Efficiency of Si L 2,3 x-ray generation in the SiO2/Si system by electron impact

The paper reports a study of the depth profile of the generation efficiency and escape of the ultrasoft silicon L _2,3 x-ray radiation excited by electrons of various energies. The generation function describing the excitation efficiency is the kernel of an integral equation determining the dependence of x-ray emission intensity on primary-electron energy. To determine the form of this function, a study was made of the dependence of the Si L _2,3 x-ray spectral intensity and of its silicon L _2,3 component bands, from crystalline silicon and amorphous dioxide SiO₂, on primary-electron energy in samples made from dioxide layers of various thicknesses grown on crystalline silicon. These experiments permitted investigation of the generation-function cross sections at the depth of the Si-SiO₂ interface. The theoretical simulation of the generation function made use of the simplest laws governing electron interaction with solids and of the cross section of the inner-level ionization by electron impact in its most general form. A comparison of the experimentally obtained relative contributions of the Si and SiO₂ emissions with the calculations shows them to be in good agreement up to primary-electron energies of 2–3 keV. Fiz. Tverd. Tela (St. Petersburg) 40, 1932–1936 (October 1998)     

Electronic structure of semiconductor solutions of cadmium chalcogenides

The electronic structure of the semiconductor solid solutions CdS_xSe_1−x x=(0, 0.17, 0.33, 0.5, 0.67, 0.83, 1.0) is investigated. The experimental x-ray and x-ray-induced electronic spectra of sulfur in CdS are compared with computed spectra. The calculations are performed using the FEFF (Version 7) program and a package of programs that implements a cluster version of the local-coherent-potential approximation. The effect of an electron vacancy in the 1s level on the x-ray absorption spectrum is investigated. The band gap as a function of the anion concentration in the solution is estimated. Fiz. Tverd. Tela (St. Petersburg) 41, 41–43 (January 1999)     

Observation of standing x-rays in Bragg diffraction on high-T c superconducting crystals Nd1.85Ce0.15CuO4−δ

Curves of the secondary-fluorescence yield from the constituent elements of high-T _c superconducting crystals Nd_1.85Ce_0.15CuO_4−δ are measured in the dynamic x-ray diffraction regime. The form of the curves attests to the appearance of a standing x-ray wave in the crystal. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 703–706 (10 May 1997)     

High resolution x-ray diffractometry of the structural characteristics of a semiconducting (InGa)As/GaAs superlattice

A statistical approach is used to construct a kinematic theory of x-ray diffraction on a semiconducting superlattice with a two layer period. This theory takes two types of structural deformations into account: crystal lattice defects caused by microdefects distributed chaotically over the thickness of the superlattice, and periodicity defects of an additional superlattice potential owing to random deviations in the thicknesses of the layers of its period from specified values. Numerical simulation is used to illustrate the effect of structural defects on the development of the diffraction reflection curve. The theory is used to analyze experimental x-ray diffraction spectra of semiconducting In_xGa_1−x As/GaAs superlattices. Zh. Tekh. Fiz. 69, 44–53 (February 1999)     

X-ray spectral analysis of the degradation of Au/Si interphase boundaries in air

The dependence on the energy of primary electrons of the form of the ultrasoft x-ray Si L _2,3 and Au N _6,7 emission bands is studied in Au/Si structures interacting with the oxygen in air. A method for quantitatively estimating the phase chemical composition and its depth distribution was used, based on an analysis of the shape of the bands and employing the most general integral form for the dependence of the intensity of the x-ray spectra on the energy of the primary electrons. Oxidation was found to cause diffusion of silicon atoms to the surface and produce a fundamental change in the phase composition of the structures that were studied. A characteristic feature of the structure of all the samples was the appearance on their surface of a relatively thick layer of SiO₂, whose thickness depends on the initial phase composition of the Au/Si interface. Fiz. Tverd. Tela (St. Petersburg) 39, 2101–2105 (November 1997)     

Anharmonicity and superconductivity in Ba0.6K0.4BiO3

The temperature dependence of the x-ray absorption spectra above the L ₃ absorption edge of bismuth in the superconducting oxide Ba_0.6K_0.4BiO₃ are investigated. It is found that the local structure is different from the simple cubic structure indicated by x-ray and neutron-diffraction data. It is shown that the oxygen atoms move in an anharmonic double-well potential arising as a result of the existence of two nonequivalent types of octahedral environments of bismuth. Vibrations in such potential modulate the Bi-O bond lengths at the low frequency of the rotational (“tilting” type) mode of the oxygen octahedra and thus give rise to a strong electron-phonon interaction, which explains the quite high superconducting transition temperatures T _c ∼30 K. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 977–982 (25 June 1998)     

