Experimental and theoretical studies on X-ray induced secondary electron yields in Ti and TiO2

Generation of X-ray induced secondary electrons in Ti and TiO₂ was studied from both experimental and theoretical approaches, using X-ray photoelectron spectroscopy (XPS) attached to a synchrotron radiation facility and Monte Carlo simulation, respectively.The experiment revealed that the yields of secondary electrons induced by X-rays (electrons/photon) at photon energies to 4950 and 5000 eV for Ti and TiO₂ are δ_Ti(4950 eV) = 0.002 and δ_Ti(5000 eV) = 0.014 while those for TiO₂ are δTiO₂(4950 eV)=0.003 and δTiO₂(5000 eV)=0.018.A novel approach to obtain the escape depth of secondary electrons has been proposed and applied to Ti and TiO₂. The approach agreed very well with the experimental data reported so far. The Monte Carlo simulation predicted; and while and .An experimental examination on the contribution of X-ray induced secondary electrons to photocatalysis in TiO₂ has also been proposed.     

Microstructure of epitaxial scandium nitride films grown on silicon

Epitaxial scandium nitride films (225 nm thick) were grown on (1 1 1)-oriented silicon substrates by molecular beam epitaxy (MBE), using ammonia as a reactive nitrogen source. Film microstructure was investigated using X-ray diffraction (XRD). The (1 1 1) ω-scan FWHM of 0.551° obtained for films grown at 850 °C is the lowest reported so far for ScN thin films. The principal orientation of ScN with respect to Si is (1 1 1)_ScN//(1 1 1)_Si and []_ScN//[]_Si, representing a 60° in-plane rotation of the ScN layer with respect to the Si substrate. However, some twinning is also present in the films; the orientation of the twinned component is (1 1 1)_ScN//(1 1 1)_Si and []_ScN//[]_Si, representing a ‘cube-on-cube’ orientation. The volume percentage of these twins in the films decreases with increasing film growth temperature.     

Crystalline orientation of BiMnO3 thin films grown by rf-sputtering

BiMnO₃ thin films were deposited by means of rf-sputtering onto [1 0 0] oriented SrTiO₃ substrates. X-ray diffraction measurements revealed epitaxial growth with [0 0 l]_m orientation in the monoclinic structure representation, equivalent to the direction of the pseudocubic cationic lattice. This [0 0 l]_m orientation was obtained in a wide deposition parameters range. Preliminary magnetization measurements suggest that these films do not present ferromagnetism below the bulk Curie temperature, T_C=105 K.     

Low-temperature measurements of magnetic susceptibility and specific heat of Eu2Ti2O7—An XY pyrochlore

Magnetic susceptibility obtained from magnetization measurement (for fields H=0.1 and 1.0 T) of polycrystalline Eu₂Ti₂O₇ shows two distinct features. Firstly, increases on cooling below 300 K and attains a temperature-independent constant value at 68 K (T_max). Secondly, shows an antiferromagnetic increase below 4.9±0.1 K. The former behavior is explained by crystal field (CF) theory. CF levels and wave functions of ground and excited states are determined accurately from analyses of and earlier reported Mössbauer and optical spectra. Analysis of vs. 1/T curve at low temperatures gives the classical nearest-neighbor exchange interaction J^cl=−0.76 K and a weak dipolar interaction D_nn=0.0056 K. C_P of polycrystalline sample of Eu₂Ti₂O₇ and Y₂Ti₂O₇ are measured between 1.8-35 and 1.8-120 K respectively and θ_D vs. T (K) curves are calculated. At 4 K, θ_D of Eu₂Ti₂O₇ shows a kink and dC_P/dT curve show a maximum. Optical results show energy exchange between Eu³⁺ ions at intrinsic and extrinsic (defect) sites via super-exchange interaction at low temperature which may account for the observed anomalous behavior of and C_P.     

On the symmetry and structure of the double perovskites Ba2LnRuO6 (Ln=La, Pr and Nd)

The structures of the double perovskites Ba₂LnRuO₆ (Ln=La, Pr and Nd) at room temperature were re-investigated by profile analysis of X-ray diffraction data. It has been shown that neither the triclinic nor the monoclinic P2₁/n space group correctly describes their structures. Ba₂LaRuO₆ is actually rhombohedral, space group , cell parameters a=6.03196(10) Å and α=60.298(2)°. On the other hand, both Ba₂PrRuO₆ and Ba₂NdRuO₆ are found to be cubic, space group , with the cell parameters a=8.48416(6) and 8.47061(5) Å, respectively.     

