Experimental Compton Profiles Research Articles

N-representable one-electron reduced density matrix reconstruction with frozen core electrons.

Recent advances in quantum crystallography have shown that, beyond conventional charge density refinement, a one-electron reduced density matrix (1-RDM) satisfying N-representability conditions can be reconstructed using jointly experimental X-ray structure factors and directional Compton profiles (DCP) through semidefinite programming. So far, such reconstruction methods for 1-RDM, not constrained to idempotency, have been tested only on a toy model system (CO2). In this work, a new method is assessed on crystalline urea [CO(NH2)2] using static (0 K) and dynamic (50 K) artificial experimental data. An improved model, including symmetry constraints and frozen core-electron contribution, is introduced to better handle the increasing system complexity. Reconstructed 1-RDMs, deformation densities and DCP anisotropy are analysed, and it is demonstrated that the changes in the model significantly improve the reconstruction quality, even when there is insufficient information and data corruption. The robustness of the model and the strategy are thus shown to be well adapted to address the reconstruction problem from actual experimental scattering data.

Exploring electronic properties of zinc oxide using pure and hybrid density functional theory and Compton profiles

To check the viability and applicability of available experimental Compton profiles (CPs) of ZnO, we have computed Mulliken population (MP) and CPs using pure and hybrid density functional theory (DFT) within the basis of linear combination of atomic orbitals (LCAO) scheme. Pure DFT have been employed within local density approximation, generalized gradient approximation (GGA) and second order GGA (SOGGA) whereas for hybrid exchange and correlation energies the standard WC1LYP and B1WC hybrid schemes are adopted. A quantitative comparison of theoretical and the available experimental CPs unambiguously identify WC1LYP as a better choice for reproducing experimental momentum densities, and thereafter hybrid approach for generating accurate Eigen functions of ZnO. Further, MP analysis reports the dominancy of ionic character in the compounds. We have also scaled the CPs of ZnO and CdO using WC1LYP for equal-valence-electron-densities and concluded about more ionic character in ZnO than that in CdO.

Compton spectroscopy of hydrogen storage material LiAlH4: Experiment and DFT strategies

Electronic structure and momentum densities of three possible phases (α, β and γ) of hydrogen storage material, LiAlH4, are reported using the linear combination of atomic orbitals (LCAO) scheme within the density functional theory (DFT). Energy bands and density of states computed using generalised gradient approximation and hybrid (Hartree-Fock + DFT) potentials show that the studied phases of LiAlH4 possess wide band gaps ranging from 3.59 to 6.64 eV. The experimental Compton profile (CP) of the most stable α-phase was measured using 100 mCi 241Am γ-ray Compton spectrometer. The experimental CP is used to validate the applicability of exchange and correlation energies by comparing it with theoretical CPs deduced using various exchange and correlation potentials. The B3LYP hybrid functional-based CP is found to be in good agreement with the present experimental Compton data. Based on Mulliken's population and valence electron charge density, the trend of ionicity is found in the descending order as γ-phase > α-phase > β-phase. Moreover, the modified Becke-Johnson (mBJ) scheme of the full potential linearized augmented plane wave (FP-LAPW) method is also used to compute the electronic and optical response of LiAlH4, which show its use as absorptive material for ultraviolet radiations. The bonding nature of the Al-H bond and Li+-[AlH4]¯ is also presented using the electron localization function.

N-Representable one-electron reduced density matrices reconstruction at non-zero temperatures

This article retraces different methods that have been explored to account for the atomic thermal motion in the reconstruction of one-electron reduced density matrices from experimental X-ray structure factors (XSF) and directional Compton profiles (DCP). Attention has been paid to propose the simplest possible model, which obeys the necessary N-representability conditions, while accurately reproducing all available experimental data. The deconvolution of thermal effects makes it possible to obtain an experimental static density matrix, which can directly be compared with theoretical 1-RDM (reduced density matrix). It is found that above a 1% statistical noise level, the role played by Compton scattering data becomes negligible and no accurate 1-RDM is reachable. Since no thermal 1-RDM is available as a reference, the quality of an experimentally derived temperature-dependent matrix is difficult to assess. However, the accuracy of the obtained static 1-RDM, through the performance of the refined observables, is strong evidence that the Semi-Definite Programming method is robust and well adapted to the reconstruction of an experimental dynamical 1-RDM.