Titanium K-edge absorption structure in Ti1−x NbxO2

The fine structure in the titanium x-ray K-edge absorption has been measured in Ti_1−x Nb_xO₂ mixed dioxides (x=0–0.1) with rutile structure in a laboratory-type spectrometer by total electron quantum-yield measurement. The position of the XANES lines is shown to be in good agreement with classical x-ray absorption spectra obtained in transmission. The structure and main features of the XANES spectra, including the effects of impurities and manyelectron excitations, are discussed. It is suggested that the intensity of the B peak characteristic of the titanium K edge depends on the Nb concentration and correlates with the charge state of titanium ions. Fiz. Tverd. Tela (St. Petersburg) 41, 894–896 (May 1999)     

Interaction of high-power x rays with sapphire crystals and quartz based materials

The interaction of high-power (10¹¹ W/cm²) soft (1–3 keV) x-rays with inorganic oxides (Al₂O₃ and SiO₂) is studied. It is found that when the wavelength of the x rays is comparable to the lattice constant of the crystal, besides generation of a high concentration of hot electrons and holes, there is broadening of the 2 p O²⁻-subband in the upper valence band owing to the local action of the strong x-ray field on regular oxygen sites. As a consequence, depending on the intensity of the x-ray pulses, a broadening of the fast (<1 ns) core-valence x-ray luminescence spectra is observed. Fiz. Tverd. Tela (St. Petersburg) 39, 286–289 (February 1997)     

Influence of crystal dimensions on interatomic distances in dispersed carbon

The relationship between the interatomic distances and crystal dimensions in dispersed carbon is studied by x-ray structure analysis and by performing model calculations. It is established that the interatomic distances in dispersed carbon are determined by the dimensions of the crystals along the crystallographic a axis (L _a). Small crystal dimensions dictate smaller interatomic distances than in graphite; an increase in the crystal dimensions leads to a corresponding increase in this parameter. The interatomic distances in dispersed carbon depend on the degree in covalency of the bonding, which is a function of L _a. Fiz. Tverd. Tela (St. Petersburg) 41, 744–747 (April 1999)     

Static compression of rare earth metal holmium to 282 GPa

ABSTRACT

The phase transitions and equation of state measurements were carried out on rare earth metal Holmium (Ho) to 282?GPa using toroidal diamond anvils thereby doubling the pressure range to which it has been studied previously. The first set of experiment employed standard beveled diamond anvils utilizing copper as an x-ray pressure standard to 217?GPa. The second set of experiment employed toroidal diamond anvils utilizing platinum as an x-ray pressure standard to 282?GPa. The recently proposed 16-atom orthorhombic structure (oF16) appeared to be stable between 103 and 282?GPa. The scaled axial ratio (c/a) shows a narrow range of variation of 1.58?±?0.05 for the five known crystalline phases of Ho to 282?GPa. The experimental equation of state of Ho is presented up to a threefold volume compression V/V_o?=?0.322.     

X-ray diffractometry and electrical conductivity of the ceramics LaSrAl(Cu,Ni)O4

A continuous series of solid solutions LaSrAL_1−x Me_xO₄ (Me = Cu, Ni) has been investigated by x-ray diffractometry and by electrical conductivity measurements. The results are of interest from the standpoint of the theory of close-packing, percolation theory, and the theory of the cooperative Jahn-Teller effect. Fiz. Tverd. Tela (St. Petersburg) 39, 985–990 (June 1997)     

Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO₂ and Ar clusters with high-intensity femtosecond laser pulses with q _las≃10¹⁸ W/cm² are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is observed to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relative number of excited ions (≃10⁻²–10⁻³) with energies of 0.1–1 MeV in such a plasma. Possible mechanisms underlying the acceleration of ions to high energies are discussed. It is shown that the experimental results can be attributed to the influence of ponderomotive forces in standing waves generated by the reflection of laser radiation from the clusters. Zh. éksp. Teor. Fiz. 115, 2051–2066 (June 1999)     

X-ray diffraction investigations of the crystallographic parameters and thermal expansion of β-BaB2O4 crystals

The lattice parameters a and c of β-BaB₂O₄ crystals have been measured in the temperature range 80–300 K by the x-ray diffraction method. The thermal expansion coefficients α are calculated from the measured values of the parameters. A substantial anisotropy of the thermal expansion is found. It is shown that the thermal expansion coefficient α _c along the c axis is an order of magnitude greater than the thermal expansion coefficient α _a in a plane perpendicular to this axis. It is established that α _a becomes negative in the temperature range 80–190 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1038–1040 (June 1997)     

Theory of chemical bonds in metalloenzymes XX: magneto-structural correlations in the CaMn4O5 cluster in oxygen-evolving complex of photosystem II

