Crystal structure of the high-pressure modification of LaNbO4

Abstract

The crystal structure of the metastable LaNbO₄ high-pressure modification (HPM) has been studied by X-ray powder diffraction. It was found that the crystal structure of HPM LaNbO₄ belongs to- the room temperature BaMCF₄ -type structure (space group Cmc2₁, a = 3,941(1)Å, b= 14,460(4) Å, c = 5,681(2) Å, Z = 4).     

Boson peak of AsxS1−x glasses and theoretical calculations of low frequencies clusters vibrations

The results of comparison of spectral position of the LF bands of g-As₂S₃ and g-As₂₂S₇₈ with theoretical calculations of vibrational spectra of different clusters (As₂S₄, As₂S₅, As₂S₆, AsS₃-As₃) in the LF region (10-85 cm⁻¹) are given. The torsion type vibrations of the small chain-like clusters are located in the same spectral region as Boson peak of As-S glasses system. They can make several contributions to the LF spectrum. The cluster lengths by weight distribution functions f(L)*L are from 5.5 to 10.1 Å and from 4.5 to 7.5 Å for g-As₂S₃ and g-As₂₂S₇₈, respectively. The lognormal fittings of the f(L)*L functions give the most probably values about the 7.6 Å (for g-As₂S₃) and 6.2 Å (for g-As₂₂S₇₈).     

用高压显微光谱系统研究非晶态As2S3的光学折射率随流体静压力变化的规律

用金刚石对顶砧高压显微光谱系统在高达66kbar的流体静压力和光谱波段为400—900nm范围内,用透射光干涉谱法测量了非晶态As₂S₃(a-As₂S₃)的光学折射率n与压力p以及波长λ的变化关系。a-As₂S₃的折射率对压力极为敏感,在波长为650nm,压力从1bar变到66kbar时,它增加35％。在计算机上用最小二乘法对实验点进行拟合的结果得到:对某一波长λ来说,遵循n(P)=n(0)+Ap+Bp²的非线性关系。其中n(p)和n(0)是p压和常压下的折射率,A和B是与波长有关的系数,文中给出了A和B的具体参数。这个关系与Weinstein和Galkiewicz等人公布的n(p)和p之间遵循线性关系不同。 关键词：     

ESR in X-irradiated As2O3 glass

X-irradiation of glassy As₂O₃ at 77K or 300K produces an unusually large density (～5×10¹⁸ cm^-3) of paramagnetic centers which are stable at 300K. The average spin-Hamiltonian parameters (g = 1.998, g_⊥ = 1.984, A₆ = 243G, A_⊥ = 114G) indicate that these centers are analogous to those previously observed in As₂Se₃ and As₂S₃ glasses and that they consist of unpaired electrons localized on a non-bonding 4p orbital of an As atom. Unlike the results obtained for As₂Se₃ and As₂S₃, the concommitant holes in As₂O₃ are trapped on Fe²⁺ impurity sites which become Fe³⁺ and not on non-bonding oxygen p orbitals. The radiation induced ESR is also accompanied by a stable optical absorption tail which lies within the band gap and increases exponentially with energy. This absorption can be partially bleached with the application of sub-band-gap light.     

Orientational and positional disorder of ions and its freezing in the CsNO2-TlNO2 system2

The heat capacities of Cs_0.695Tl_0.305NO₂ (Specimen I) and Cs_0.385Tl_0.615NO₂ (Specimen II) have been measured between 14 and 350 K. Specimen I underwent a phase transition at (197.7 ± 0.1) K, with ΔS = (19.2 ± 1.5) JK^?mol^?, and specimen II at (214.5 ± 0.2) K, with ΔS = (5.4 ± 1.0) JK^?1mol^?1, respectively. Above the phase transition, an exothermic temperature drift due to phase separation was observed. Annealing of the sample at 203 K for 300 hr brought about complete phase separation. The solid solution system annealed at 203 K gave two heat capacity peaks at (203.3 ± 0.1) K, with ΔS = (13.8 ± 0.8) JK^?1 mol^?1, and (242.4 ± 0.2) K, with ΔS = (10.6 ± 1.3) JK^?1 for Specimen I, and at (203.0 ± 0.1) K with ΔS = (6.7 ± 0.5) JK^?1 mol^?1, and (257.5 ± 0.2) K with ΔS = (17.9 ± 1.7) JK^?1 mol^?1 for Specimen II. The phase diagram of the CsNO₂-TlNO₂ binary system was constructed on the basis of DTA, heat capacity and dielectric measurements. In the metastable phase, the existence of a residual entropy due to the freezing of a random distribution of Cs⁺₁ and Tl⁺ cations in addition to the orientational disorder of the NO₂^?1 ion was confirmed by a comparison of entropies of the stable and the metastable phases.     

