Formation and growth of an α-PbF2 phase during the plastic deformation of β-PbF2 crystals

The microstructure of strained β-PbF₂ crystals is investigated by optical and scanning electron microscopy. It is established that the α-PbF₂ previously observed by x-ray diffraction methods during the straining of cubic PbF₂ nucleates predominantly on structural defects like slip lines and bands. The kinetics of the growth of α-PbF₂ particles during the straining of β-PbF₂ crystals and during a postdeformation anneal is investigated. The results of the experiments point to the involvement of diffusion in the growth of α-PbF₂. Fiz. Tverd. Tela (St. Petersburg) 39, 640–646 (April 1997)     

First-principles calculations of the electronic structure of fluorite-type crystals (CaF2, BaF2, SrF2, and PbF2) containing Frenkel defects. Analysis of optical and transport properties

The electronic structure of the alkali-earth fluorides CaF₂, BaF₂, SrF₂, and PbF₂ with Frenkel defects is investigated in the tight-binding approximation by the LMTO method. The defect formation and migration energies are calculated. The electronic structure and optical excitations of a H center in a defective fluorite structure are examined. It is shown on the basis of calculations of the binding energies that CaF₂, BaF₂, and SrF₂ are ionic compounds, while the chemical bond in PbF₂ is partially covalent. Possible methods of displacement of interstitial fluorine atoms that lead to the observed optical spectra of an H center are investigated. Fiz. Tverd. Tela (St. Petersburg) 40, 2019–2025 (November 1998)     

Atomic and electronic structures of GaAs (001) surface

The atomic and electronic structures of α, β, β₂, and ζ reconstructions for the Ga-terminated GaAs (001)-(4×2) surface are investigated in the framework of the pseudopotential approach. Total, surface, and local densities of electron states, electron-energy spectra, and relative surface energies of the structures under consideration are calculated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 44–52, October, 2006.     

Mass spectrometric investigation of low-temperature diffusion and solubility of helium in lead fluoride crystals

Thermal desorption of helium from presaturated crystals was used for mass spectrometric investigations of the diffusion and solubility of helium in lead fluoride crystals in the temperature range 606–714K which precedes the transition of the crystal to the superionic state. The experimental apparatus and measurement method are described and mechanisms for the diffusion and solubility of helium in PbF₂ are discussed. Zh. Tekh. Fiz. 68, 85–89 (December 1998)     

On bound state computations in three- and four-electron atomic systems

A variational approach is developed for bound state calculations in three- and four-electron atomic systems. This approach can be applied to determine, in principle, an arbitrary bound state in three- and four-electron ions and atoms. Our variational wave functions are constructed from four- and five-body Gaussoids that respectively depend on six (r ₁₂, r ₁₃, r ₁₄, r ₂₃, r ₂₄, r ₃₄) and ten (r ₁₂, r ₁₃, r ₁₄, r ₁₅, r ₂₃, r ₂₄, r ₂₅, r ₃₄, r ₃₅ and r ₄₅) relative coordinates. The approach allows operating with the more than one electron spin functions. In particular, the trial wave functions for the ¹ S states in four-electron atomic systems include the two independent spin functions χ₁ = αβαβ + βαβα − βααβ − αββα and χ₂ = 2ααββ + 2ββαα − βααβ − αββα − βαβα − αβαβ. We also discuss the construction of variational wave functions for the excited 2³ S states in four- electron atomic systems.     

External bremsstrahlung spectra of the Sr source in some lead compounds measured using NaI detector

The spectral distributions of external bremsstrahlung (EB) excited by beta particles from a ⁹⁰Sr/⁹⁰Y source in thick target compounds PbCl₂, PbF₂, Pb(NO₃)₂ and CdO were measured using a 3.8 cm × 3.8 cm NaI(Tl) crystal. The spectra, unfolded using the Liden-Starfelt procedure, showed fairly good agreement with theory (Tseng and Pratt) at low energies and some deviation (less than 15%) at higher energies. The discrepancy between theory and experiment increases with modified atomic number of the target compound and photon energy. The application of the measured/theoretical spectrum to estimate the bremsstrahlung dose is also discussed.     

Low-temperature X-ray and photoluminescence of lead halide crystals

The emission spectra of PbF₂, PbCl₂ and PbBr₂ monocrystals are measured under optical and X-ray excitation at liquid helium temperature. Certain emission bands are attributed to the radiative transitions in the self-trapped cation exciton (excited Pb²⁺ion). The increase of trapped exciton via electron-hole recombination is discussed.     

X-ray determination of thermal expansion of cadmium fluoride and lead fluoride

The lattice parameters of CdF₂ andβ-PbF₂ have been determined over the temperature range 300–670 K. The coefficient of expansion at room temperature is 21·3 × 10⁻⁶ K⁻¹ and 25·4 × 10⁻⁶ K⁻¹ for CdF₂ and PbF₂ respectively and it increases linearly with temperature over the range of temperature covered. The Grüneisen parameter decreases with temperature in both the crystals.     