Adsorption characteristics of atomic nitrogen on ruthenium surfaces

We have studied the adsorption, vibration, and diffusion of N atoms on Ru(0 0 0 1), , and surfaces by means of the 5-parameter Morse potential (5-MP) of interaction between atomic nitrogen and a metal surface. The adsorption sites, adsorption geometry, binding energy and eigenvibration of atomic nitrogen on the different ruthenium surfaces are calculated. It is shown that atomic nitrogen always preferably occupies the high coordination sites on Ru surfaces. The 4-fold site is the preferable adsorption site for atomic nitrogen on both open and surfaces while 3-fold site is the most stable adsorption site for atomic nitrogen on both Ru(0 0 0 1) and surfaces. Moreover, we find the lowest energy pathway of diffusion and diffusion barriers of atomic nitrogen on the surfaces.     

Combined neutron and synchrotron X-ray diffraction study of Sr/Mg-doped lanthanum gallates up to high temperatures

Combined neutron diffraction and high-resolution synchrotron X-ray powder diffraction methods have been used to examine the crystal structures of two sample sets of Sr/Mg-doped Lanthanum gallate with the compositions La_0.9Sr_0.1Ga_1−yMg_yO_{3−0.5(0.1+y)} (y=0, 0.1, 0.2) and La_0.8Sr_0.2Ga_1−yMg_yO_{3−0.5(0.2+y)} (y=0.15, 0.2) up to 900 °C. At room temperature all samples of the first series exhibit orthorhombic structures with space group Imma: La_0.9Sr_0.1GaO_2.95: , , ; La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9: , , ; La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85: , , . The samples of the second series have the cubic perovskite structure with space group at room temperature: La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825: ; La_0.8Sr_0.2Ga_0.8Mg_0.20O_2.80: . Samples of the first series transform from the orthorhombic to a rhombohedral (Imma→) structure at ∼170 °C for La_0.9Sr_0.1GaO_2.95, at ∼430 °C for La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9, and between 600 and 700 °C for La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85. Both La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825 and La_0.8Sr_0.2Ga_0.8Mg_0.2 show no structural deviations from the cubic aristotype over the whole temperature range. The room temperature Imma structures of the first series are justified by a domain model and are rationalized in terms of static disorder increasing with Mg content, thus driving the phase transition temperatures to higher values in agreement with tolerance factor considerations. The distortion of the rhombohedral high-temperature phases (octahedra tilting and compression) and the effect of phase transitions on the ionic conductivity are discussed.     

On the formulation of D = 11 supergravity and the composite nature of its three-form gauge field

The underlying gauge group structure of the D = 11 Cremmer-Julia-Scherk supergravity becomes manifest when its three-form field A₃ is expressed through a set of one-form gauge fields, , , η_1α, and E^a, ψ^α. These are associated with the generators of the elements of a family of enlarged supersymmetry algebras parametrized by a real number s. We study in detail the composite structure of A₃ extending previous results by D’Auria and Fré, stress the equivalence of the above problem to the trivialization of a standard supersymmetry algebra E^(11|32) cohomology four-cocycle on the enlarged superalgebras, and discuss its possible dynamical consequences. To this aim we consider the properties of the first order supergravity action with a composite A₃ field and find the set of extra gauge symmetries that guarantee that the field theoretical degrees of freedom of the theory remain the same as with a fundamental A₃. The extra gauge symmetries are also present in the so-called rheonomic treatment of the first order D = 11 supergravity action when A₃ is composite. Our considerations on the composite structure of A₃ provide one more application of the idea that there exists an extended superspace coordinates/fields correspondence. They also suggest that there is a possible embedding of D = 11 supergravity into a theory defined on the enlarged superspace .     

FTIR, XRD, SEM and solid state NMR investigations of carbonate-containing hydroxyapatite nano-particles synthesized by hydroxide-gel technique

Nano-crystalline hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂ has been synthesized by a precipitate conversion technique using hydroxide gel at lower temperatures, e.g. 80 °C. HA crystallizes in hexagonal structure (space group: P6₃/m) having lattice parameters: and and around 17 nm in crystallite-size for the 80 °C-heated sample. SEM micrographs show hexagonal crystallites of average particle dimensions approximately 50×20 nm for 80 °C heated sample. The structure analysis by XRD, FTIR, ¹H and ³¹P MAS NMR show the existence of structural disorder at the particle surface that either does not form hydrogen bonding due to lack of adequate bonding parameters or forms a very weak dipolar bonding. The structural disorder has been explained as a result of chemical interactions between the phosphate groups either with the surface adsorbed water or the hydroxyl groups at the surface of the nano-particles.     

