Influence of γ irradiation on the optical and mechanical properties of langasite

The optical (transmission and circular dichroism) spectra and mechanical (Vickers microhardness and fracture toughness K _1c ) properties of langasite La₃Ga₅SiO₁₄ crystals have been studied after γ irradiation and exposure for a month. It is shown that, as a result of irradiation crystals become more transparent in the range 310–640 nm, nonstructural defects with the energies of excited states in the range 2.06–4.13 eV decay and new structural defects with the excited-state energies in the range 4.14–5.00 eV are formed. Irradiation does not lead to a change in microhardness, while the coefficient K _1c increases from 0.32 to 0.36 MPa m^1/2.     

Calculation of the Fourier Coefficients of the Generalized Spherical Functions

Generalized spherical functions which are used in texture analysis may be developed into Fourier series. The coefficients a of this series may be expressed as a simple product of much smaller a number of other coefficients Q. This may greatly simplify the numerical calculation of these functions and hence also of the texture function itself.     

Crystal chemistry and prediction of compounds with a structure of skutterudite type

Crystallochemical analysis of skutterudite-type structures in the BX ₃, AB ₄ X ₁₂, and AB′₃ B ₄O₁₂ compositions (A and B are metals; X = P, As, Sb) has been performed. Probable regions of structure formation are determined, thereby indicating that more than 270 new compounds of the AB ₄ X ₁₂ composition can be synthesized.     

Correlation between the lattice parameters of crystals with perovskite structure

The correlation \((\bar a_2 + \bar a_3 ) = (\bar a_1 + \bar a_4 )\) has been established for the mean parameters of the reduced perovskite unit cells, \(\bar a_1 ,\bar a_2 ,\bar a_3 \) and \(\bar a_4 \) of four arbitrary compounds with the perovskite structure having the compositions (1) ABX₃, (2) A′BX₃, (3) AB′X₃, and (4) A′B′X₃, where A, A′ and B, B′ are the pairs of metal atoms with equal valences and X is an oxygen or halogen atom. The above correlation is consistent with the model of harmonic quasi-elastic atomic interactions.     

Quantum levitation of nanoparticles seen with ultracold neutrons

Analyzing new experiments with ultracold neutrons (UCNs) we show that physical adsorption of nanoparticles/nanodroplets, levitating in high-excited states in a deep and broad potential well formed by van der Waals/Casimir-Polder (vdW/CP) forces results in new effects on a cross-road of the fields of fundamental interactions, neutron, surface and nanoparticle physics. Accounting for the interaction of UCNs with nanoparticles explains a recently discovered intriguing so-called “small heating” of UCNs in traps. It might be relevant to the striking conflict of the neutron lifetime experiments with smallest reported uncertainties by adding false effects there.     

Investigation of X-ray natural circular dichroism in a CsCuCl<Subscript>3</Subscript> single crystal: Theory and experiment

X-ray natural circular dichroism in the near-Cu K edge in a CsCuCl₃ single crystal, caused by dipole-quadrupole (E1E2) transitions between electronic states of different parity, has been experimentally and theoretically investigated. Numerical simulation of the experimental X-ray absorption spectrum and circular dichroism has been performed. It is established that a circular dichroism signal is observed in the pre-edge spectral region; therefore, it is mainly due to the localized electronic p-d states.     

Crystal structures of double cesium zirconium and barium zirconium orthophosphates

Double cesium zirconium and barium zirconium orthophosphates with the compositions CsZr₂(PO₄)₃ and Ba_0.5Zr₂(PO₄)₃, respectively, were prepared by thermal treatment of stoichiometric gels. The samples were characterized by electron-probe microanalysis and powder X-ray diffraction analysis. The crystal structures of both phosphates were refined by the Rietveld method within the space groups R\(\bar 3\)c and R\(\bar 3\), respectively, with isotropic atomic displacement parameters for all atoms.     

Investigation of zirconium phosphate Zr<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>4</Subscript> during heating

Zirconium phosphate Zr₃(PO₄)₄ has been synthesized by the sol-gel technique and investigated using X-ray powder diffraction, IR spectroscopy, and differential scanning calorimetry. It has been established that the symmetry of the unit cell, R \(\bar 3\) c, which is characteristic of the NaZr₂(PO₄)₃ (NZP) family, is lowered to P \(\bar 3\) c. The behavior of the zirconium phosphate during heating has been examined using high-temperature X-ray diffraction at temperatures ranging from 25 to 575°C. It has been revealed that the structure of the zirconium phosphate is hardly subjected to expansion due to heating in the temperature ranges 25–125°C (α _a < 1 × 10^?6 K^?1, α _c < 1 × 10^?6 K^?1, Δα < 1 × 10^?6 K^?1) and 325–575°C (α _a = ?1.4 × 10^?6 K^?1, α _c < 1 × 10^?6 K^?1, Δα < ?2.4 × 10^?6 K^?1). In the temperature range 125–325°C, the synthesized compound undergoes a second-order phase transition (upon heating), which is accompanied by the contraction of the structure along all crystallographic directions. Upon cooling in the range from 75 to 25°C, the phase transition is accompanied by the expansion of the structure.     