Solvothermal preparation of nano-β-HgS from a precursor,bis(dibenzyldithiocarbamato)mercury(II)

A simple solvothermal method for the selective synthesis of β-HgS (meta cinnabar) nanoparticles in aqueous solutions is reported with bis(dibenzyldithiocarbamato)mercury(II) as the precursor. Crystal structure, size, morphology and composition of the products are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, high-resolution transmission electron microscopy (HRTEM), SAED and X-ray photoelectron spectroscopy (XPS). PXRD shows (111), (220), (200), (311), (222), (400), (331), (420) reflections characteristic of β-HgS. SEM micrographs display the spherical nature of the nano-β-HgS. EDX analysis showed the presence of Hg and S. HRTEM images indicate the spherical nature of the nanoparticles with their size in the range of 10–15 nm and the FFT pattern shows the crystalline nature of the spherical particles. The results are in agreement with those estimated from the XRD pattern. XPS signals observed at 162.6 and 162.8 eV are due to S2p _3/2 and S2p _1/2 electrons and the S2s was observed at 222.3 eV. The band gap of nano-β-HgS has been found to be 3.6 eV from the UV–visible spectral measurement. The blue-shifted band gap compared to the bulk HgS is a consequence of “size quantization” effect. A comprehensive characterization of the precursor by IR and single crystal X-ray crystallography shows the presence of HgS₄ coordination environment, with a distinct Hg–S bond asymmetry.     

Crystal structure of ethyl (2Z)-2-(3-methoxy-4-acetyloxy benzylidene)-7-methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate

The title compound, C₂₆H₂₄N₂O₆S, (I), crystallizes in the monoclinic space group, P2₁/c, with cell parameters a = 16.248(1), b = 7.927(1), c = 19.371(4) ?, β = 105.295(2)°, Z = 4. The central pyrimidine ring in the compound (I) is significantly puckered, assuming a screw-boat conformation. The C11–C16 benzene ring stands vertical while thiazole and C18–C23 benzene rings are coplanar to the mean plane of pyrimidine ring having dihedral angles of 87.48(12), 3.63(11) and 0.94(12)°, respectively. In the absence of potential hydrogen bonding interaction, the crystal packing is influenced by intramolecular C-H…S interaction and intermolecular C-H…π interactions.     

Electronic and electronic-vibrational phenomena in the new organic conductor ϰ-(ET)2[Hg(SCN)2Cl] with a metal-insulator transition

The reflection spectra and optical conductivity spectra of the new organic conductor ϰ-(ET)₂[Hg(SCN)₂Cl] with a metal-insulator transition in the spectral regions 700–5500 and 9000–40 000 cm⁻¹ have been studied in polarized light at 300 K. A comparisonis made between the spectra obtained and the corresponding spectra of related isostructural conductors based on the ET molecule, and also the properties of the crystal structure of the investigated compounds. An electronic transition between the ET molecules of the dimer (ET) ₂ ⁺ in the spectral region 700–5500 cm⁻¹ has been identified, as have the features of the electronic-vibrational structure arising as a consequence of the interaction of this transition with the completely symmetric intramolecular vibrations of the ET molecule. It is found that the conductor with the stronger dimer interaction between the ET molecules has the higher the transition temperature. Fiz. Tverd. Tela (St. Petersburg) 39, 1313–1319 (August 1997)     

Focal switch of cosine-Gaussian beams

The focal switch of cosine-Gaussian (CsG) beams passing through a system with the aperture and lens separated is studied analytically and numerically. It is shown that the focal switch of CsG beams can appear not only for the apertured case, but also for the unapertured case. The necessary condition for the focal switch is that truncation parameter α > α_c and the beam parameter β > β_c, α_c, β_c being the corresponding critical values. There exists a maximum of the relative transition height Δz _sw as α varies, and Δz _sw increases with increasing β and decreasing N _w. The normalized axial intensity minimum I _min / I _max decreases with an increase of α and β, and I _min / I _max remains unchanged as N _w varies.     

Cohesion in lead chloride

The Born model is used as a method to assess the ionicity and the extent of covalent contribution to the binding in lead chloride. In the process, the Madelung constant (11·2429 with respect to cube root of unit cell volume) and cohesive energy (W _L) (484 kcal/mole) are evaluated. The theoreticalW _L, when compared with that derived from a thermochemical cycle indicates that PbCl₂ can be considered to be ‘predominantly ionic’.     

