The static Debye-Waller factor of defect-induced lattice displacements

The attenuation of Bragg scattering intensities due to static displacements is discussed for interstitially dissolved point-defects. It is shown that in the single defect approximation the static Debye-Waller factor (DWF) exp (–2L) decreases linearly with the defect concentrationc and quadratically with the scattering-vectorK (for smallK andc). From a model calculation of the displacements around H in Nb the static DWF is shown to be primarily due to the displacements closest to the impurity. Therefore the measurement of the static DWF can be used as a method to determine these displacements. From X-ray measurements of the integrated Bragg intensities of NbH _x single crystals we obtained the displacements of the four nearest neighbours around a tetrahedral interstitial site to beu ₁=0.1Å. This is in good agreement with the results of our model calculation.     

X-ray superstructure determination of ordered oxygen and Cu-displacements in a single domain of YBa2Cu3O6.35

The superstructure of ordered oxygen atoms in a single domain of YBa₂Cu₃O_6.35 has been determined by X-ray diffraction. The 45^o rotated superstructure cell is orthorhombic—spacegroup Pbam, lattice constants , witha=3.8652(3) Å,c=11.815(2)Å the dimensions of the fundamental tetragonal cell. The arrangement of the oxygen atoms in the basal plane minimizes their electrostatic repulsion by avoiding near and next-near neighbors. There are displacements of copper atoms of 0.13 Å in the Cu(1) plane and 0.06 Å in the Cu(2) plane. Only 12% of the oxygen atoms in the basal plane contribute to the superstructure.     

The Debye-Waller factor and the diffusion process for hydrogen in Nb,Ta, and V single crystals investigated by neutron spectroscopy

Quasielastic scattering of slow neutrons on hydrogen diffusing in the-phase of NbH_0.02, TaH_0.13 and VH_0.07 single crystals was investigated in a wide range of temperatures and scattering vectorsQ (0.5Q2.5 Å^–1). The incoherent scattering lawS _d (Q,) for four different diffusion models was consistently compared with the measured lineshapes. At elevated temperatures one had to introduce correlated jumps to describe the experimental data, whereas at room temperature a model with jumps between adjacent sites is sufficient. The integrated quasielastic intensityI(Q) obtained from the fit ofS _d (Q,) with the measured spectra follows an isotropic Debye-Waller factor with mean square amplitudes u ²=0.02–0.04 Ų for H in Ta (20°C–500°C), and u ²=0.03–0.04 Ų for H in Nb (20°C–300°C). For H in V,I(Q) obtained from the analysis of the quasielastic scattering deviates from a normal Debye-Waller factor behaviour. This effect is assumed to be due to the flight process between the interstitial sites. On the other hand, a normal Debye-Waller factor was obtained from theQ-dependence of the inelastic scattering of the band modes, with values of u ²=0.02–0.04 Ų (135°C–500°C). The observed values of u ² were compared with theoretical calculations.     

Superconductivity,crystallization, and structural relaxation of the new metallic glass system La1−x Si x (0.15≤x≤0.27)

New metallic glass alloys have been prepared by melt spinning of La–Si samples in a pumped system. The superconducting transition temperature,T _c , increases linearly with the La concentration, from 3.00 K at 73 at. % La to 3.80 K at 85 at. %. Three new metastable phases: -, -, and -La₃Si, were formed by annealing and crystallizing amorphous La₃Si. Their crystal structures are orthorhombic (a=6.32 Å,b=8.06 Å,c=9.96 Å), hcp (a=10.55 Å,c=5.05 Å), and tetragonal (a=6.92 Å,c=5.05 Å) resp.T _c increased to 3.75 K, 6.00 K and 6.80 K, resp. During low temperature anneals of an amorphous La₃Si alloy,T _c changed logarithmically with time.On leave from Institute of Physics, Academica Sinica, Beijing, China     

The crystal structure of the prototypic ceramic superconductor BaPbO3: An X-ray and neutron diffraction study

The structure of the distorted perovskite BaPbO₃ was studied with high-resolution X-ray diffraction at 300 K and 26 K and with neutron diffraction at 300 K. Simultaneous refinement of the neutron and X-ray data sets (300 K) using the Rietveld method yields a monoclinic structure with the space groupI 2/m and lattice parametersa=6.0278 (1) Å,b=6.0664(1) Å,c=8.5109(1) Å, and =90.083 (2)^o. The tilting of the oxygen octahedra is given asa ^– a ^– c ⁰ in Glazer's notation [11]. The monoclinic angle corresponds to the angle between the cubic directions [110] _c and _c . This is in contrast to the observations in BaBiO₃. The structure of BaBiO₃ has the same space groupI 2/m, the similar dimensions of the unit cell and the same tilt system, but = ([110] _c , [001] _c ) as the monoclinic angle. As a consequence there is only one type and size of, PbO₆ octahedra but two types of octahedra in BaBiO₃. This fact may influence the occurrence of superconductivity in solid solutions (Ba(Pb_1–x Bi _x )O₃ containing a large fraction of lead by enhancing valence fluctuations.     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

Domain-wall energy in sintered Nd15Fe77B8 permanent magnet

From measurements of the magnetic domain widthsD versus grain thicknessL, we have determined the domain wall energy in the demagnetized state for a sintered Nd₁₅Fe₇₇B₈ magnet: = 56 erg/cm². Using this wall energy and the published magnetocry-stalline constantK ₁=4.9×10⁷ erg/cm³, we have calculated the exchange constantA=4.0×10^–6 erg/cm, domain wall thickness _B=89 Å, and the critical diameter for single domain particlesD _c=0.67 m.     

