I. I. Zver’kova

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB2) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB2 with Tc=5.5 K, while NbB2 and TaB2 have been observed to be nonsuperconducting up to 0.37K. Hc2(T) is linear in temperature below Tc, leading to a rather low Hc2(0)=0.1 T. At T close to Tc, Hc2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB2 samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB2, NbB2, and MgB2, respectively, rather than T2 and T3, as previously reported for MgB2.

Nanocrystallization of an amorphous Fe80B20 alloy during severe plastic deformation

The structural evolution of an amorphous Fe80B20 alloy subjected to severe plastic deformation at room temperature or at 200°C was studied. Deformation leads to the formation of α-Fe nanocrystals in an amorphous phase. After room-temperature deformation, nanocrystals are localized in shear bands. After deformation at 200°C, the nanocrystal distribution over the alloy is more uniform. Possible causes of the crystallization of the amorphous phase during severe plastic deformation are discussed.

Phase segregation and crystallization in the amorphous alloy Ni70Mo10P20

Structural evolution of the amorphous alloy Ni70Mo10P20 has been studied by x-ray diffraction, and by following transmission and high-resolution electron microscopy annealing both above and below the glass-transition temperature. When annealed above this temperature, the amorphous phase undergoes segregation into regions about 100 nm in size having different chemical composition. Diffraction from such samples produces diffuse rings, and the scattering vector corresponding to the maximum intensity varies from point to point within the interval of 4.88 to 4.78 nm−1. When occurring between the glass-transition and crystallization temperatures, crystallization produces groups of nanocrystals, 20–30 nm in size, which are in direct contact with one another and form a polymorphic mechanism. The crystallization mechanism changes when the annealing temperature is brought below the glass-transition point. At these temperatures the amorphous matrix crystallizes entectically with formation of eutectic colonies.

Synthesis of ruthenium-based layered cuprates in ammonium nitrate melt and their characterisation

Synthesis in the ammonium nitrate melt was used to obtain polycrystalline samples of complex copper–ruthenium oxides RuSr2Ln1þxCe1� xCu2O10� y (Ru-1222) and RuSr2LnCu2O8 (Ru-1212) (Ln ¼ Sm, Gd and Eu), as well as Ru1� xAxSr2GdCu2O8 (A ¼ Pd, Os, Rh; x ¼ 0:2, 0.4). Magnetic and superconducting properties of these samples were studied. The superconducting transition temperature in the RuSr2Ln1:5Ce0:5Cu2O10� y compound was found to be higher compared with the samples obtained using the solid state synthesis. At the same time, the new synthesis process has a much shorter duration. RuSr2GdCu2O8 compound is also produced faster in the ammonium nitrate melt. Doping with Os, Rh, Pd expedites the development of superconductivity in these samples. 2002 Elsevier Science B.V. All rights reserved.

Synthesis of α-SiC nanocrystals by carbothermal reduction of spherical nanoparticles of amorphous silicon dioxide

The method for carbothermal reduction of spherical particles of amorphous silicon dioxide is developed, and hexagonal α-SiC polytype nanocrystals are synthesized. The prepared samples are characterized by X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, and electron microscopy. The silicon carbide nanocrystals prepared have sizes in the range 5–50 nm depending on the diameter of initial silicon dioxide particles. A detailed analysis of the positions of the lines in the Raman spectra, their broadening, and shift makes it possible to reliably establish that the samples under investigation predominantly contain the 6H and 4H silicon carbide polytypes and insignificant amounts of the 2H and 3C phases. The 15R and 21R polytypes in the samples are absent. It is noted that the samples are characterized by a substantial size effect: the luminescence intensity of small silicon carbide nanocrystals is more than three times higher than that of large SiC nanocrystals.

Electron transport, penetration depth, and the upper critical magnetic field in ZrB12 and MgB2

We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field Hc2, for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature TR = 280 K (to be compared to TR = 900 K for MgB2). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB12 at Tc = 6.0 K. Although a clear exponential λ(T) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate an s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. In disagreement with conventional theories, we found a linear temperature dependence, of Hc2(T) for ZrB12 (Hc2(0) = 0.15 T).

Mössbauer and X-Ray studies of the dynamics of phase transformations and suppression of polymorphism in the LaMn1 − xFexO3 + δ compound (x = 0.015–0.500)

The effect of iron impurity on the structural transformations and the suppression of polymorphism of the LaMn1 − xFexO3 + δ compound (x = 0.015–0.500) was studied by X-ray diffraction and Mössbauer spectroscopy. It is shown that the iron impurity greatly influences the polymorphic transformations of the manganite. At low Fe impurity contents (up to 10 at %), the PnmaI phase is locally formed in the PnmaII phase. The mechanisms of suppression of the polymorphism are discussed.

Evolution of the spectral characteristics upon annealing of lithium borate glasses containing europium and aluminum

The spectral and structural characteristics of lithium borate glasses containing europium and aluminum have been investigated upon annealing at different temperatures. It has been found that the spectral characteristics of the studied system change nonmonotonically with an increase in the annealing temperature. After annealing at a temperature of 600°C, the luminescence spectra of the glasses exhibit broad structureless bands that are specific for the amorphous phase containing Eu3+ ions. Then, after annealing at T = 700°C, narrow lines appear in the wavelength ranges 585–595 and 610–620 nm, which correspond to the luminescence of the Eu(BO2)3 and EuAl3(BO3)4 borates. A further increase in the annealing temperature (T = 800–900°C) leads to the disappearance of europium aluminum borate. In the luminescence spectra of these samples, there are narrow bands in the wavelength range λ = 585–595 nm, which are typical of europium metaborate. Finally, at a temperature of 1050°C, these bands disappear and narrow lines appear again in the wavelength range 610–620 nm, which are characteristic of the EuAl3(BO3)4 borate. Thus, the temperature annealing makes it possible to purposely change the spectral characteristics of the studied system in the wavelength range 590–615 nm.

Synthesis of a periodic SiC/C nanostructure

Highly porous periodic structures consisting of a three-dimensional replica of pores in the initial opal lattice have been synthesized by high-temperature thermochemical treatment of opal matrices filled with carbon compounds, followed by dissolution of silicon dioxide. It has been shown that the main phases of the composite are carbon and silicon carbide. Based on the X-ray diffraction, Raman, and IR spectroscopy data, it has been assumed that the composite contains fragments of hexagonal diamond. The photoluminescence and optical reflection spectra of the composites have been measured.

Photoluminescence properties of core-shell SiO2/Lu2O3: Eu monodisperse heteronanoparticles

Core-shell monodisperse heteroparticles of the composition SiO2/Lu1.86Eu0.14O3 have been synthesized using the developed technique for preparing spherical colloidal silicon dioxide particles with the size dispersion in the range 2.0–2.5% and the procedure for producing nanocoatings on the surface of spheres by codeposition. The structure of heteroparticles has been investigated, their excitation and photoluminescence spectra have been analyzed, and the lifetime of the 5D0 excited state of Eu3+ ions has been examined. It has been revealed that the luminescence decay time for heteroparticles increases by a factor of approximately two compared to that for a powdered luminophor Lu2O3: Eu (7 at %) prepared and treated under the same temperature conditions as the SiO2/Lu2O3: Eu (7 at %) heteroparticles. This effect has been attributed to the change in the effective refractive index and the local density of photon states in luminophor nanolayers of heteroparticles.