : 2 to 1 : 6 are (in order of increasing LiOH concentration) GeO₂ (rutile structure), Li₂Ti^[5]Ge^[4]O₅ + Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂, and Li₂Ti^[5]Ge^[4]O₅ + Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂ + Li₂Ge^[4]₃ + Li₃HGe₄ ^[6]Ge₃ ^[4]O₁₆ · 4H₂O. The phases crystallizing in the Sn system are Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂, Li₃HGe₄ ^[6]Ge₃ ^[4]O₁₆ · 4H₂O, and Li₂Ge^[4]O₃. Tin is present in the form of SnO₂ only. The structures of Li₂Ti^[5]Ge^[4]O₅ and Li₂Ge^[6]Ge₂ ^[4]O₆(OH)₂ are characterized by the simplest type of matrix assembly: direct packing of substructural precursor units and their two-dimensional growth in various crystallographic directions.     

Effect of rare-earth (Eu, Yb and Ag) substitutions on superconducting properties of the Bi1.7Pb0.3 Sr2Ca2−xRx(R = Eu, Yb, and Ag)Cu3OY system

The superconducting properties of Bi^1.7Pb_0.3Sr₂Ca^2–xR^x(R = Eu, Yb and Ag)Cu₃O_Y have been investigated by X-ray diffraction, electrical resistance and the peritectic transition of these superconductors was studied kinetically under different atmospheres and temperature gradients. X-ray diffraction results show that the volume fraction of the high-T_c (2 2 2 3) phase decreases and that of the low-T_c (2 2 1 2) phase increases as Eu, Yb and Ag concentrations increase. The resistivity measurements reveal that the T_c onset decreases down to 100 K for Eu, 85 K for Yb and 106 K for Ag concentrations. These results are explained on the basis of possible variations of hole concentration with trivalent rare earth ion substitutions. Activation energies and frequency factors for crystallisation were determined by non-isothermal differential thermal analysis (DTA), employing different models. It was found that both peritectic transition and reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2 2 1 2 phase was greatly enhanced under oxygen atmosphere.     

Mössbauer spectroscopic studies of the crystallization of amorphous Fe80B20−x Si x (x=0, 2, and 8) alloys

The crystallization process of amorphous Fe₈₀B_20–x Si _x (x=0, 2, and 8) ferromagnetic alloys has been studied by using ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction studies. Results for samples heat treated at different temperatures for different times show that the crystallization of Fe₈₀B_20–x Si _x samples having x=0 and 2 leads to -Fe and t-Fe₃B, while for x=8, it leads to -Fe, t-Fe₂B, and perhaps Fe-Si. It is further observed that the addition of silicon to the Fe-B system improves the thermal stability of the system.     

Effect of the addition of Ca2+ on the structure, microstructure and ferroelectric properties of (Pb1 − y Ca y )(Zr0.8Ti0.2)O3 system with y = 0.0, 0.02, 0.05 and 0.1

This work studies the relationship between ferroelectric properties and structural features of the (Pb_{1 – y} Ca _y )(Zr_0.8Ti_0.2)O₃ system with y = 0.0, 0.02, 0.05 and 0.1. All of the samples were prepared by hot-pressing of PZT calcined powder, prepared by conventional ceramic route. Single phase compositions of rhombohedral (RH) structure were observed for the whole series of the samples studied in this work. However, a linear decrease of RH lattice constant a _p is seen with increasing the Ca²⁺ concentration with a subsequent decrease of lattice strain expressed by (90 – _p). Further, a sharp decline is also observed in the magnitudes of saturation polarization (P _s) of the samples doped with 2 mol% Ca²⁺. However, the rate of decrease slows down for Ca²⁺ concentrations above this level. A diffuse phase transition was observed in the case of 10 mol% Ca²⁺-doped PZT(0.8) which is believed to be possibly due to the distribution in coupling strengths created from a random compositional fluctuations of Ca²⁺ and Pb^2– ions on A-sites. The results are interpreted on crystal-chemical grounds. It is proposed that the long range coupling of (Zr/Ti)O₆ coordination octahedra and Pb²⁺ ions, due to the covalency of the Pb–O bond, is interrupted by the incorporation of Ca²⁺. By considering the effect of calcium on coupling strength and coupling distribution, the observed dielectric response for the above compositions is qualitatively explained.