Influence of parity-forbiddenness in the onset of mixed valence state in Sm1−x GdxS

The electronic structure of Sm and Gd in Sm_1−x Gd_xS has been experimentally studied by the x-ray shift method. Besides the well known strong increase of Sm valence at x≈0.15 (associated with the electronic transition to the mixed valence state), a decrease of Sm valence for x⩾0.9 has been observed for the first time. This phenomenon is explained as due to the 4f-5d hybridization on neighboring Sm atoms involved in the onset of the mixed valence state, and is interpreted as a manifestation of parity violation forbiddenness. Fiz. Tverd. Tela (St. Petersburg) 39, 1017–1019 (June 1997)     

Study of matter at high pressure using small angle scattering and depolarization of neutrons

Abstract

The method of studying matter under high pressure using the measurement of the transmitted neutron beam is proposed. Method is based on the very small angle scattering and depolarization of neutrons combined with the anvil pressure technique. Test experiments have been carried out demonstrating feasibility of the method for studying phase transitions accompanied with the change of density or magnetization and equations of state. The obtainable pressure range of the method is discussed.     

Electronic structure and spectra of NH-isomers of isobacteriochlorin

Using the restricted and unrestricted Hartree-Fock method (RHF and UHF) with the AM1 Hamiltonian, we have calculated the geometric structure of the NH-isomers of isobacteriochlorin and its meso-cyano derivative. We have used the CNDO/S method for all the considered structures to calculate the transitions to excited electronic states. We have analyzed the effect of taking into account electron correlation on the results of the calculations of the chemical and geometric structure and the electronic spectra. For the a isomer with the imino hydrogens opposite each other, we obtained a planar structure corresponding to literature x-ray diffraction data. For the aromatic c isomer and other isomers with the imino hydrogens adjacent to each other, the calculations lead to a substantial nonplanarity of the macrocycle (defined mainly by the tilt of the pyrrole rings) and also deviation of the NH bond from the plane of the pyrrole ring. The ground-state energies of the a and c isomers have close values, which generally explains the observed NH tautomerism. We calculated the dipole moments of the isomers in the ground state and the excited state. The results of the calculation of the characteristics of the excited states with UHF geometry on the whole seem to be preferred over those with RHF geometry. We show that only for the aromatic c isomer can we describe the electronic absorption spectrum within a four-orbital model. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 232–245, March–April, 2006.     

Electronic structure of phosphate glasses with a complex oxygen sublattice structure

Calculations of the electronic structure of phosphate glasses are performed in the MO LCAO x _α approximation discrete variation method. On the basis of an analysis of theoretical and experimental electronic spectra of the system BeO-P₂O₅ regularities are found in the formation of the valence band of alkaline-earth phosphate glasses with different types of anion sublattice. Data on the electronic structure are used to refine the models of short-range order; in particular, the possibility of oxygen in the threefold coordination state is confirmed. With the features of the spectrum of electronic states taken into account, localization of charge carriers, the nature of the optical transitions, and hole-transport phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1366–1372 (August 1997)     

Effect of the degree of ordering of structural vacancies on the fine structure of the valence-band top in cubic boron nitride

The electronic energy structure of the defect system of c-BN_x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic structure of nonstoichiometric boron nitride c-BN_0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation. Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997)     