Electronic structure of tantalum dichalcogenide using Compton scattering technique and density functional theory

Compton spectroscopy measurements on tantalum dichalcogenide TaS0.5Se1.5 using a 100 mCi Am-241 γ-ray Compton spectrometer are presented. Experimental Compton profile (CP) is compared with those computed using linear combination of atomic orbitals (LCAO) method within the density functional theory (DFT) and hybrid functionals of Hartree-Fock and DFT. Hybrid functional namely WC1LYP based CP is found in better accordance with the experimental CP. The relative nature of bonding in TaS0.5Se1.5 and TaSSe is interpreted by scaling the experimental CP on equal-valence-electron-density and Mulliken's population analysis. Origin of metallic behaviour of TaS0.5Se1.5 is discussed in terms of energy bands and density of states. It is seen that the 5d states of Ta dominate around the Fermi level leading to metallic character of the compound. Fermi surface topology and optical response like dielectric constants, absorption coefficients, etc. of TaS0.5Se1.5 are also described using modified Becke-Johnson theory of full-potential linearized augmented plane wave approach.

59.54 keV’lik Gama-ışınları Kullanılarak Ar ve O2’nin Compton Saçılma Ölçümleri

In this study, it has been investigated of the Compton profiles (CP) of the Ar and O2 gases using a Compton spectrometer having annular 241Am radioactive source. The photons emitted from the radioactive source and scattered photons from the samples were recorded using HPGe detector. In order to obtain experimental Compton profiles of the samples the Warwick codes were used. The experimental results obtained were compared with the theoretical results calculated by Hartree-Fock method. The results clearly show that the experimental and theoretical results are in good agreement, especially in the high momentum region. It was observed that while the difference between the experimental and theoretical results of Ar and O2 gases in the pz = 0 region is about 4%, this difference is below 1% after pz = 2.

Electronic properties of lithium tantalate using Compton spectroscopy

We have measured the electron momentum density (EMD) of LiTaO3 using 100 mCi 241Am Compton spectrometer. The experimental Compton profile (CP) is compared with theoretical EMDs deduced using linear combination of atomic orbitals (LCAO) method with different exchange and correlation potentials namely PBE, PWGGA and VBH. On the basis of χ2 fit (which scales goodness of fitting) and difference profiles between experimental and theoretical CPs, LCAO-PWGGA potential based CP is found close to experimental results. Electronic properties like energy bands and density of states of LiTaO3 using LCAO-PWGGA potential show a band gap of 4.85 eV of LiTaO3, thereby depicting its semiconductor nature with wide band gap. It is observed that upper valence band region is mainly dominated by 2p states of O and 5d states of Ta, although there is small contribution of 2s states of Li in this region.

Electronic response of hydrogen storage intermetallics LaNi5 and LaNi4.5Co0.5: inelastic scattering experiment and ab initio calculations

Electron momentum densities of intermetallic compounds LaNi5 and LaNi4.5Co0.5 using inelastic scattering of 662 keV γ-rays are presented. To check the usefulness of different exchange and correlation potentials for such type of hydrogen storage compounds, the resulting experimental Compton profiles (CPs) have been compared with the Compton lines deduced using linear combination of atomic orbitals (LCAO) scheme. It is seen that the LCAO with HISS scheme (middle range separated hybrid approach) leads to a better agreement with the experimental CPs than other considered schemes based on LDA, GGA, SOGGA, B3LYP and short range separated hybrid HSE06. In addition, energy bands and density of states of these systems derived using LCAO and the full potential linearized augmented plane wave methods are discussed. Anisotropies in momentum densities with respect to [001] direction namely (J001−J110 and J001–J100) show almost similar trend for both the intermetallics. It is noticed that the energy states corresponding to d states of Ni (2c), Ni (3 g) and Co atoms dominate near Fermi level (EF). The Fermi surface (FS) topology and real space CPs of LaNi5 and LaNi4.5Co0.5 show non-spherical character of FS and failure of free electron theory for such intermetallics.

Performance of hybrid functional in linear combination of atomic orbitals scheme in predicting electronic response in spinel ferrites ZnFe2O4 and CdFe2O4

Pure and hybrid density functional theory (DFT) schemes within linear combination of atomic orbitals (LCAO) have been employed to compute Mulliken population (MP), energy bands, density of states (DOS) and electron momentum densities (EMDs) of TMFe2O4 (TM = Zn and Cd). Pure DFT calculations were performed within local density and generalized gradient approximations, while Hartree–Fock exchange contribution is added to DFT for hybrid calculations (B3LYP and PBE0). To validate the performance of hybrid functionals, we have also performed EMD measurements using 661.65 keV γ-rays from 137Cs source. Chi-square test predicts an overall better agreement of experimental Compton profile data with LCAO–B3LYP scheme-based momentum densities leading to usefulness of hybrid functionals in predicting electronic and magnetic response of such ferrites. Further, LCAO–B3LYP-based majority- and minority-spin energy bands and DOS for ZnFe2O4 and CdFe2O4 predict semiconducting nature in both the compounds. In addition, MP data and equal-valence-electron-density scaled EMDs show more covalent character of ZnFe2O4 than that of CdFe2O4. A reasonable agreement of magnetic moments of both the ferrites with available data unambiguously promotes use of Gaussian-type orbitals in LCAO scheme in exploring magnetic properties of such ferrites.

