B. A. Gizhevskii

Red Shift of Absorption Edge and Nonmetal–Metal Transition in Single Crystals La1—xSrxMnO3 (x = 0.1, 0.2, 0.3)

The red shift of the fundamental absorption edge (mo.16 eV) of a La 0.9 Sr 0.1 MnO 3 single crystal occurs as the crystal is cooled into the ferromagnetic phase. This result shows that there exist spin-dependent states. The reflection and absorption spectra of La 1-x Sr x MnO 3 single crystals (x = 0.1, 0.2, 0.3) indicate that the red shift may be the reason for the appearance of free charge carriers participating in the colossal magnetoresistance phenomenon. The nonmetal-metal type transition below T C confirms this.

Nanocrystalline copper oxide for selective solar energy absorbers

The high-energy edge of the transparency window in nanocrystalline copper monoxide (CuO) can be shifted to 0.6 eV (against 1.45 eV in single crystals). This shift is related to a modification of the shape of the fundamental absorption edge of CuO without any change in the refractive index. The obtained nanocrystalline CuO is recommended for use as a selective solar energy absorber.

Electronic and structural properties of micro-and nanometre-sized crystalline copper monoxide ceramics investigated by positron annihilation

Electronic and structural properties of copper monoxide (CuO) sintered as a common ceramic and nanoceramic are studied by positron annihilation spectroscopy. A CuO nanoceramic with crystallite size ranging from 15 to 90 nm was prepared from a common one by shock-wave loading. It is found that the momentum distribution of valence electrons in CuO is shifted, as compared with metallic copper, towards higher momentum values. This result is related to the effect of the Cu 3d–O 2p hybridization in the Cu–O ionic covalent bond formation. It is found that open volumes, identified mainly as small agglomerates of oxygen vacancies, appear at the nanoceramic crystallite interfaces. The degree of the Cu–O bond covalency decreases locally at the crystallite interfaces because of an oxygen deficit. The nanocrystalline state in CuO is shown to be thermally stable up to 700 K.

Oxygen Isotope Exchange between Gaseous Phase Enriched with 18O Isotope and Nanocrystal Oxides LaMnO3+δ Obtained by Severe Plastic Deformation

The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have been evaluated at 500 °C. They are equal to 3.5·10 −20 and 1.5·10 −13 cm2/s, respectively.

Optical spectra of nanoceramics of yttrium-iron garnet Y3Fe5O12 obtained by the method of intense plastic deformation

The features of the optical properties of nanostructured samples of iron-yttrium garnet Y3Fe5O12 (YIG) in the range of 0.5 to 4.7 eV, which includes both the fundamental-absorption region and low-energy electron excitations, have been investigated by spectroscopic ellipsometry. The results are discussed in comparison with the measurement data of a YIG single crystal. The dispersion of the optical functions in nanostructured samples is significantly different from that for the single crystal: the spectral density is redistributed from the energy region above the fundamental absorption edge to the region below the edge. It has been shown that the energy positions of the main electronic transitions in nanostructured samples are on the whole the same as in the single crystal; at the same time, the intensity of low-energy transitions increases. The possible causes of this increase and the resolution of the fine absorption structure in the bandgap of nanostructured Y3Fe5O12 are discussed.

Grain boundary self-diffusion of tracer 18O atoms in nanocrystalline oxide LaMnO3 + δ

Diffusion coefficients of tracer 18O atoms at boundaries of nanograins of LaMnO3 + δ oxide have been measured in the temperature range of 400–500°C. The samples of the nanocrystalline oxide are prepared with the use of the shockwave loading method. The concentration profile of the tracer atoms after diffusion annealing is measured with the use of the nuclear microanalysis method. The activation energies of the grain boundary diffusion amounts to about 2 eV and the boundary width is ∼0.05 nm. The measured coefficients of the grain boundary diffusion at 500°C exceed the corresponding coefficients of the volume diffusion by seven orders of magnitude.

Phase transformations in CuO caused by bombardment by He+ ions and by the action of spherical shock waves

The valence state of copper ions and the phase composition of copper monoxide CuO subjected to bombardment by He+ ions and explosive shock waves are studied by the methods of x-ray photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES). Measurements of photoelectron Cu 2p and emission O Kα spectra revealed a decrease in the concentration of Cu2+ ions and partial reduction of CuO to Cu2O as a result of both ion bombardment and shock-wave loading. The concentration of the Cu2O phase attained values of 10–15%. The Cu2O phase is revealed by the XPS and XES methods even at concentrations lower than its threshold concentration for detection by x-ray diffraction measurements. This points to the effectiveness of XPS and XES techniques in studying nanocrystalline materials and defect structures containing finely dispersed inclusions. A model for the emergence of Cu2O due to the formation of charged clusters under the action of stress waves is proposed.

Charge states of cobalt ions in nanostructured lithium cobaltite: X-ray absorption and photoelectron spectra

The charge states of ions in nanostructured lithium cobaltite prepared by severe plastic deformation under pressure have been determined using X-ray absorption spectroscopy and photoelectron spectroscopy, as well as calculations of the atomic multiplets with allowance for the charge transfer. It has been found that small deformations (pressures up to 5 GPa and angles of anvil rotation up to 30°) lead to the generation of lithium vacancies in the bulk of the nanostructured material and the formation of the Li2O phase on the surface. The charge compensation occurs at the expense of holes in oxygen 2p states; the electronic configuration of cobalt ions is 3d6L, where L is a hole in oxygen 2p states. It has been shown that nanostructured lithium cobaltite belongs to the class of insulators with a negative charge transfer energy. An increase in the degree of deformation of lithium cobaltite (at a pressure up to 8 GPa) leads to the formation of Co2+ ions (with the electronic configuration 3d7).

Charge state of manganese ions and the nonstoichiometry of Ca1 − xLayMnO3 − δ single crystals

The charge states and effective spins of manganese ions in nonstoichiometric single crystals of Ca1 − xLayMnO3 − δ (x = 0, 0.05, 1) manganites have been determined from the exchange splitting of Mn 3s X-ray photoelectron spectra. The chemical composition of the manganite samples under investigation, including the concentration of oxygen, has been determined using the data of the X-ray microanalysis of cations. It has been found that the manganite single crystals under investigation have large defectiveness in both the cation and oxygen sublattices. The efficiency of the complex use of 3s photoelectron spectroscopy and X-ray microanalysis in the determination of the charge state of manganese ions and violations of stoichiometry in doped and defected manganites has been demonstrated.

Application of X-ray absorption spectroscopy to the investigation of charge states of iron ions in iron borate nanoceramics

The method of X-ray absorption spectroscopy has been used for the investigation of charge states of iron ions in iron borate nanoceramics prepared by shear deformation under pressure. The experimental Fe 2p X-ray absorption spectra have been presented in comparison with the calculation of atomic multiplets of iron ions taking into account the charge transfer from the 2p orbitals of oxygen to the 3d orbitals of iron and the crystal-field splitting of the 3d orbitals of iron. Our results indicate that, in addition to iron ions in the ground charge state Fe3+, nanostructured FeBO3 contains a few percent of Fe2+ ions. It has been found that an increase in the degree of plastic deformation (the rotation angle of the anvils) leads to a decrease in the size of crystallites and to an increase in the concentration of Fe2+ ions without the formation of new phases. The results of this work agree with the magnetic and optical measurements and confirm high defectness of FeBO3 nanoceramics.