Features of magnetic properties of La x MnO3 + δ (0.815 ≤ x ≤ 1.0)

Magnetic and resonance studies of the system of polycrystalline samples of self-doped manganites La _x MnO_{3 + δ} (x = 0.815, 0.90, 0.94, 0.97, and 1.0) have been performed in a temperature range of 77–300 K. According to ⁵⁵Mn NMR data, all samples contain a ferromagnetic phase at 77 K. As the defect density increases (x changes from 1.0 to 0.815), samples become more magnetically ordered. In this case, the ferromagnetic state of the system gradually changes from a mixed state in which both ferromagnetic insulating (basic) and ferromagnetic metal (for x = 0.97 and 1.0) phases coexist to only the ferromagnetic metal state (for x = 0.815 and 0.90). It has been shown that both ferromagnetic metal and ferromagnetic insulating phases are inhomogeneous, and either phase consists of two phases with different dynamics of nuclear spins and different Curie temperatures. The diagram of the magnetic phase state of the La _x MnO_{3 + δ} system (x = 0.815, 0.90, 0.94, 0.97, 1.0) has been constructed for a temperature range of 120–240 K and Mn⁴⁺ contents of 12–30%.     

Luminescence properties of Sr3−x−3y/2MxCeyAlO4F (M=Ca,Ba, 0≤x≤0.9, 0.001≤y≤0.05) phosphors

Luminescent materials composed of Sr_3?x?3y/2M_xCe_yAlO₄F (M=Ca, Ba, 0≤x≤0.9, 0.001≤y≤0.05) were prepared by the solid-state reaction method. X-ray diffraction (XRD) patterns of the obtained oxyfluorides are exhibited for indexing peak positions. Dynamic excitation and emission spectra of the Ce³⁺-activated oxyfluoride phosphors are clearly monitored. The critical emission quenching as a function of Ce³⁺ contents in Sr_2.5?3y/2M_0.5Ce_yAlO₄F phosphors is revealed at quite low concentrations of the activator. CIE coordinates of blue and green Sr_2.5?3y/2M_0.5Ce_yAlO₄F phosphors are clearly measured. The relative quantum efficiency of Sr_2.4985Ca_0.5Ce_0.005AlO₄F based on the integrated emission is determined. The Sr_3?x?3y/2M_xCe_yAlO₄F phosphors excited near 410 nm light could be prominent phosphors in applications of NUV-LED.     

Synthesis and native defectivity of Zn1−x V x O (0 ≤ x ≤ 0.03) photocatalysts

Nanodisperse solid solutions Zn_1?x V _x O (0 ≤ x ≤ 0.03) with high numbers of defects in an oxygen sublattice are synthesized via the precursor technique. ESR analysis reveals that V _O ⁺ oxygen vacancies are the main defects of the oxygen sublattice in the Zn_1?x V _x O structure. The Zn_1?x V _x O (0 < x ≤ 0.15) solid solutions exhibit high photocatalytic activity during hydroquinone oxidation in water upon irradiation with UV and visible light.     

The influence of praseodymium concentration on the luminescence properties of Ca1 ? x Pr x F2 + x (0.002 ≤ x ≤0.35)

The possibility of implementing photon cascade emission in CaF₂-PrF₃ crystals by increasing the PrF₃ concentration in the CaF₂ matrix is studied. It is shown that an increase in the Pr³⁺ content leads to redistribution of radiative transitions in favor of the ¹ S ₀-luminescence. The decrease in the efficiency of the second step of photon cascade emission at high PrF₃ concentrations is caused by quenching of the ³ P ₀ ?? 4f ² transitions as a result of cross relaxation.     