ABSTRACT

Magneto-structural correlations in oxygen-evolving complex (OEC) of photosystem II (PSII) have been elucidated on the basis of theoretical and computational results in combination with available electron paramagnetic resonance (EPR) experimental results, and extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) results. To this end, the computational methods based on broken-symmetry (BS) UB3LYP solutions have been developed to elucidate magnetic interactions in the active manganese catalyst for water oxidation by sunlight. The effective exchange interactions J for the CaMn(III)Mn(IV)₃O₅(H₂O)₃Y(Y = H₂O or OH^?) cluster (1) model of OEC of PSII have been calculated by the generalised approximate spin projection (GAP) method that eliminates the spin contamination errors of the BS UB3LYP solution. Full geometry optimisations followed by the zero-point energy (ZPE) correction have been performed for all the spin configurations of 1 to improve the J values that are compared with accumulated EPR in the S₂ state of Kok cycle and magnetic susceptibility results of Christou model complex Ca₂Mn(IV)₃O₄ (2). Using the calculated J values, exact diagonalisation of the spin Hamiltonian matrix has been carried out to obtain excitation energies and spin densities of the ground and lower excited states of 1. The calculated excitation energies are consistent with the available experimental results. The calculated spin densities (projection factors) are also compatible with those of the EPR results. The calculated spin densities have been used to calculate the isotropic hyperfine (A_iso) constants of ⁵⁵Mn ions revealed by the EPR experiments. Implications of the computational results are discussed in relation to the structural symmetry breaking (SSB) in the S₁, S₂ and S₃ states, spin crossover phenomenon induced by the near-infrared excitation and the right- and left-handed scenarios for the O–O bond formation for water oxidation.     

Electronic structure of Y and Pr in Laves phases with Mg, Al, Fe, Co, and Ni

The electronic structure (population of the valence d and s shells) of Y and Pr in the RM₂ Laves phases with Mg, Al, Fe, Co, and Ni has been determined experimentally by the x-ray K-line shift method. It was found that the Y and Pr electronic structures in RM₂ with Mg and Al are practically the same as in metals, but the distribution of the three outer electrons between the d and s shells in Y is essentially different from that in Pr. In the Laves phases with 3d metals (Fe, Co, or Ni), the outer-electron distributions between the d and s shells in Y and Pr are practically identical and do not depend on the d metal. Fiz. Tverd. Tela (St. Petersburg) 41, 1721–1723 (October 1999)     

Evolution of the population of outer 6s and 5d shells in rare-earth metals

The shifts of the Kα ₁ and Kβ ₁ lines of all rare-earth (RE) metals (from La to Lu) have been measured experimentally by the x-ray shift method. The population of the RE-metal 6s and 5d shells has been determined by comparing the experimental and theoretical shifts obtained within the Dirac-Fock (Koopmans) model. Trivalent metals exhibit a monotonic cross-over from the 6s ^≈25d ^≈1 to 6s ^≈15d ^≈2 configuration with increasing atomic number. Fiz. Tverd. Tela (St. Petersburg) 41, 1361–1362 (August 1999)     

X-ray studies of magnetic phase transitions in the intermetallic compounds RMn2Ge2 (R=La, Sm, Gd, Nd, Tb, and Y)

The lattice parameters a and c of the tetragonal intermetallic compounds RMn₂Ge₂ (R=La, Sm, Gd, Nd, Tb, and Y) have been measured by x-ray diffraction in the temperature interval 10–800 K. Anomalies are observed in the temperature dependence of a and c due to phase transitions from the paramagnetic to the magnetically ordered state in the Mn subsystem, transitions between various magnetically ordered phases due to a change in the magnitude and sign of the Mn-Mn exchange interaction, and magnetic transitions caused by ordering of the rare-earth subsystem leading to a rearrangement of the magnetic structure of the Mn subsystem. It is concluded that, along with the lattice parameter a, the lattice parameter c also has an influence on the Mn-Mn exchange interaction. Fiz. Tverd. Tela (St. Petersburg) 41, 2053–2058 (November 1999)     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)     

Spectra of low-energy electrons excited by soft x rays

A study is made of the x-ray photoemission spectra of low-energy electrons (E _kin=0–30 eV) and its dependence on the way in which the potentials are applied to the entrance system of the analyzer. It is shown that under certain experimental conditions the spectrum consists solely of electrons emitted from the sample. The x-ray photoemission spectrum of pure silver is found to have a fine structure, which disappears when a surface layer of carbon or oxygen is deposited on the surface. The spectrum then takes on the “cascade” form. A possible interpretation of the fine structure is suggested in the framework of band theory. Zh. Tekh. Fiz. 67, 73–77 (March 1997)