A necessary condition for the formation of a bisystem molecular crystal

A hypothesis is formulated which states that the necessary condition for the existence of a bisystem molecular crystal is the approximate equality of the total energies of the interactions of two symmetrically independent molecules N and N′ with their environment in the crystal structure (U_Σ). The hypothesis was tested on four crystalline organic compounds of the structure class P2₁/c, Z = 8(1, 1). The computations in the atom-atom approximation demonstrated that the energies U_Σ(N) and U_Σ(N′) have very close values despite the different coordination numbers of the molecules and a difference in the structures of the molecular agglomerates that include the molecules N and N′.     

X-Ray Natural Circular Dichroism in Langasite Crystal at the Ga and La Edges

X-ray natural circular dichroism (XNCD) and its structural nature have been investigated in a langasite (La₃Ga₅SiO₁₄) crystal at an incident radiation energy close to the Ga K-absorption edge and the La L_2,3-absorption edges. An XNCD signal was observed mainly beyond the absorption edge, which confirmed the existence of delocalized mixed p–d electronic states in Ga and d–f and d–p electronic states in La. Calculations with application of the multiple scattering method have made it possible to separate the contributions from three crystallographically nonequivalent Ga sites to the absorption spectrum and the XNCD signal and explain adequately the largest contribution of the Ga atom occupying the 1a site to the XNCD signal.     

Single Crystal X-ray Diffraction Study of a Pure Natrolite Sample

The crystal structure of a pure natrolite sample, Na₂(Al₂Si₃O₁₀)·2H₂O, coming from Asheken, Ethiopia, has been analysed by single crystal X-ray diffraction. It crystallizes within the orthorhombic space group Fdd2, with the following cell constants: a = 18.2930(2) Å; b = 18.6430(5) Å; c = 6.5860(5) Å; V = 2246.07(18) Å³. The three-dimensional framework of this hydrated aluminosilicate zeolite is made up by chains of corner-sharing SiO₄ and AlO₄ tetrahedra down c; the chains are held together by sharing the external vertices of tetrahedra; water molecules and Na⁺ extra-framework cations fill up the resulting cavities, the latter forming irregular NaO₆ octahedra. Hydrogen bonds complete the array.     

On the existence of declared 9<Emphasis Type="Italic">R</Emphasis> phase in Fe–Ni invar alloy

An analysis of recently reported electron diffraction patterns suggests that metastable austenitic Fe–32Ni alloy subjected to α → γ transformation upon slow heating does not exhibit any signs of formation of the 9R phase; the conventional nanocrystalline γ phase with an fcc lattice is formed instead. Extended lamellae with a layered structure, erroneously identified as a new phase of the (3R + 9R) type in Fe–32Ni alloy, are conventional twinning (midrib) regions of each initial α crystal, in which γ-phase twin nanolamellae are formed upon heating.     

Formation conditions and temperature dependence of forbidden reflections during resonant X-ray diffraction in zinc oxide single crystals

Different mechanisms of formation of purely resonant reflections hhl (h = 2n + 1) during resonant diffraction of X-ray synchrotron radiation in zinc oxide, whose intensity very strongly changes with an increase in temperature, are considered. Such reflections, forbidden in the dipole-dipole scattering, can be excited owing to the dipole-quadrupole scattering and the anisotropy of X-ray susceptibility induced by thermal motion of atoms and point defects. Different contributions are theoretically considered and it is shown that they can be separated owing to their different temperature dependences.     

Study of the Radiation Resistance of Endohedral Fullerenes of Rare-Earth Elements and Their Water-Soluble Derivatives

Endohedral metallofullerenes Me@C_2n (n = 30–50), their hydroxylated derivatives Me@C_2n(OH)_38–40 (Me = Tm, Ho, Eu, Sm, Co), and fullerenol С₆₀(OH)₃₈ have been synthesized The radiation resistance of these structures under irradiation in reactor by thermal and fast neutron fluxes in the fluence range of 1018–1019 cm^–2 (cadmium ratio of ~10) has been analyzed. Endofullerenols are shown to be more stable in comparison with initial endohedral fullerenes; the molecules containing Eu and Sm (atoms characterized by the largest slow-neutron absorption cross section) turned out to be most stable. The mechanism of the formation of secondary Eu and Sm endofullerenols is discussed taking into account the neutron capture and emission of γ quanta by excited nuclei, which acquire the recoil energy.     