Structural deformations in metastable cubic compounds Ni(1 − x)Zn x O (0.60 ≤ x ≤ 0.99)

X-ray diffraction has been used for studying the fine crystal structure of metastable cubic oxide compounds Ni_{(1 ? x)}Zn _x O (0.60 ≤ x ≤ 0.99) obtained from the initial hexagonal phase by quenching of the samples at a high temperature and under external hydrostatic pressure. It has been found that the diffraction patterns of these compounds include a system of diffuse superstructure maxima, whose number and intensity essentially depend on the composition. The origin of this superstructure has been discussed.     

Ellipsometry study of interband transitions in TlGaS2xSe2(1−x) mixed crystals (0≤x≤1)

In this paper, the spectroscopic ellipsometry measurements on TlGaS_2xSe_2(1?x) mixed crystals (0≤x≤1) were carried out on the layer-plane (001) surfaces with light polarization E⊥c^? in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the dielectric function, refractive index and extinction coefficient were calculated from ellipsometric data using the ambient-substrate optical model. The critical point energies in the above-band gap energy range have been obtained from the second derivative spectra of the dielectric function. Particularly for TlGaSe₂ crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure. The effect of the isomorphic anion substitution (sulfur for selenium) on critical point energies in TlGaS_2xSe_2(1?x) mixed crystals was established.     

Pre-exponential factor for non-isothermal crystallization of glassy Se85− x Te15Sb x (0 ≤ x ≤ 10) alloys

We report observations of the Meyer–Neldel rule for the non-isothermal crystallization of glassy Se_{85? x} Te₁₅Sb _x (x =?0, 2, 4, 6, 8, 10) alloys. We found a strong co-relation between the pre-exponential factor K ₀ of the rate constant K(T) for crystallization and the activation energy of crystallization E _c. This indicates the presence of a compensation effect for the non-isothermal crystallization process in this glassy system. The composition dependence of the crystallization temperature T _c and the activation energy for crystallization E _c is discussed.     

Fine structure and magnetism of bulk Zn1 − x Cr x Se single-crystal cubic compounds (0 ≤ x ≤ 0.045)

The character of structural and magnetic features of the cubic lattice of bulk Zn_{1 ? x} Cr _x Se crystals (0 ≤ x ≤ 0.045) has been investigated using thermal neutron diffraction and magnetic measurements. It has been found that the diffraction scans of doped crystals contain effects of nuclear diffuse scattering caused by local static atomic displacements in the face-centered cubic (fcc) lattice. Results of magnetic measurements of doped crystals indicate the presence of weak antiferromagnetic correlations, which are a consequence of structural features of these compounds.     

Effect of Nd on the Superconductivity of Nd1+x Ba2-x Cu3O y with 0.1 ≤ x ≤ 0.35

The effect of Nd substitution at the Ba sites has been studied in Nd_1+x Ba_2-x Cu₃O _y system for 0.1 x 0.35. Orthorhombic to tetragonal structural phase transition occurs at about x = 0.17. With the increase of Nd, the T _c value decreases and T increases, where T is the difference between the onset and critical temperatures and indicates the sharpness of the transition. These changes are explained by ac-susceptibility measurements on Nd_1.1Ba_1.9Cu₃O_7.12 at 0.10 Oe. The susceptibility curves show the presence of about four phases, which are not detectable by X-ray diffraction technique.     

Transport properties of the single-crystal La2−xBaxCuO4 (0≤x≤0.11)

We report on the temperature dependence of the in-plane electrical resistivity ρ_ab and the in-plane Hall coefficient R_H in various magnetic fields of the single-crystal La_2−xBa_xCuO₄ with x=0.083 and 0.11. In x=0.11, which is close to x=1/8, where the superconductivity is strongly suppressed, a clear jump in ρ_ab and a drop in R_H have been observed at T_d2∼53 K, where the structural phase transition between the orthorhombic mid-temperature and tetragonal low-temperature phases occurs. Moreover, a sign reversal of R_H has been observed below ∼25 K and the magnitude of the sign reversal increases with increasing magnetic field. In x=0.083, on the other hand, there is neither jump in ρ_ab nor drop in R_H at T_d2, and also no sign reversal in R_H at low temperatures even in magnetic fields up to 9 T. In conclusion, there is no doubt that a static stripe order of holes and spins, observed in La_1.6−xNd_0.4Sr_xCuO₄ with x∼1/8, is formed below T_d2 also in La_2−xBa_xCuO₄ with x∼1/8. The R_H in the stripe-ordered state has a negative value, which is consistent with the recent theory by Prelovšek et al.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

