Compton Profiles Research Articles

A high energy Compton scattering study of magnetocaloric HoAl2

We present the first-ever Compton profile of HoAl2 at an intermediate resolution of 0.36a.u. using a 740GBq 137Cs Compton spectrometer. Charge reorganization on the formation of compound is depicted by significant deviation between the experimental Compton profile of HoAl2 and superposition profile (deduced from individual Compton measurements of Ho and Al). Further analysis of experimental momentum densities of HoAl2 shows a charge transfer of 0.32±0.03e‾ from Al→Ho. The electronic occupancies obtained from the present measurements are very close to those computed using spin-polarized relativistic Korringa–Kohn–Rostoker method. Electronic properties of HoAl2 are also explained on the basis of energy bands and integrated density of states.

Electron momentum density, band structure, and structural properties of SrS

The electron momentum density, the electronic band structure, and the structural properties of SrS are presented in this paper. The isotropic Compton profile, anisotropies in the directional Compton profiles, the electronic band structure and density of states are calculated using the ab initio periodic linear combination of atomic orbitals method with the CRYSTAL06 code. Structural parameters of SrS—lattice constants and bulk moduli in the B1 and B2 phases—are computed together with the transition pressure. The computed parameters are well in agreement with earlier investigations. To compare the calculated isotropic Compton profile, measurement on polycrystalline SrS is performed using 5Ci-241Am Compton spectrometer. Additionally, charge transfer is studied by means of the Compton profiles computed from the ionic model. The nature of bonding in the isovalent SrS and SrO compounds is compared on the basis of equal-valenceelectron-density profiles and the bonding in SrS is found to be more covalent than in SrO.

Gamma-Ray Compton Spectroscopy for Determination of Electron Momentum Distributions in Iron

Compton profile measurement of iron polycrystalline sample has been performed with 662 keV γ-radiation from a caesium-137 source. The spectrometer calibration and data corrections for the high energy experiment are discussed. The data are compared with the augmented-plane-wave (APW) and linear combination of atomic orbitals (LCAO) band theoretical Compton profiles of iron. Both theoretical predictions show the band theories overestimate the momentum density at low momenta and underestimate it at intermediate momenta.

Gamma-Ray Compton Spectrometers

General aspects of different gamma ray Compton scattering techniques are introduced. Comparison between the performances of different Compton spectrometers in terms of the parameters: resolution, statistical accuracy, and signal-to-noise ratio are presented. In addition to that, the most relevant parameters for further improvement in the quality of Compton profile measurements are suggested.

Electron momentum density in liquid silicon

The electron momentum density (EMD) in liquid silicon (Si) has been measured by synchrotron-based Compton scattering. The observed variation in the valence EMD upon melting, reflecting a semiconductor-metal transition of Si, is well explained by the collapse of the Jones zone of crystalline Si. However, the shape of the EMD of liquid Si is considerably broad and retains fairly solid ($\ensuremath{\alpha}$-Si)-like characteristics. The analysis of the Fourier-transformed Compton profiles reveals that the valence electronic state in liquid Si exhibits a marked deviation from the free-electron gas features.

Perpendicular Magnetic Anisotropy in Fe/MgO Multilayer Film Measured by Magnetic Compton Scattering

We have measured magnetic Compton scattering (MCS) for an Fe/MgO multilayer film at several magnetic field applying perpendicular to film plane. A spin specific magnetic hysteresis (SSMH) loop is obtained by the MCS for the Fe/MgO multilayer film. A knickpoint is observed in the SSMH loop around the magnetic field of 0.5 T. Orbital magnetization is enhanced within the magnetic field from-0.5 T to 0.5 T. A decomposition analysis for magnetic Compton profiles shows the suppressed |m|=0 states and enhanced |m|=1 and 2 states within the magnetic field from-0.5 T to 0.5 T. Heremdenotes magnetic quantum number. The knickpoint corresponds to a perpendicular magnetic anisotropy, which comes from the enhanced |m|=1 and 2 state and orbital magnetization in the Fe/MgO multilayer.