Structural properties of vapor-grown C60 crystals

Vapor grown crystals of C₆₀ with thin flat triangular, rhombic or trapezoidal shapes of size to 1×2×0.001 mm as well as prismastic crystals typically 0.5×0.4×0.2 mm have been grown using a high temperature vapor transport method. Room temperature X-ray precession photography shows these crystals exhibit diffraction patterns consistent with those for either (a) single or (b) twinned crystals of the previously reported face-centered cubic structure or (c) a more complex cell of hexagonal symmetry with a=10.010(2) Å and c=49.064(11) Å. This latter from actually is a multiple twin containing both the face-centered cubic and the close-packed hexagonal structure types. The sharp diffraction maxima for either the single or the twinned crystals demonstrate that they consist of large coherent domains and are essentially free of planar defects parallel to the twin planes.     

Preparation and crystal structure of C60S16

A new fullerene-containing van der Waals-compound, C₆₀S₁₆, has been synthesized and its crystal structure determined from single-crystal X-ray diffraction data collected at room temperature. The compound is monoclinic, space group C 2/c with a=20.867(4) Å, b=21.062(4) Å, c=10.508(2) Å, =111.25(7)° and four formula units per cell. The C₆₀ molecules form a three-dimensional framework with one-dimensional channels along c which contain crown-shaped S₈ rings. The structure has been determined by direct methods and has been refined to atomic resolution on the C₆₀ molecule. The two independent C-C bond distances, averaged under the idealized point symmetry of the free C₆₀ molecule, amount to 1.340(8) Å and 1.448(8) Å, corresponding to a bond alternation as large as 0.108(8) Å. The bond lengths are compared with the results of theoretical calculations of the molecular structure of C₆₀ as well as with bond lengths from various experimental sources.     

Real-space renormalization group investigation of pure and disordered mixed spin Ising models ond-dimensional lattices

The mixed spin Ising model (spins =1/2 andS=1) ond-dimensional hypercubic lattices with nearest-neighbour exchange interactions is studied via a renormalization group transformation in position space. The phase diagrams in (L, K) space, i.e. in dependence of the bilinear (K) and the biquadratic (L) interaction coefficients, are qualitatively different ford=2 andd>2. For any dimensiond however it is found that all transitions are of second order. At zero-temperature (K=,L=), the ferromagnetic order disappears at (L/K)₀=2, which does not depend ond. Using an extension of this real-space renormalization group analysis we study the two-dimensional random disordered version of the above model.L is kept homogeneous and the bilinear interactionsK _ij are assumed to be independent random variables with distributionP(K _ij )=p(K _ij –K)+(1–p)(K _ij –K); whereK>0. The phase diagrams for different values ofp are obtained. At zero temperature, it is found that in the bond diluted model (=0) the value (L/K)₀ depends continuously onp, whereas in the random ±K interactions (=–1) (L/K)₀ is unique and does not depend onp.Supported by the agreement of cooperation between the DFGW. Germany and the CNR-Maroc     

Monoclinic symmetry in LaSrCu0.5Co0.5O4−z found by neutron,Guinier-and synchrotron-X-ray powder structure analysis

High resolution X-ray and neutron powder diffraction data of semiconducting LaSrCu_0.5Co_0.5O_3.7 have been collected in the temperature range of 5 to 300 K. A monoclinic structure with space groupI 2/m a=3.7952(1) Å,b=3.7902(1) Å,c=12.6507(1) Å, =90.074(1) Deg was refined with the Rietveld method for individual data sets as well as for the collection of all data simultaneously. The structureI 2/m results from an internal shearing of the octahedra and-unlikeCmca in superconducting La_1.85Sr_0.15CuO₄-not from a second order phase transition inI4/mmm. An alternative model with two tetragonal latticesa ₁=3.7902,a ₂=3.7952,c=12.6507 Å is discussed.     

Structural study by X-ray diffraction and Mössbauer spectroscopy of a new synthetic iron phosphate: K4MgFe3(PO4)5

A new iron phosphate K₄MgFe₃(PO₄)₅ has been synthesized by the flux method and characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. It crystallizes in the tetragonal system with the space group and the unit cell parameters a=9.714(3) Å and c=9.494(5) Å. The crystal structure is of a new type. It exhibits a three-dimensional framework built up from corner-sharing MO₅ (M=0.75Fe+0.25Mg) trigonal bipyramids and PO₄ tetrahedra. The K⁺ ions are occupying large eight-sided tunnels running along c. A room temperature Mössbauer study confirmed the +3 valence state of iron and its five-coordination.     