K-doping effect of the superconductivity in K2xFeTe1-xSx (0.07≤x≤0.3)

Bulk samples of K doping K_2xFeTe_1-xS_x with x = 0.07, 0.1, 0.2, 0.3 are successfully prepared by using easy-to-use stable compound K₂S as the reactant. The lattice constant calculated from X-ray diffraction patterns indicate that K ions enter the Fe-Te-S layers. K doping is beneficial enhance the superconductivity transition temperature from the R-T curves. The apparent diamagnetic signal is observed in M-T curves when the content of K is smaller than 0.1. However, differential curves (dM/dT) in K-rich samples appear sharp slope mutations, which means that the Meissner effect signal is covered by the increased excess ferromagnetic ions. The number of excess Fe magnetic ions is proportional to K content, which may play an important role in determining the superconductivity.     

High temperature thermoelectric properties of Ca1−xBixMn1−yV yO3−δ (0≤x=y≤0.08)

CaMnO_3?δ with complex additives Bi₂O₃–V ₂O₅ were prepared by the solid-state reaction. The crystal structures of the Ca_1?xBi_xMn_1?yV _yO_3?δ (0≤x=y≤0.08) solid solutions were determined by means of the powder X-ray diffraction (XRD) using Rietan 2000 program and the high temperature thermoelectric properties were also investigated. Perovskite-type Ca_1?xBi_xMn_1?yV _yO_3?δ solid solutions are n-type semiconductors. The lattice parameters increase with increasing dopant level. The high temperature thermoelectric properties are improved due to Bi₂O₃–V ₂O₅ simultaneous doping. A maximum ZT value reaches to 0.21 for electron-doped Ca_0.96Bi_0.04Mn_0.96V _0.04O_3?δ at 1050 K, which is about twice as high as that of CaMnO_3?δ. The thermal shock resistance at temperatures between 20 and 450 ^°C is also highly improved.     

Effect of Nd doping on the magnetic properties of charge-ordered Bi0.6−x Nd x Ca0.4MnO3 (0.0≤x≤0.6) perovskite manganites

We have studied structure, magnetic and transport properties of polycrystalline Bi_0.6?x Nd _x Ca_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6). Substitution of Nd at Bi sites induces a strong interplay between the magnetic and charge ordering. The charge-ordering temperature (T _CO) decreases with increasing x. Further, the antiferromagnetic ordering temperature (T _N) increases sharply at both extremes and remains nearly constant for x=0.2–0.4. At T<T _N a transition to a metamagnetic glass-like state is also seen. Nd doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.3 and then an increase in resistivity up to x=0.5. Furthermore, Nd doping promotes an antiferromagnetic to ferromagnetic type fluctuation in the materials at room temperature, as evidenced by the change in the value of the paramagnetic Curie temperature. We find that the local lattice distortion induced by the size mismatch between the A-site cations and the 6s² character of Bi³⁺ lone pair electrons explains the observed peculiarity in magnetic and transport properties of Nd-doped Bi_0.6Ca_0.4MnO₃.     