Investigations of EPR parameters and impurity structure for Co<Superscript>2+</Superscript> in Ca(OH)<Subscript>2</Subscript> crystal

The electron paramagnetic resonance (EPR) parameters (g _‖ andg _⊥ factors and hyperfine structure constantsA _‖,A _⊥) for Co²⁺ in Ca(OH)₂ are studied from the second-order perturbation formulas on the basis of the cluster approach. In these formulas, the contributions to EPR parameters from the state interactions and covalency effects are considered and the parameters related to both effects are obtained from the optical spectra and impurity structure of the studied system. From the study, it is found that the β angle between the metal-ligand bond and the C₃ axis changes from 61° in a pure crystal to 53.68(26)° in the impurity center of a Co²⁺-doped Ca(OH)₂ crystal because of the impurity-induced local lattice relaxation. The reduction of the angle β in the impurity center is also supported by the result obtained by analyzing the EPR zero-field splitting for Mn²⁺ in the same Ca(OH)₂ crystal. The EPR parameters of Ca(OH)₂:Co²⁺ are also reasonably explained by considering the suitable local lattice relaxation.     

Effect of water vapors on the luminescence of cation-exchange membranes modified by Pt(II) and Ru(II) complexes and Nile blue

The surface of a cation-exchange membrane was modified by the [PtEnPpy]⁺, [PtEnBt]⁺, [PtEnTpy]⁺, [RuBpy₃]⁺², and NB⁺, (En is ethylenediamine; Ppy, Bt, Tpy are α-deprotonated forms of 2-phenylpyridine, 2-phenylbenzothiazole, and 2-(2′-thienyl)pyridine, respectively; Bpy is 2,2′-bipyridyl, and NB⁺ is Nile blue) ions, which exhibit intense luminescence. It is found that the quenching of the luminescence of the modified cation-exchange membrane by water vapors depends on the nature of the excited electronic state of the immobilized cation.     

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)     

Influence of thermal treatment in oxidizing and reducing atmospheres on the composition and properties of solid solutions based on bismuth vanadate: (Bi,La)4(V,Me)2O11?y (Me = Ga, Zr, Cu)

Ceramic solid solutions (Bi_{1 − y} La _y )₄(V_{1 − x} Me _x )₂O_{11 − z} (x, y < 0.2: Me = Zr, Ga, Fe, Cu) were prepared by solid-state reaction. It was shown that the annealing (973 K, reducing atmosphere H₂/Ar (20/80)) of the samples whose compositions belong to the stability domains of α, β, and γ′ polymorphs increases their electronic conductivity by six orders of magnitude. The samples with low concentrations of dopant cations exhibited good compositional stability and a reversible change in their structure parameters. At the same time, the solid solutions with a high concentration of lanthanum cations and/or copper cations (y, x ∼ 0.1) underwent partial decomposition.     

A Possible Nuclear Spin Dewar. Hyperfine Interactions of Short-Lived β Emitter 8Li and 12B in TiO2

It was confirmed by detecting the β-NMR of ¹²B (I ^π=1⁺, T _1/2=20.2 ms) in a TiO₂ (rutile) crystal that the nuclear spin polarization of ¹²B was totally maintained in the crystal as produced through a nuclear reaction before implantation. Two locations, site 1 and site 2, were found with the relative populations 9 and 1, respectively, and the electric field gradients (EFGs) at those sites were obtained to be q(site 1)=+(37.1±0.5)10¹⁵ V/cm², η(site 1)<0.03, q(site 2)=+(185±5)10¹⁵ V/cm² and η(site 2)=0.62±0.02. We also found that about 30% of the initial polarization of ⁸Li (I ^π=2⁺, T _1/2=838 ms) was maintained in the crystal. Since the polarizations of other β emitting nuclei, ^12,16N, ^13,19O, and ⁴¹Sc were also totally maintained in the crystal, the crystal can be a “Spin Dewar” in which many short-lived nuclides can be implanted with their polarizations totally maintained during their lifetimes for the studies not only on the electronic structure of impurities in it but also on the nuclear properties. This revised version was published online in September 2006 with corrections to the Cover Date.     

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)     

Third-order elastic moduli of single-layer graphene

In the framework of the Keating model with allowance made for the anharmonic constant of the central interaction between the nearest neighbors μ, analytical expressions have been obtained for three third-order independent elastic constants c _ijk (μ, ζ) of single-layer graphene, where ζ = (2α − β)/(4α + β) is the Kleinman internal displacement parameter and α and β are the harmonic constants of the central interaction between the nearest neighbors and the noncentral interaction between the next-nearest neighbors, respectively. The dependences of the second-order elastic constants on the pressure p have been determined. It has been shown that the moduli c ₁₁ and c ₂₂ differently respond to the pressure. Therefore, graphene is isotropic in the harmonic approximation, whereas the inclusion of anharmonicity leads to the appearance of the anisotropy.