First-principle study of structural and electronic properties of ternary layered Ta2AlC

The structural and electronic properties of ternary layered Ta₂AlC ceramics have been studied using the first-principle method based on the density-functional theory. We have obtained the equilibrium lattice parameters and the equilibrium atomic positions in the unit cell. The equilibrium lattice parameters are computed to be a=b=3.15 Å and c=13.95 Å. The internal coordinates of Ta are determined to be (1/3, 2/3, 0.092). The band structure and density of states reveal that Ta₂AlC is an electronic conductor. The charge density distribution shows that the Ta and C atoms form a strong Ta-C-Ta covalently bonded chain.     

The crystal structure and magnetic properties of the 1-dimensional dihalide-bridged polymers dichlorobis(thiazole)cobalt(II) and dibromobis(thiazole)-cobalt(II)

The isostructural polymeric compounds Co(thiazole)₂X₂ (X=Cl (1), Br(2)) have been synthesised by the addition of thiazole to an ethanolic solution of the corresponding anhydrous cobalt halide. Powder X-ray and neutron diffraction measurements were used for structural determination. The structures were determined using powder neutron diffraction data and Rietveld techniques: (1) C2/c, a=17.806(2) Å, b=3.6806(6) Å, c=14.807(3) Å, β=94.78(1)°, V=967.1(3) ų, Z=4; (2) C2/c, a=18.079(3) Å, b=3.8138(8) Å, c=15.022(4) Å, β=92.71(1)°, V=1034.6(4) ų, Z=4. Each linear polymer chain is composed of pseudo-octahedral, high-spin Co²⁺ centres, doubly linked by halide bridges. Magnetisation measurements of 1 and 2 at 5 K between 0 and 10 kG reveals a metamagnetic transition between antiferromagnetic and ferromagnetic states. Low temperature susceptibility data have been fitted to a one-dimensional Ising model with a mean field correction and were found to be anisotropic with ferromagnetic intrachain interactions along the b-axis and weaker antiferromagnetic interchain interactions.     

Laser-induced changes in the electron density distribution of crystals: Ca3Cr2Si3O12

Difference of electron density distribution between a crystal being He–Ne laser-irradiated and non-irradiated has been investigated by in situ X-ray diffraction methods. Difference Fourier analysis between these data revealed electron density decrease of 2.1 eÅ^–3 at the trivalent site occupied 64% by Cr³⁺, the absorbant; while the calcium, silicon, and oxygen sites remained practically unchanged. Crystal data: [Natural uvarovite from Outokumpu, Finland; Ca₃(Cr_1.284Al_0.692Fe_0.024)Si₃O₁₂ (EPMA analysis); Cubic garnet structure; ;Z=8;a ₀=11.936(1) Å;V=1700.5 ų D _x =3.775 Mgm^–3; (MoK)=0.71069 Å; =4025 m^–1;F(000)=1900;T=293 K].     

Synthesis of Co2N by a simple direct nitriding reaction between nitrogen and cobalt under 10 GPa and 1800 K using diamond anvil cell and YAG laser heating

Synthesis of cobalt nitrides has been tried in a supercritical nitrogen fluid at high pressure (about 10 GPa) and high temperature (about 1800 K) using diamond anvil cell and YAG laser heating system. We have succeeded to synthesize a single phase of the CFe₂-type Co₂N easily in a short time. This is the first synthesis by a simple reaction between the pure cobalt and pure nitrogen (supercritical fluid nitrogen). The cell parameters of the synthesized Co₂N are a=4.662(9) Å, b=4.332(5) Å and c=2.749(9) Å, respectively.     

Crystallographic and magnetic structures of highT c related (Bi/Pb)2Sr2FeO6.25 determined by neutron powder diffraction

The structure of (Bi/Pb)₂Sr₂FeO_6.25 was refined from high resolution neutron powder diffraction data taken at 300 K and 5 K. The average structure is well described in the orthorhombic spacegroup Amaa witha=5.4254 Å,b=5.4909 Å andc=23.2245 Å. The FeO₆ octahedra are tilted by 2.7° towards (0 1 0). The oxygen positions in the (Bi/Pb)O-layers are consistent with the occurrence of (Bi/Pb)–O–(Bi/Pb) chains. Excess oxygen was located between these chains. Antiferromagnetic 3-dim ordering of the Fe-moments withM=(2, 1, 2) _B was observed at 5 K with the wavevector pointing along (1 0 0).     

Über dieKα-Satelliten von Fluor in Lithiumfluorid

Using a focussing crystal spectrometer with photoelectric registration theK-emission spectrum of fluorine in solid lithium fluoride (=18.3 Å) was studied. If the spectrum is excited by Co or CuL-radiation or bremsstrahlung of a W-target the satellitesK ₃ andK ₄ appear with considerable intensity, whereas they appear only faintly ifL-radiation of Fe is used for excitation. In agreement with these observations the calculated threshold energies of the satellites are near the photon energy of FeL _{1, 2}. The measured energies of the components agree with values obtained from the energies of the singly and doubly ionized F^– ion as calculated by means of optical data. The results show that the satellitesK ₃ andK ₄ in the spectrum of fluorine in solid LiF correspond to transitions in the doubly ionized F^–-ion.