Superconductivity,crystallization, and structural relaxation of the new metallic glass system La1−x Si x (0.15≤x≤0.27)

New metallic glass alloys have been prepared by melt spinning of La–Si samples in a pumped system. The superconducting transition temperature,T _c , increases linearly with the La concentration, from 3.00 K at 73 at. % La to 3.80 K at 85 at. %. Three new metastable phases: -, -, and -La₃Si, were formed by annealing and crystallizing amorphous La₃Si. Their crystal structures are orthorhombic (a=6.32 Å,b=8.06 Å,c=9.96 Å), hcp (a=10.55 Å,c=5.05 Å), and tetragonal (a=6.92 Å,c=5.05 Å) resp.T _c increased to 3.75 K, 6.00 K and 6.80 K, resp. During low temperature anneals of an amorphous La₃Si alloy,T _c changed logarithmically with time.On leave from Institute of Physics, Academica Sinica, Beijing, China     

Spin-reorientation, ferroelectricity, and magnetodielectric effect in YFe(1-x)Mn(x)O3(0.1 ≤ x ≤ 0.40)

We report the observation of magnetoelectric and magnetodielectric effects at different temperatures in Mn-substituted yttrium orthoferrite, YFe(1-x)Mn(x)O(3)(0.1 ≤ x ≤ 0.40). Substitution of Mn in antiferromagnetic YFeO(3)(T(N) = 640 K) induces a first-order spin-reorientation transition at a temperature, T(SR), which increases with x whereas the Néel temperature (T(N)) decreases. While the magnetodielectric effect occurs at T(SR) and T(N), the ferroelectricity appears rather at low temperatures. The origin of magnetodielectric effect is attributed to spin-phonon coupling as evidenced from the temperature dependence of Raman phonon modes. The large magnetocapacitance (18% at 50 kOe) near T(SR) = 320 K and high ferroelectric transition temperature (～115 K) observed for x = 0.4 suggest routes to enhance magnetoelectric effect near room temperature for practical applications.     

Optical properties of polyvinyl alcohol doped (Se80Te20)100–xAgx (0 ≤ x ≤ 4) composites

Polyvinyl alcohol (PVA) doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films were prepared by the spin-coating technique on a quartz substrate. The optical parameters of PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) composites at the same chalcogen concentration (S₀ = 0.1 mg ml^?1) and PVA/(Se₈₀Te₂₀)₉₆Ag₄ composites at three different chalcogen concentrations viz. S₁ = 0.3 mg ml^?1, S₂ = 0.6 mg ml^?1 and S₃ = 1 mg ml^?1 have been studied. The semi-crystalline nature of the as-deposited thin filmsisdetermined by X-ray diffraction. The transmission and reflection spectra of PVA-doped Se–Te–Ag thin films were obtained in a 350–650 nm spectral region. The optical-band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical-band gap increases, but the refractive index, extinction coefficient, and the real and imaginary parts of the dielectric constant decrease, with increase in Agcontent in PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films. Such type of behavior is explained on the basis of decrease in density of the defect states. However, the optical-band gap has been found to be decreased and all other optical parameters show increase in their values with increase in concentration of (Se₈₀Te₂₀)₉₆Ag₄ glass in PVA-doped composites. The results have been explained on the basis of cluster-size formation at the time of dissolution. This study shows that the optical properties of new composites are affected by the change in silver and chalcogen concentration.     

Preparation and characterization of Al and La co-doped (Ce1−x−y Al x LayO2-(x+y)/2) ceria

Effect of co-doping of lanthanum and aluminum on ionic conductivity of CeO₂ for its use as a solid electrolyte in intermediate temperature solid oxide fuel cells has been studied. A few compositions in the system Ce_1???x???y Al _x La_yO_{2???(x?+?y)/2} (CAL) with x?=?0.05, 0.025, and 0.00 and y?=?0.00, 0.025, and 0.05 such that x?+?y?=?0.05 are prepared by auto-combustion method. X-ray diffraction patterns show that all the synthesized samples are ceria-based solid solutions. Microstructures of thermally etched samples have been studied by scanning electron microscope. To determine the contribution of grains, σ _g and grain boundaries, σ _gb to the total conductivity, σ _t impedance is measured in the frequency range 1–1 MHz at different steady temperatures in the range 250–500 °C. It has been found that conductivity of the co-doped composition is more than singly doped with Al but less than singly doped lanthanum composition.