The magnetic structure of neodymium crystal below the néel temperature

The phase transition in neodymium crystal from the paramagnetic to the magnetically ordered phase at the temperature T _N = 19.9 K is discussed in the terms of Landau's theory. It is shown that the calculated six-dimensional active representations at the point q = vb ₁, with—0.5 < v < 0.5, v ≠ 0, induce transitions to “single-q”, “double-q” and “triple-q” spin structures. A general formula for the spin vector S(r) for T < T _N is derived, from which a variety of spin structures can be obtained depending on the choice of the free parameters. The spin structure calculated by Bak and Lebech (1978) is obtained from that formula as a particular solution which is compatible with Landau's theory. An antiferromagnetic spin structure which represents a modification of that given by Bak and Lebech (1978) is calculated. The corresponding cross-section for neutron elastic magnetic scattering fulfils the experimental conditions listed by Lebech (1981). It is shown that the magnetic satellites of the type (h ± v, 0, 0) cannot appear for any type of structure following from our general formula for S(r). These satellites therefore can be connected only with representations at the point q = μ₁ b ₁ + μ₂ b ₂,—0.5 < μ₁, μ₂ < 0.5, μ₁, μ₂ ≠ 0, μ₁ ≠ μ₂.     

Phase coexistence in proton glass

Abstract

Proton glasses are crystals of composition M_1?x(NW₄)_xW₂A0₄, where M = K,Rb,Cs, W = H,D, A = P, As. For x = 0 there is a ferroelectric (FE) transition, while for x-1 there is an antiferroelectric (AFE) transition. In both cases, the transition is from a paraelectric (PE) state of tetragonal structure with dynamically disordered hydrogen bonds to an ordered state of orthorhombic structure. For an intermediate x range there is no transition, but the hydrogen rearrangements slow down, and eventually display nonergodic behavior characteristic of glasses. We and others have shown from spontaneous polarization, dielectric permittivity, nuclear magnetic resonance, and neutron diffraction experiments that for smaller x there is coexistence of ferroelectric and paraelectric phases, and for larger × there is coexistence of antiferroelectric and paraelectric phases. We present a method for analytically describing this coexistence, and the degree to which this coexistence is spatial and/or temporal. We discuss also the experimental determination of these coexistence parameters.     

Thorium: Phase transformations and equation of state to 300 GPa

Abstract

Equation of state and phase transformations of thorium metal have been investigated to 300 GPa at 300 K in a diamond anvil cell using energy dispersive X-ray diffraction employing synchrotron source. Phase transformations in the 70–100 GPa range indicative of 5f-electron bonding are observed and thorium metal is isostructural with its 4f counterpart cerium at ultra high pressures. The measured static equation of state of thorium to 300GPa (volume fraction V/V _o = 0.40) at 300K is given. At high pressures, the sd to f electronic transfer has significant influence on the measured equation of state of thorium.     

Temporal electronic structure of non-resonant Raman excited virtual state of P-nitroaniline by 514 nm excitation via bond polarisabilities

We have studied the temporal bond polarisabilities of para-nitroaniline from the Raman intensities by the algorithm proposed by Wu et al.in 1987 (Tian B,Wu G,Liu G 1987 J.Chem.Phys.87 7300).The bond polarisabilities provide much information concerning the electronic structure of the non-resonant Raman excited virtual state.At the initial moment by the 514.5 nm excitation,the tendency of the excited electrons (mapped out by the bond polarisabilities) is to spread to the molecular periphery,and the electronic structure of the Raman virtual state is close to the pseudo-quinonoidic state.When the final stage of relaxation is approached,the bond polarisabilities of those peripheral bonds relax faster than those closer to the molecular core,the phenyl ring.The molecule is in the benzenoidic form as demonstrated by the bond polarisabilities after relaxation.     

Ultrahigh pressure equation of state of tantalum to 310 GPa

ABSTRACT

The isothermal compression of transition metal tantalum (Ta) was studied in a diamond anvil cell by X-ray diffraction utilizing rhenium (Re) and gold (Au) as internal X-ray pressure standards. The Re pressure marker was employed during non-hydrostatic compression to pressures up to 310?GPa while the Au pressure marker was used during quasi-hydrostatic compression in a neon pressure-transmitting medium to 80?GPa. Two ultra-high pressure experiments were conducted on Ta and Re mixtures utilizing focused-ion beam machined toroidal diamond anvils with central flats varying from 8 microns to 16 microns in diameter. The Ta metal was observed to be stable in the body-centered-cubic phase to a volume compression V/V₀?=?0.581. The measured equations of state (EOS) of Ta using two different calibrations of the Re pressure marker are compared with the ambient temperature isotherm derived from shock compression data. We provide a detailed analysis of EOS fit parameters for Ta under quasi-hydrostatic and non-hydrostatic conditions.     