Spin polarization of conduction electrons and electronic structure of gadolinium

The structure of magnetic moment in metallic gadolinium is studied with special attention paid to the conduction electron spin-polarization. The theoretical electronic structure, Fermi surface topology, neutron magnetic form-factor and magnetic Compton profiles of ferromagnetic gadolinium are analysed. The calculated band structure is in good agreement with the results of a photoemission experiment, especially in the position of the 4f band. Some tendency to parallelization behaviour in the Fermi surface was observed. The maximum entropy reconstruction of the magnetization density from neutron diffraction data is also presented and compared with the theoretically predicted distribution. It is shown that in contrast to the old interpretation of the magnetic form factor data, the negative spin polarization in the interstitial regions may contain contribution coming from d-type electrons. The negative magnetization within the muffin-tin should characterize semi-core electrons 5s and 5p. Presence of this magnetization can hardly be found in neutron measurements, yet it is possible to observe it in accurate magnetic Compton profiles. The latter are calculated and discussed.

Compton Profile Study and Electron Momentum Density Reconstruction in Hexagonal Mg

Four directional Compton profiles (CPs) of a hexagonal magnesium single crystal have been measured with high momentum resolution (0.12 a.u.) using 115.6 keV synchrotron radiation in SPring-8. Experimental CP anisotropies were compared with augmented plane waves (APW), Korringa–Kohn–Rostocker (KKR), and the latest density functional theory (DFT) band structure calculations. In order to reconstruct the full three-dimensional valence electron momentum density distribution, maximum entropy method (MEM) was applied. 3D electron momentum densities obtained using MEM were compared (after appropriate integration) with 2D electron momentum density reconstructed using Cormack's method. MEM has proved its advantage as a technique for reconstructing density distributions when initial information is insufficient and noisy. The 3D valence electron momentum density of hexagonal Mg was successfully reconstructed using as few as four experimental or five theoretical Compton profiles. As is shown, despite a clear anisotrop...

Analysis of the electronic structure of ZrO2 by Compton spectroscopy

The electronic structure of ZrO2 is studied using the Compton scattering technique. The first-ever Compton profile measurement on polycrystalline ZrO2 was obtained using 59.54 keV gamma-rays emanating from the 241Am radioisotope. To explain the experimental data, we compute theoretical Compton profile values using the method of linear combination of atomic orbitals in the framework of density functional theory. The correlation scheme proposed by Perdew-Burke-Ernzerhof and the exchange scheme of Becke are considered. The ionic-model-based calculations for a number of configurations, i.e., Zr+x (O−x/2)2 (0 ≤ x ≤ 2), are also performed to estimate the charge transfer on compound formation, and the study supports transfer of 1.5 electrons from Zr to O atoms.

Electronic Properties of RDX and HMX: Compton Scattering Experiment and First-Principles Calculation

The first-ever electron momentum density (EMD) measurements of explosive materials, namely, RDX (1,3,5-trinitro-1,3,5-triazacyclohexane, (CH2-N-NO2)3) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, (CH2-N-NO2)4), have been reported using a 740 GBq (137)Cs Compton spectrometer. Experimental Compton profiles (CPs) are compared with the EMDs derived from linear combination of atomic orbitals with density functional theory. It is found that the CPs deduced from generalized gradient approximation (GGA) with Wu-Cohen exchange energies give a better agreement with the corresponding experimental profiles than those from local density approximation and other schemes of GGA. Further, Mulliken population, energy bands, partial and total density of states, and band gap have also been reported using GGA calculations. Present ground state calculations unambiguously show large band gap semiconductor nature of both RDX and HMX. A similar type of bonding in these materials is uniquely established using Compton data and density of states. It is also outstandingly consistent with the Mulliken population, which predicts almost equal amount of charge transfer (0.84 and 0.83 e(-)) from H1 + H2 + N2 to C1 + N1 + O1 + O2 in both the explosives.