Compton profile of few-layer graphene investigated by electron energy-loss spectroscopy

In this paper, acquisition of the valence Compton profile of few-layer graphene using electron energy-loss spectroscopy at large scattering angle is reported. The experimental Compton profile is compared with the corresponding theoretical profile, calculated using the full-potential linearized augmented plane wave method based on the local-density approximation. Good agreement exists between the theoretical calculation and experiment. The graphene profile indicates a substantially greater delocalization of the ground state charge density compared to that of graphite.

Magnetic properties and spin momentum densities of Ni excess Ni-Mn-Sn Heusler alloys

Abstract The role of excess Ni in Ni2+xMn1−xSn (x = 0.05, 0.1) Heusler alloys has been reported using temperature dependent magnetic Compton spectroscopy. The experimental Compton profiles from magnetic electrons have been analyzed to quantify the contribution of Mn 3d electrons, Ni sites and sp-electrons in building-up the total spin moments. The experimentally deduced spin momentum densities, total and element-specific spin moments are examined using theoretical investigations within the spin polarized relativistic Korringa-Kohn-Rostoker method. The build-up of spin moments has been explored in terms of total and projected density of states. Site-dependent Ni magnetic moment is also discussed. Heisenberg exchange coupling parameters are also computed to study the change in Ruderman-Kittel-Kasuya-Yosida type exchange interaction on increasing Ni concentration in the Ni-Mn-Sn system.

Performance of hybrid exchange-correlation potential for photocatalytic silver chromate and molybdate: LCAO theory and Compton spectroscopy

Linear combination of atomic orbitals (LCAO) method has been employed to compute the Mulliken's population (MP), energy bands, density of states (DOS), band gaps and electron momentum densities (EMDs) of Ag2TMO4 (TM = Cr and Mo). The LCAO prescription has been considered within the framework of density functional theory (DFT) and a newly suggested Hartree-Fock incorporated DFT scheme namely PBESOL0. These approximations for exchange and correlation potentials have been validated by comparing the deduced EMDs with the experimental Compton profiles using 137Cs radio isotope. It is observed that PBESOL0 scheme based EMDs show better agreement with the experimental profiles than other approximations considered in the present work. The energy bands and DOS confirm the semiconducting nature of both the Ag-based oxides. In addition, the relative nature of bonding in both the iso-electronic compounds has also been compared on the basis of equal-valence-electron-density profiles and MP data, which depict that Ag2MoO4 has more ionic character than that of Ag2CrO4.

Electronic and optical properties of CaCl2 using Compton scattering and density functional theory

In this paper electron momentum density of CaCl2 using 241Am Compton spectrometer is reported. The experimental Compton profile is compared with the corresponding theoretical profiles, computed using different exchange and correlation potentials within linear combination of atomic orbitals (LCAO) scheme. The hybrid WC1LYP approximation within LCAO scheme shows a better agreement with experimental CP than other reported theoretical profiles. Going beyond Compton line shapes, the energy bands, density of states using LCAO scheme and modified Becke-Johnson (mBJ) potential within full potential linearized augmented plane wave (FP-LAPW) method are also reported. The optical properties like, dielectric tensors, absorption spectra, reflectivity and refraction are also discussed using FP-LAPW method. Energy band gaps computed using LCAO-WC1LYP and FP-LAPW-mBJ methods are found to be 6.89 and 6.76 eV, respectively which are in reasonable agreement with the available experimental data.

Compton profiles of some composite materials normalized by a new method

Recently, we have shown that as a novel approach, in the case of samples which can be treated as pure incoherent scatterers, the effective atomic number Zeff itself could be conveniently used to normalize their un-normalized Compton profiles. In the present investigation, we have attempted to examine the efficacy of this approach. For this purpose, we have first determined the single differential Compton scattering cross sections (SDCS) of the elements C and Al as well as of some H, C, N and O based polymer samples such as bakelite, epoxy, nylon and teflon which are pure incoherent scatterers. The measurements were made at 120° in a goniometer assembly that employs a high resolution high purity germanium detector. The SDCS values were used to obtain the Zeff and the un-normalized Compton profiles. These Compton profiles were separately normalized with their Zeff values (for Compton scattering) as well as with the normalization constant obtained by integrating their Hartree-Fock Biggs et al Compton profiles based on the mixture rule. These two sets of values agreed well within the range of experimental errors, implying that Zeff can be conveniently used to normalize the experimental Compton profiles of pure incoherent scatterers.

Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice

The hydrogen mean force from experimental neutron Compton profiles is derived using deep inelastic neutron scattering on amorphous and polycrystalline ice. The formalism of mean force is extended to probe its sensitivity to anharmonicity in the hydrogen-nucleus effective potential. The shape of the mean force for amorphous and polycrystalline ice is primarily determined by the anisotropy of the underlying quasi-harmonic effective potential. The data from amorphous ice show an additional curvature reflecting the more pronounced anharmonicity of the effective potential with respect to that of ice Ih.

Temperature dependent magnetic Compton profiles and first-principles strategies of quaternary half-Heusler alloy Co1−xCuxMnSb(0 ⩽ x ⩽ 0.8)

Temperature dependent experimental Compton profiles of quaternary alloys Co1−xCuxMnSb (x = 0.0, 0.2, 0.6 and 0.8) when decomposed into constituent profiles show that the sp-electron spin polarization is antiferromagnetically coupled to Mn-3d moments. The orbital magnetic moments derived from combination of magnetic Compton profiles (MCPs) and magnetization measurements are found to be small. Moreover, the first-principles full potential-linearized augmented plane wave (FP-LAPW) calculations have been performed to validate the experimental investigations of spin moments and half-Heusler properties. Present experimental and theoretical work show major role of Mn atoms in building-up the absolute spin moments. Our MCP data and spin-projected density of states derived from FP-LAPW computations show an increase in sp-d interaction in conduction region on increasing the Cu concentration. Further, Ruderman–Kittel–Kasuya–Yosida-type hybridization and antiferromagnetic superexchange interactions are witnessed in the reported alloys.

Spin density in YTiO3 : II. Momentum-space representation of electron spin density supported by position-space results

Unpaired electrons in YTiO 3 ferromagnetic crystal (below 30 K) were studied by polarized neutron diffraction (PND) and incoherent x-ray magnetic Compton scattering (MCS). These experiments provide both position and momentum representations of the electrons at the origin of the magnetic behavior, mostly those in the t 2g state of Ti atoms. A two-dimensional reconstruction was conducted from experimental and theoretical directional magnetic Compton profiles to obtain the two-dimensional magnetic electron momentum density. A superposition method is proposed to examine the coherence between results for position and momentum spaces, respectively. This model-free approach allows a straightforward cross-checking of PND and MCS experiments. An isolated Ti model is proposed to emphasize the role played by O 1 in the ferromagnetic coupling between Ti and its neighboring atoms.

Structural, electronic and elastic properties of Zn 3 As 2

Abstract In this paper, we report ab initio studies of structural, electronic and elastic properties of a II-V group semiconductor compound, Zn3As2 along with its experimental Compton profile (CP). For experimental CP, 59.54 keV gamma ray (radiation source:5 Ci 241Am) spectrometer is used and to deal with the theoretical part, density functional theory, Hartree-Fock as well as the hybrid functional theory, implemented in CRYSTAL code, are used. A full structure optimization following an equation of state calculation is performed to get the structural parameters. Further directional CPs, band structure, density of states, Mulliken population analysis and elastic properties have also been computed. A concurrence-test with experimental CP proves hybrid scheme to provide the best results in the present case.

Investigation of Compton Profile of Molecular Propane**Supported by the National Natural Science Foundation of China under Grant Nos U1332204, 11274291 and 11504361, and the experiment was carried out in a beam time approved by Shanghai synchrotron radiation facility under Proposal Nos 11sr0210, 12sr0009 and 13SRBL15U15487.

The experimental Compton profile of the propane molecule is measured at an incident photon energy of 20 keV based on the third generation synchrotron radiation, and the statistical accuracy of 0.2% is achieved near pz = 0. The calculated Compton profile by the density functional theory with aug-cc-pVTZ basis set reproduces the experimental observation very well. The joint experimental and theoretical investigation provides the benchmark data of the electronic structure of propane.

High energy Compton spectroscopy and electronic structure of Laves phase ZrFe 2

Abstract We present the first-ever experimental Compton profile of Laves phase ZrFe2 using indigenous 20 Ci 137Cs Compton spectrometer. To annotate the experimental electron momentum density, we have calculated the theoretical Compton profiles using density functional theory (DFT) and hybridization of Hartree–Fock and DFT within linear combination of atomic orbitals (LCAO) method. The spin-polarized energy bands and density of states are computed using LCAO and full potential-linearized augmented plane wave methods. The revised Perdew-Burke-Ernzerhof functional (for solids) based theoretical profile gives a marginally better agreement with the experimental profile as compared to other approximations considered in the present work. The Fermi surface topology of ZrFe2 is explained in terms of majority- and minority-spin energy bands.