Formation of ultracold molecules via photoassociation with blue detuned laser light

We propose a new possibility to form ultracold molecules, via photoassociation of a pair of cold atoms into vibrational levels of the external well of an excited electronic state located at intermediate interatomic distance ( ≈ 20 Bohr radii), and embedded in the dissociation continuum above its dissociation limit. The existence of such a well is demonstrated by conventional free-free absorption spectroscopy at thermal energies. Estimation for cold atom photoassociation and cold molecule formation rates are obtained within a perturbative approach [Drag et al., IEEE J. Quant. Electr. 36, 1378 (2000)], and are found observable for usual conditions of photoassociation experiments. Received 1st March 2001     

Electronic structure and molecular spectroscopic constants of ScN and ScP investigated by several quantum chemistry methods

The electronic structure of the ScN and ScP molecules is a subject of controversy and turns out to be a challenging problem in quantum chemistry. We show that the ground-state electronic structure for both molecules depends critically on the choice of methods used which incorporate different ways of accounting for electron correlation. A parallel ab initio, DFT and TD-DFT study is performed for this purpose and uses sufficiently flexible basis sets able to reproduce accurate electronic structures, as well as correct spectroscopic constants.

In the ab initio methodology, results have been obtained with methods such as Hartree-Fock (HF), M?ller-Plesset perturbation theory (MPn), direct configuration interaction (CI), quadratic configuration interaction (QC), coupled cluster configuration interaction (CC), complete active space self-consistent field (CASSCF) and multireference configuration interaction (CIPSI) methods. In the DFT methodology, various ‘pure’ and ‘hybrid’ density functionals are used and the corresponding results are compared to sophisticated ab initio methods and to available experimental data.

All the methods used show that the ground state of both molecules is ¹Σ⁺, but two electronic structure natures, ¹Σ⁺ open-shell or ¹Σ⁺ closed-shell, are competitive and depend on the method employed. All the ab initio methods based on a single determinant wavefunction suffer seriously in predicting clearly the exact nature of the ground state or its correct structural and spectroscopic parameters. However, the ab initio methods based on a multiconfigurational wavefunction appear to be successful in describing correctly, within one shot, the electronic structure and the molecular spectroscopic constants. The ground state, particularly for the ScN molecule, presents an unusual electronic structure: the presence of degenerate determinants, quasidegeneracy with other states and one avoided crossing in the region around the equilibrium distances. The bonding of the ground state is a two open-shell ¹Σ⁺ state described as a π double bond and a Σ dative bond; the real triple bond ¹Σ⁺ state, i.e. closed-shell state, is found to lie higher in energy. The potential energy curves of the lowlying electronic states, the derived electronic structures and various molecular spectroscopic constants are presented and discussed for each method employed.     

硫化锰电子结构的软X射线共振非弹性散射研究

利用软X射线共振非弹性散射谱(resonant inelastic soft X-ray scattering, RIXS)对3d过渡金属硫化物中的硫化锰(MnS)电子结构进行了研究.通过分析Mn²⁺的2p⁶3d⁵→2p⁵3d⁶→2p⁶3d⁵二次光子过程，得到了共振非弹性散射谱中的两类非弹性峰，d-d电子跃迁和电荷转移(charge-transfer)跃迁.这两部分跃迁分别共振增强于L边附近及伴随峰附近.基于Hartree-Fock方法的多重态计算分别模拟了原子近似下和立方体O_h对称群下共振非弹性散射谱及吸收谱.计算得MnS实际晶体场10Dq值介于0.80eV—0.85eV之间.对MnS和MnO CT跃迁差异的讨论表明MnS较强的CT跃迁来源于其较窄的能隙宽度. 关键词： 软X射线共振非弹性散射 软X射线吸收谱 d-d跃迁 电荷转移     