Experimental observations of Z-dependence of saturation thickness of 662 keV gamma rays in metals and glasses

In this paper the energy, intensity and angular distributions of multiple scattering of 662 keV gamma photons, emerging from targets of aluminium, copper, iron and glass samples are studied as a function of target thickness. An intense collimated beam obtained from a 137Cs source of 5.8 mCi is allowed to impinge on samples of varying thickness. The scattered photons are detected by a properly shielded 76 mm × 76 mm NaI (Tl) scintillation detector. In Compton scattering experiments, it is observed that the number of multiple backscattered photons increase with an increase in target thickness and become almost constant for particular target thickness called saturation thickness. The saturation thickness decreases with increasing atomic number. The multiple scattering, an interfering background noise in Compton profile, has been successfully used to assign effective atomic number (Z eff ) to composite materials. The experimental results are compared with the Monte Carlo calculations using Monte Carlo nuclear particle code.

Electron Momentum Density and Phase Transition in ZnS

The electron momentum density distribution and phase transition in ZnS are reported in this paper. The calculations are performed on the basis of density functional theory (DFT) based on the linear combination of atomic orbitals (LCAO) method. To compare the theoretical Compton profile, the measurement on polycrystalline ZnS has been made using a Compton spectrometer employing 59.54 keV gamma rays. The spherically averaged theoretical Compton profile is in agreement with the measurement. On the basis of equal valence-electron-density Compton profiles, it is found that ZnS is less covalent as compared to ZnSe. The present study suggests zincblende (ZB) to rocksalt (RS) phase transition at 13.7 GPa. The calculated transition pressure is found in good agreement with the previous investigations.

Study of spin and orbital magnetization in Dy- and Gd-doped Co ferrite using magnetic Compton scattering

Temperature dependent experimental magnetic Compton profiles (MCPs) of inverse spinel CoFe2−xRExO4 (x = 0.05; RE = Dy, Gd) have been decomposed into constituent profiles to determine site-specific spin moments. A comparison of MCPs of doped and undoped CoFe2O4 shows a decrease in spin moment on 5% doping of Dy and Gd. Reduction in spin moment is explained on the basis of migration of Co2+ ions from octahedral to tetrahedral sites, which is also supported by photoelectron spectroscopy measurements. The orbital moments deduced from combination of spin momentum density and magnetization data are found to be almost similar in doped ferrites.

What we can learn from magnetic Compton scattering: application to the determination of spin polarization

Studies of spin-resolved electron momentum densities involve the measurement of the so-called magnetic Compton profile. This is a one-dimensional projection of the electron momentum distribution of only those electrons that contribute to the spin moment of a sample. The technique is applicable to ferri- and ferromagnetic materials. The profile is obtained via the inelastic 'Compton' scattering of high energy X-rays. Since electrons originating from different atomic orbitals have specific momentum densities, it is often possible to determine the origin of the magnetism present. Typically, interpretation requires the use of electronic structure calculations using molecular orbital and band structure approaches. Here, we highlight the application of the technique to the determination of the Fermi level spin polarization, the knowledge of which is important to the development of novel spintronic materials.

Ab-initio study of phase transition and momentum density in PbTe

In this article, the high-pressure structural phase transition and electron momentum density in PbTe have been studied by means of first-principles total energy calculations which are based on the linear combination of atomic orbitals method within generalized gradient approximation. It is observed that the stable phase of PbTe is rocksalt (B1) type and it transforms to an orthorhombic Pnma (B27, FeB type) structure at 6.77 GPa. Further, structural phase transition from the Pnma phase to the cesium chloride (B2) phase has been observed at 13.04 GPa. The anisotropies in theoretical directional Compton profiles indicate larger occupied states along the [100] direction. On the basis of equal-valence-electron-density profiles, it is found that PbTe shows less ionicity as compared to PbS and PbSe.

Compton scattering study and electronic structure of different phases of NH4NO3

In this paper, we present energy bands, density of states and the nature of hydrogen bonding in different temperature-dependent phases of NH4NO3, namely phases II (357–398 K), III (305–357 K) and IV (257–305 K), using the full potential linearized augmented plane wave and linear combination of atomic orbitals (LCAO) methods. Both band structure calculations show the insulating character of the II–IV phases of NH4NO3. In addition, we have reported the isotropic Compton profile of NH4NO3(IV) measured using 661.65 keV γ-radiation from a 137Cs isotope. The experimental momentum density has been compared with that based on density functional theory (DFT) and Hartree–Fock schemes within the LCAO method. It is seen that the LCAO with hybridization of DFT and Hartree–Fock (so-called B3LYP) gives better agreement with the present Compton profile measurement.

