W. Prandl

Magnetic moment distribution in Fe3−xCrxSi alloys

Abstract Polarized and unpolarized neutron diffraction and magnetization studies in the temperature range 10–300 K are report for the Fe 3− x Cr x Si ( x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0) system. The experiments indicate that the single phase having the fcc DO 3 -type structure is maintained for Fe 3 -x Cr x Si for x ≤0.4, while the sample with x = 0.5 shows a small ( 3 Si-type phase, the amount of which increases linearly with Cr concentration. It is inferred from the dependence of the lattice constant on Cr content measured separately by X-ray diffraction that the phase with x = 0.35 ± 0.05 is always formed. Therefore, for the heat treatment reported here the concentration x = 0.4 seems to be the limit of solubility for Cr in DO 3 -type Fe 3 -x Cr x Si system. It was found that Cr is randomly distributed among (A, C) and B sites, while D sites are fully occupied by Si. Room-temperature magnetization measurements show a linear dependence of magnetization on Cr concentration. The Curie temperature varies as (843.0 ± 1.5) K, (740.0 ± 1.5) K and (712.0 ± 1.5) K for x = 0.1, 03. and 0.4, respectively. Also a linear composition dependence of the one-site magnetic moments μ(A, C) and μ(B) was observed. At room temperature the μ(B) moment decreases from 2.61 μ B for x = 0.1 to 2.08 μ B for x = 0.4, while the μ(A, C) moments decrease from 0.95 μ B for x = 0.1 to 0.72 μ B for x = 0.4. Assuming the constancy of magnetic moments one obtains μ Fe A = (1.04±0.04) μ B and μ Fe B = (2.77±0.10) μ B , while μ A Cr = -(1.27±0.14) μ B and μ B Cr = −(2.57±0.38) μ B . A weak temperature dependence of μ(B) moment dμ(B)/d T = (-2.4±0.1)×10 -4 μ B /K in the range 10–300 K was found for samples with x = 0.1 and 0.3.

Proton density and orientational potential in nickelhexammine salts: a thermodynamic analysis of rotation-translation coupling

The scattering densities of the orientationally disordered protons and deuterons in Ni(NX3)6Y2 (X=H or D; Y=Br, I, NO3 or PF6) are obtained from neutron single-crystal diffraction data at room temperature. While we observe a nearly circular density distribution for Y=PF6, a nuclear density distribution with four maxima on a square shows up for Y=Br, I and NO3. This is a pronounced deviation from the circular distribution that is expected from uniaxial rotational diffusion or reorientational jump models for the dynamics of the orientational disordered ammonia groups. All observed density distributions are consistently explained as the consequence of rotation-translation coupling in an anharmonic crystal potential. The calculated potential parameters, i.e. anharmonicity and strength of the coupling, depend on the type of anion present in the crystal frame. The weak anisotropy in the Y=NO3 and PF6 compounds is the consequence of a nearly eightfold non-crystallographic symmetry of the atoms establishing the hindrance potential.

The isotope effect and phase transitions in ammonium hexachloropalladate studied by neutron tunnelling spectroscopy

Above 30% deuteration, ammonium hexachloropalladate undergoes the phase transition Fm3m P21/n, with Tc increasing to 30.2 K for pure (ND4)2PdCl6. Rotational tunnelling of NH4 and NH3D was used to study this phase transition. The transition energies are related to the tunnelling matrix elements hi.NH3D can be described as a one-dimensional rotor, with the one-dimensional matrix elements being almost exactly the averages of the three-dimensional ones. In the high-temperature phase the intensities of the respective tunnelling transitions confirm a statistical occurrence probability of the isomers. In the low-temperature phase the strength of the rotational potential increases by about 30%. A broadening of the NH3D tunnelling line with decreasing temperature above the crystallographic phase transition is interpreted as an intermediate glass phase which evolves due to long-range dipolar coupling combined with positional disorder.

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

AbstractThe 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 γ=90.083 (2)o. The tilting of the oxygen octahedra is given asa−a−c0 in Glazers notation [11]. The monoclinic angle corresponds to the angle between the cubic directions [110]c and

The onset of oxygen diffusion in YBa2Cu3O7 − δ: An investigation by neutron diffraction

[1\bar 10]

Magnetisation phenomena due to magnetic disorder in the partly frustrated compounds (FexCr1−x)2Ca3(GeO4)3

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 β=∢ ([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(Pb1−xBix)O3 containing a large fraction of lead by enhancing valence fluctuations.

Inelastic neutron scattering by the quasi two-dimensional librational modes in K2SnCl6

The ceramic superconductor BaPb0.75Bi0.25O3 is monoclinic atT<430 K with space groupI2/m. Lattice constants and the atomic coordinates result from 20 reflection groups (12<2Θ<120°, λ=1.1267 Å) collected with a high resolution powder diffractometer using a synchrotron radiation source and a full neutron powder pattern (6<2Θ<140, λ=1.909 Å). Synchrotron and neutron data were simtultaneously refined with SIMREF, a new profile refinement program, based on the Rietveld method. Group theoretical analysis of the distortion field leading from the ideal perovskite structure toI2/m exhibitsR4+ symmetry. A comparison is made with the corresponding distortions in nonsuperconducting BaBiO3 where three different symmetry types are identified. The space groupI2/m found for BaPb0.75Bi0.25O3 allows only one type of averaged (Pb/Bi)O6 octahedron in the structure: this may support valence fluctuations and hence contribute to the mechanism of superconductivity in this particular compound.

Neutron diffraction study of the orientational disorder in Ni(ND3)6Br2

Abstract In order to obtain information on the oxygen mobility in orthorhombic YBa 2 Cu 3 O 7 − δ , we carried out in situ neutron diffraction measurements at the D2B high resolution diffractometer at Grenoble in the temperature range 300 K ⩽ T ⩽ 1000 K under a constant pressure of 1 atm O 2 . From the data evaluation using the Rietveld method we conclude a maximum oxygen occupancy at T ≈ 600 K with δ = 0.09. The mean square amplitude B 11 of the O4 site rises almost linearly up to 800 K and then increases sharply. We take this temperature as the onset of enhanced oxygen diffusion occurring at the order-disorder phase transition within the orthorhombic phase. We use the temperature dependence of the mean square delocalization of O4 to obtain an estimate of the energy barrier ΔV PER hindering diffusion. ΔV PER starts to decrease appreciably as soon as the temperature exceeds 800 K and comes down to a value ΔV PER ≈ 1200 K at a temperature of 950 K. Thermal activation is therefore strong enough for diffusion to occur.