Features of the electronic structure of the intermetallic Co3Ti in an atomically disordered state

Using a method based on incorporating statistics into the theory of multiple scattering via the distribution function of atoms on lattices, the electronic structure of the intermetallic Co₃Ti is calculated for both complete atomic order and complete disorder. An unusual feature of the electronic structure of this intermetallic when it is disordered in the solid state is identified. Possible consequences of this behavior are analyzed. Fiz. Tverd. Tela (St. Petersburg) 39, 809–810 (May 1996)     

Crystal imperfections and structural phase transitions in perovskites

High‐resolution X-ray scattering measurements of the antiferrodistortive phase transitions in the perovskites SrTiO₃, RbCaF₃ and KMnF₃ have recently revealed the existence of two length scales in the critical scattering above T_e . Here we review these observations and discuss how they might be related to the well known existence of two time scales (the "central peak" problem) in the critical dynamic response function above T_e . We reach the tentative conclusion that within a few degrees of T_e both the time and length scales of the fluctuations are strongly influenced by defects and that the intrinsic critical behaviour is swamped by these effects in most crystals.     

Nature of the electronic states involved in the chemical bonding and superconductivity at high pressure in SnO

We have investigated the electronic structure and the Fermi surface of SnO using density functional theory calculations within recently proposed exchange-correlation potential (PBE + mBJ) at ambient conditions and high pressures up to 19.3 GPa where superconductivity was observed. It was found that the Sn valence states (5s, 5p, and 5d are strongly hybridized with the O 2p states, and that our density functional theory calculations are in good agreement with O K-edge X-ray spectroscopy measurements for both occupied and empty states. It was demonstrated that the metallic states appearing under pressure in the semiconducting gap stem due to the transformation of the weakly hybridized O 2p-Sn 5sp subband corresponding to the lowest valence state of Sn in SnO. We discuss the nature of the electronic states involved in chemical bonding and formation of the hole and electron pockets with nesting as a possible way to superconductivity.     

New ferromagnetic compound CaCo2 (C15) synthesized at high pressure

The compound CaCo₂ with the C15 cubic Laves phase structure and an estimated density of 5.21 g/cm³ has been synthesized at 8.0 GPa pressure. Magnetization measurements showed that the compound CaCo₂ is a ferromagnet with Curie temperature 528 K and magnetic moment per Co atom 1.75 μ_B at T=4.2 K. LMTO calculations of the electronic band structure showed that CaCo₂ forms as a result of an s-d electronic transition of Ca and in the ground state it is a ferromagnet with a high magnetic moment per Co atom. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 864–869 (25 December 1998)     

Influence of pressure on electron spectra of metals and semiconductors

Abstract

We have already reported the results of direct observations of electron-topological phase transition (ETT) in cadmium'. The appearance of new dHvA-frequencies corresponding to the Fermi surface (FS) change, i.e. restoring of folding of hole “monster” and electron “needle” appearance is observed under pressure. In t h i s report we are going to enlarge on the ETT consequences study in cadmium-on the advent of anomalous electronic features in transverse magnetoresistance and thermoelectric power.     

Volume dependence of the raman frequencies in Y Ba2Cu3Ox single crystals with different oxygen content

Abstract

The Raman spectra of Y B _a2C_u3O_x, single crystals (x=6.25; 6.75; 7.0) were measured at pressures up to 22 GPa (35 GPa for x=6.25) at room temperature in nearly hydrostatic conditions. The frequency-volume curves for most of the Raman-active fundamental vibrations were derived from the present data making use of a previous high-pressure study of the equations of state of Y Ba₂ Cu ₃ O _x compounds.