Refinement of the composition and structure of YBaCo<Subscript>4−<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>7+δ</Subscript> crystals

YBa(Co_{4 ? x} Al _x )O_{7 + δ} (114Y) crystals have been grown in the Y-Ba-Co-O system by spontaneous crystallization from a slowly cooled nonstoichiometric melt. To change the oxygen content, the crystals were isothermally annealed in air in the range of 280–490°C. The crystals grown were characterized by scanning electron microscopy and powder X-ray diffraction. According to the data of an X-ray spectroscopic quantitative microprobe analysis, the average compositions of “as-grown” and oxygen-saturated crystals were Y_1.04Ba₁Co_3.54Al_0.50O_7.8 and Y_1.02Ba₁Co_3.55Al_0.51O_8.4, respectively. The refinement of the crystal structure after saturation on an automatic Bruker X8APEX diffractometer with a CCD detector (MoKα radiation, graphite monochromator, θ_max = 32.54°, sp. gr. P6₃ mc, a = 6.2746(9), c = 10.257(3) Å, V = 349.71(13) ų, Z = 2, d _calcd = 5.220 g/cm³) reveals the location of Al in two independent positions of Co atoms and yields the general formula of the compound as YBaCo_3.26Al_0.74O₇. Problems related to the difference in the compositions obtained by different methods are discussed.     

Conditions for single-crystal formation during bridgman growth

The microstructure of IV–VI crystals grown by varying the shape of the conical bottom of a ampoule, the melt superheating ΔT⁺ with respect to the liquidus temperature of a blend, the axial temperature gradient in a furnace ΔT, and the pulling rate of a ampoule v were studied. It was found that the nucleation of a single-crystal seed is most likely under the following conditions: ΔT ≤ 20 K/cm, ΔT⁺ < 30 K, and v < 0.4 mm/h. The conical bottom of a ampoule should have walls of constant thickness. The principle of geometric selection of a single-crystal seed is not efficient. The obtained results are discussed with regard to data on the structure and behavior of IV–VI melts upon heating and cooling.     

Structural chemistry of peroxo compounds of group VI transition metals: I. Peroxo complexes of chromium (a review)

The specific features revealed in the structure of the d ³ Cr(III), d ² Cr(IV), d ¹Cr(V), and d ⁰ Cr(VI) peroxo complexes with the ratios M:O₂ = 1:1, 1:2, and 1:4 are considered. It is noted that, in eleven compounds of the general formula Cr(O₂)_nO_m A _p (n = 1, 2, 4; m = 0, 1; p = 0–4), the metal atoms can be in four oxidations states: +3 (d ³), +4 (d ⁴), +5 (d ¹), and +6 (d ⁰). This property distinguishes chromium peroxo compounds from molybdenum and tungsten dioxygen complexes, which, with one exception, are represented by the d ⁰ M(VI) compounds.     

Comparative analysis of spatial organization of laccases from <Emphasis Type="Italic">Cerrena maxima</Emphasis> and <Emphasis Type="Italic">Coriolus zonatus</Emphasis>

Laccase (oxygen oxidoreductase, EC 1.10.3.2) belongs to the multicopper oxidase family. The main function of this enzyme is to perform electron transfer from the oxidized substrate through the mononuclear copper-containing site T1 to the oxygen molecule bound to the site T3 in the trinuclear T2/T3 cluster. The structures of two new fungal laccases from C. maxima and C. zonatus were solved on the basis of synchrotron X-ray diffraction data. Both laccases show high structural homology with laccases from other sources. The role of the carbohydrate component of laccases in structure stabilization and formation of ordered protein crystals was demonstrated. In the structures of C. maxima and C. zonatus laccases, two water channels of functional importance were found and characterized. The structural results reported in the present study characterize one of the functional states of the enzyme fixed in the crystal structure.     

Relationship between the structure and luminescent properties for two crystal modifications of N-butyl-2-cyano-3-[4-(dimethylamino)phenyl]-2-propenamide

Two crystal modifications (1o and 1y) of N-butyl-2-cyano-3-[4-(dimethylamino)phenyl]-2-propenamide, which differ in the color of crystals and the color of luminescence, have been studied by X-ray diffraction and spectral-luminescence methods. The corresponding bond lengths and bond angles in the molecules of two crystal modifications are virtually identical. In both crystal structures, there are two systems of weak intermolecular interactions: π-stacking interactions and -CN…H-N hydrogen bonds involving nitrile and NH groups. In the crystal structures, two hydrogen bonds connect pairs of molecules into centrosymmetric dimers. The N…H distances are 2.21 and 2.41 Å in 1o and 1y, respectively. The interplanar distances in the π-stacked systems of 1o and 1y are 3.33 and 3.41 Å, respectively. Both types of weak interactions are stronger in 1o than in 1y, which accounts for a larger shift of absorption and luminescence bands for the former compound.