Room temperature stable structures and phase transition of Na2Al2B2O7

By Rietveld refinement of the X-ray diffraction (XRD) data of powdered Na₂Al₂B₂O₇ samples aged for over 3 months, we found that Na₂Al₂B₂O₇ at room temperature is a mixture of two phases with space group and P6₃/m, respectively. The structures of the two phases can be refined with identical cell parameters of a=4.80760(11) Å, c=15.2684(5) Å and are composed by [Al₂B₂O₇]²⁻_∝ double layers stacking alternatively with Na⁺ ions along the c-direction, but differ at in-plane bond orientations of the BO₃/AlO₄ groups within the double layers: in P6₃/m phase B-O₁/Al-O₁ bonds of the two layers are perfectly aligned, whereas in phase they are twisted by 46.4/41.6° around c-axis against each other. It is also found that a freshly prepared sample contains only the phase, but part of the phase will transfer to P6₃/m phase slowly at room temperature and the transition can be reversed by heating the aged sample above 220 °C.     

Low-temperature magnetic structures in the DySi FeB-type compound 27.03.09

The low-temperature magnetic ordering of the dimorphic DySi compound has been studied at 1.5 K by neutron diffraction on two polycrystalline samples. The samples comprise various amounts of the two orthorhombic modifications: CrB-type (Cmcm Nr. 63, all atoms at 4c site: (0, y, )) and FeB-type (Pnma Nr. 62, all atoms at 4c site: (x, , z)), both order antiferromagnetically (T_N≈38 K). The CrB-type phase orders with a uniaxial structure with the wave vector q₁=(0, 0, ) requiring a doubling of the c-axis. The Dy moments point along the linear chain with the shortest distance c. At 1.5 K, the ordered moment value is 8.57(1) μ_B/Dy atom.Two symmetry independent wave vectors describe the 1.5 K magnetic ordering of the FeB-type phase: q₂=(0, , ) and q₃=(0, 0.484(1), 0.0892(1)), coexisting in form of domains. In both structures the magnetic moments are confined to the (0 0 1) plane at an angle of 2(2)° and 22(3)° from the shortest axis b, respectively. Both structures correspond to sine wave modulations. The amplitude of the q₂ wave is m_o=7.5(1) μ_B/Dy atom and that of q₃ 8.2(1) μ_B/Dy atom. The wave vector q₂ when referring to the (a, 2b, c) cell and the wave vector q=(0, 0, ) corresponds to a transversal modulation, which by a proper origin choice can be also described as an antiphase domain structure with two amplitudes. The moments point to the b-axis and are stacked in the sequence (+m_o/2, −m_o/2, −m_o, −m_o/2, +m_o/2, +m_o, …) along the c-direction, while t_b acts as an antitranslation. For the q₃ phase, the local moment value depends on the atom position in the wave. We also discuss the case where q₃ and q₂ act simultaneously in physical space.     

Crystal field effect and geometric frustration in Er2Ti2O7 —an XY antiferromagnetic pyrochlore

Magnetic susceptibility of powder Er₂Ti₂O₇ (ErT) is measured between 300 K and 80 K. shows a Curie-Weiss (CW) type behaviour with   ErTiO_3.5 and . A crystal field (CF) analysis of our experimental data, g-values (g_∥=0.27 and g_⊥=7.8) and the positions of two CF levels (reported earlier from an inelastic neutron scattering study) provide CF parameters and CF levels of the ground ⁴I_15/2 and excited multiplets of ErT. The theoretical follows a CW-type behaviour, with . Single-ion magnetic anisotropy (χ_⊥−χ_∥) is 9500×10⁻⁶ emu/mol ErTiO_3.5 at 300 K, which increases by ∼54 times at 10 K and ErT resembles an XY planar system. It can be inferred from CF analysis that the earlier observed change of from −13 K to −22 K below 50 K is not due to the CF effect. Nuclear hyperfine (HF) levels of ¹⁶⁷ErT and ¹⁶⁶ErT are calculated and the theoretical curve of vs. T (K) for T<T_N matches the observed results. Mössbauer lines expected for ¹⁶⁶ErT are also predicted.