Transitions de phase destructive et non destructive dans le cristal moleculaire bromo-2 naphtalene

The polymorphism of 2-bromonaphthalene has been studied by both crystallographic and energetical methods. Three crystalline forms have been characterized, respectively denoted Br_III, Br_II and Br_I. The first one is the stable form up to 319 K. At this temperature a first order phase transition occurs which breaks the single crystal and leads to a disordered structure Br_I isotypic with the naphthalene one. The Br_II form is a metastable one which appears when cooling the Br_I, form; when so formed, the Br_II phase can give again the Br_I one by annealing; so that the Br_I-Br_II transition is reversible. This second order transition extends over a large temperature range (275–319 K), it does not destroy the single crystal and corresponds to a substructural modification along the c axis of the monoclinic cell. The anomalies of the Cp curve and the variations of the principal components of the thermal expansion tensor suggest that there are two competitive temperature dependent molecular reorientations. The transition Br_II→Br_III which leads to the stable ordered form is spontaneous at room temperature, it takes place sluggishly and destroys the single crystal.Cell parameters of the Br_III form have been determined from powder data using an automatic indexing method: a = 9.578(1) Å, B = 9.601(1) Å, c = 10.303(1) Å, α = 100.79(1)°, β = 109.06(1)°, γ = 101.77(1)°. (Figure of merit M₂₀ = 86.)     

Multiphonon absorption in As2S3As2Se3 glasses

Multiphonon absorption in As₂S₃ and As₂Se₃ glasses is well explained by a molecular model in terms of combination bands of high frequency vibrational modes of pyramidal AsY₃ and bent AsYAs groups (Y = SorSe). Multiphonon absorption coefficients in mixed As₂S₃As₂Se₃ glasses are nearly additive in terms of the pure components, suggesting a high degree of non-random mixing.     

Second order resonance Raman scattering in As2S3 glass

First and second order resonance enhanced Raman scattering has been observed for As₂S₃ glass. First order scattering is dominated by a peak at 340 cm^-1 due to the symmetric stretch of the AsS₃ pyramidal group. Two partially resolved second order peaks are observed at about 630 and 710 cm^-1 whose relative intensities are highly temperature dependent and which are due respectively to overtones of the antisymmetric and symmetric AsS₃ stretching modes.     

Micro‐Raman spectroscopic investigations of extremely scarce Pb–As sulfosalt minerals: baumhauerite,dufrénoysite,gratonite, sartorite,and seligmannite

The overmuch rare lead arsenic sulfosalts, baumhauerite (Pb₃As₄S₉), dufrénoysite (Pb₂As₂S₅), gratonite (Pb₉As₄S₁₅), sartorite (PbAs₂S₄), and seligmannite (PbCuAsS₃), were investigated by Raman spectroscopy. Raman spectra of the investigated minerals arise from the stretching and bending modes of the isolated and interconnected AsS₃ pyramidal groups. The As–S stretching vibrations of the studied minerals occur between 390 and 340 cm⁻¹, whereas those of S–As–S bending modes between 335 and 220 cm⁻¹. The shorter bond distances of the As–S readily elucidate the blue shift from seligmannite to gratonite, sartorite, dufrénoysite, and baumhauerite and the increase in full width at half maximum in similar modes. Copyright © 2016 John Wiley & Sons, Ltd.     