Cross section of the impact single ionization of B2+ by H+

The ionization process of B2+ by H+ impact is studied using the continuum-distorted-wave eikonal-initial-state (CDW-EIS) method and the modified free electron peak approximation (M-FEPA), respectively. Total, single-, and double-differential cross sections from 1s and 2s orbitals are presented for the energy range from 10 keV/u to 10 MeV/u. Comparison between the results from the two methods demonstrates that the total and single-differential cross sections for the high-energy incident projectile case can be well evaluated using the simple M-FEPA model. Moreover, the M-FEPA model reproduces the essential features of the binary-encounter (BE) bump in the double-differential cross sections. Thus, the BE ionization mechanism is discussed in detail by adopting the M-FEPA model. In particular, the double- and singledifferential cross sections from the 2s orbital show a high-energy hip, which is different from those from the 1s orbital. Based on Ref. [1], the Compton profiles of B2+ for 1s and 2s orbitals are given, and the hips in DDCS and SDCS from the 2s orbital are explained.

Electronic Origin of the Structural Anomalies of Zinc and Cadmium

AbstractThe electron distributions in position and in momentum space of the hcp metals magnesium and zinc are investigated experimentally and compared to results of quantum‐chemical calculations. Furthermore, a survey is given on recent analyses of the bonding properties of zinc and cadmium, using the method of increments. The experimental deformation densities were obtained by refining multipole models to X‐ray diffraction data sets measured at 100 K with either Mo‐Kα (Mg) or Ag‐Kα (Zn) radiation. The final RF values (Valray/Jana2006) are 0.0028/0.0034 (Mg) and 0.0068/0.0068 (Zn). The differences to deformation densities obtained from periodic density functional calculations are discussed. The effect of dynamical electron correlation on the electron density was analyzed, using cluster models. Compton profiles were measured with 88.67 keV synchrotron radiation at beamline ID15B at the ESRF in Grenoble. Varied orientations of the samples allowed for probing the projected momentum distribution along the [100], [423] and [001] directions. Fourier transforms of the computed reciprocal form factor B(r) resulted in the corresponding theoretical Compton profiles. It is suggested that the anomalous hcp structure of zinc is favored by a kinetic balancing of the valence electrons, i.e. correlation mediated 4s‐3d interactions.

Magnetic Compton scattering studies of magneto-dielectric Ba(Co0.85Mn0.15)O3−δ

We revealed that the Ba(Co0.85Mn0.15)O3-δ ceramic samples exhibited ferromagnetic-dielectric behavior below the magnetic transition temperature of about 35 K. The origin of their magnetic ordering was expected to super-exchange coupling of Co4+(d5)-O2--Mn4+(d3) with bonding angle of 180° and/or Mn4+(d3)-O2--Mn4+(d3) with bonding angle of 90°. The magnetic spin momentum estimated by the magnetic Compton profiles (MCP) of the samples had similar temperature dependence as that determined by the temperature dependence of magnetic moment by superconducting quantum interference device, which meant that the observed magnetic moments could be ascribed to the spin moment. The shapes of the MCPs of the samples were completely same regardless of the temperature measured. This result indicates that there are no changes of the momentum space distribution of spin density between ferromagnetic and paramagnetic states. So, this magnetic transition is simply caused by a thermal fluctuation of the spin.

Directional Compton profiles of CdTe using a low intensity 241Am source

We report the first-ever experimental Compton profiles (CPs) of CdTe measured along [100] and [111] directions using 100mCi 241Am Compton spectrometer. The experimental Compton data have been interpreted in terms of theoretical anisotropy in the momentum densities and also energy bands computed using density functional theory with local density approximation, generalized gradient approximation (GGA) and second order GGA. It is found that DFT–GGA based CPs are in marginally better agreement with the experimental data in comparison to other DFT based profiles. The anisotropy in CPs is found to be in tune with the energy bands. The bond length (5.23±0.11a.u.) deduced from the oscillation in the experimental anisotropy reconciles well with the theoretical Cd–Te bond length (5.30a.u.) which establishes the role of γ-ray Compton measurements in determination of structural parameters.