High-pressure phases in the GaSb-Mn system

The effect of high pressure (6 GPa) on the formation of new phases in a polycrystalline mixture GaSb: Mn = 1: 1 upon heating was studied. Sphalerite-type solid solutions with a small amount of Mn form at temperatures below 520–600 K. At higher temperatures, new crystalline GaSbMn phases are synthesized: a phase with a simple cubic structure with a lattice parameter a = 2.946 ± 0.001 Å (at 620–670 K) and a phase with a tetragonal CuAl₂-type structure (space group I4/mcm) with lattice parameters a = 6.426 ± 0.004 Å and c = 5.349 ± 0.004 Å (at 690–870 K). These new phases are metastable under normal conditions and have magnetic properties. The structure, conductivity, and thermal stability of the synthesized phases are investigated, and the products of decomposition of these new phases upon annealing are analyzed.     

Internal friction in silver-containing (Ag3AsS3) x (As2S3)100 ? x superionic glasses

The mechanical properties of glasses in the Ag₃AsS₃-As₂S₃ system have been studied by the internal friction method. Measurements of internal friction and shear modulus have been carried out in the temperature interval 80?C300 K at deformation frequencies of 10?C50 mHz. Relaxation-type maxima of internal friction have been revealed. It has been shown that the concentration-induced variations in the parameters of the mechanical relaxation process in (Ag₃AsS₃) _x (As₂S₃)_{100 ? x} glasses observed in the region of 20 at % Ag₃AsS₃ are of the threshold character. It has been established that this feature can be associated with the variation in the structural unit, which accounts for the process of mechanical energy dissipation within the temperature interval 150?C250 K.     

Effect of nonlinear refraction and two-photon absorption on the optical limiting in amorphous chalcogenide films

This paper reports on the results of investigations into the nonlinear optical characteristics of chalcogenide films (As₂S₃, As₂₀S₈₀, 2As₂S₃/As₂Se₃, 3As₂S₃/As₂Se₃). The nonlinear refractive indices and two-photon absorption coefficients for these films are measured using the Z-scan technique at wavelengths of a picosecond Nd: YAG laser (λ=1064 and 532 nm). The optical limiting due to Kerr-type nonlinearities is analyzed.     

A quantum-chemical study of intermediates of the 1O2 photogeneration sensitized by buckminsterfullerene and accompanying photochemical reactions

The intermediates of hypothetical photochemical reactions that accompany the quenching of the ³C ₆₀ ^* triplet state by triplet oxygen are studied by the (U)PBE0 quantum-chemical method. The diradical C₆₀-O-O formed from ³O₂ and photoexcited buckminsterfullerene ³C ₆₀ ^* is characterized by a negative binding energy ?1.11 eV (with respect to C₆₀ and ³O₂), the singlet-triplet splitting ΔE _ST of 0.07 eV, and the dipole moment of 3.2 D at the equilibrium internuclear separations 1.522 Å (CO) and 1.294 Å (OO). Its decay produces ¹O₂. The formation of a dioxetane circle lowers the energy by 0.8 eV. The ground-state energy of diketone C₅₈(C=O)₂ is 2.0 eV lower than the energy of C₆₀-O-O. The metastable centrosymmetric diradical C₆₀-C₆₀, formed upon ineffective light absorption by clusters (C₆₀)_N, has a single interpolyhedral C-C bond (1.657 Å). Its triplet state T ₁ lies 0.16 eV higher than the S ₁ singlet. The S ₁ → S ₀ relaxation leads to the formation of a stable C₆₀-C₆₀ dimer with a shorter (1.584 Å) bis-single exothermic (+0.24 eV) bond of polyhedra. The photoexcited C₆₀-C₆₀ dimer is able to form isomeric metastable diradicals C₆₀-C₆₀-O-O.     

Additional polymorphism and non-stoichiometry in AgF

A phase forming from B2 type AgF on decrease of pressure found recently by Vaidya and Kennedy has been studied by in situ X-ray diffraction. It is indexed as hexagonal with c = 6.226±0.45 Å and a = 3.246±.002 Å, Z = 2. This structure is probably inverse NiAs. Its pressure of formation is highly dependent on the samples history of pressure variation. All three phases, B1, B2 and hexagonal have been made to give X-ray patterns with each line split into two lines of equal intensity. We interpret this as a decomposition into two non-stoichiometric phases, one Ag rich and the other F rich. The conditions for the formation of these split phases are not always reproducible which suggests that this is a metastable phenomenon possibly due to pressure gradients on the sample. The splitting could, however, also be explained by equilibrium behavior.     

Far infrared transmission and Raman scattering in Pb-As-S semiconducting glasses

Various compositions (x = 0.1?0.625) in the pseudo-binary (As₂S₃)_1?x(PbS)_x have been prepared in the glassy form. Thermal analysis of the prepared glasses shows a multiphase non-crystalline structure. Their far-infrared transmission (50–450 cm^-1) and first order Raman scattering have been observed for the first time and the observations indicate a two mode behaviour of the glasses. The frequencies and symmetries of AsS₃ bond-stretching modes are maintained in this alloy system and are therefore determined by the local order, and not the dimensionality or longer range order of the network.     

High-pressure phases in nanocrystalline Co(C) films obtained by pulsed plasma vaporization

The phase composition of nanocrystalline Co(C) films obtained by a new pulsed plasma vaporization technique was found by studying their atomic structure and magnetic properties. The films deposited at the substrate temperature T=50°C were of heterophase structure and consisted of a supersaturated solid Co(C) solution and the metastable Co₃C carbide. The films obtained at T=150°C represented a mechanical mixture of the metastable Co₃C and Co₂C carbides. The metastable Co₃C and Co₂C carbides obtained in a nanocrystalline state were high-pressure phases (～100 kbar). The thermal stability ranges of these metastable phases were determined.     

Crystallographic and vibrational study of hexanitroethane

The crystal structure of hexanitroethane has been determined in the low-temperature ordered phase at 145 K (P2₁/c: a = 10.152(2) Å, b = 9.311(2) Å, c = 10.251(2) Å, β = 97.54(1)°, V = 960.6(3) ų, Z = 4 with 2 non-equivalent molecules in the unit cell).The far-infrared, infrared and Raman spectra of hexanitroethane in both low-temperature ordered and high-temperature disordered crystal phases were recorded. The internal modes are interpreted with the help of normal coordinate analysis and a conformational analysis with the MNDO method. The transition mechanism is discussed.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Superconductivity in Ti-doped iron-arsenide compound Sr<Subscript>4</Subscript>Cr<Subscript>0.8</Subscript>Ti<Subscript>1.2</Subscript>O<Subscript>6</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

Superconductivity was achieved in Ti-doped iron-arsenide compound Sr₄Cr_0.8Ti_1.2O₆Fe₂As₂ (abbreviated as Cr-FeAs-42622). The X-ray diffraction measurement shows that this material has a layered structure with the space group of P4/nmm, and with the lattice constants a = b = 3.9003 Å and c = 15.8376 Å. Clear diamagnetic signals in ac susceptibility data and zero-resistance in resistivity data were detected at about 6 K, confirming the occurrence of bulk superconductivity. Meanwhile we observed a superconducting transition in the resistive data with the onset transition temperature at 29.2 K, which may be induced by the nonuniform distribution of the Cr/Ti content in the FeAs-42622 phase.     

Electron spectroscopy with excited atoms and photons.: X. Halogen-substituted methanes and methyl amines

Electron binding energy spectra have been measured for CH₃Cl, CH₃Br, CH₃I, CFCl₃, CF₂Cl₂, CF₃Cl, CF₄, CH₃NH₂, (CH₃)₂ NH and (CH₃)₃N. Measurements have been made using 584 Å (21.22 eV) photons as well as with 2³S(19.82 eV) and 2¹S(20.62 eV) metastable helium atoms. Relative spectral intensities are compared for photoionization and Penning ionization.