Band 195, Heft 3-4

Two slits have been introduced into high-angle double-crystal X-ray diffractometry (HADOX): one for limiting the area of the specimen surface to be examined, and the other for defining the resolution of 2 θ . This enables us to combine ω and 2 θ scannings, and to obtain two-dimensional intensity distribution in high resolution on a reciprocal lattice plane parallel to the plane of the diffractometer. By this method, we can determine the temperature dependence of the lattice constant of the specimen crystal without being disturbed by an influence of surroundings, for expample, a distortion of a specimen holder. Moreover, measurements of the lattice constant can be made on a crystal that consists of grains. Applications are made for BaTiO 3 crystals in connection with the characterization and with the study of the structural phase transition.

The feasibility of experimental determination of multiplet phase invariants by means of four-beam ψ -scan experiments is investigated both theoretically and experimentally. There are two types of four-beam cases: systematic ones, if the ψ -rotation axis coincides with a symmetry direction of the reciprocal lattice and accidental ones for special wavelengths, if two arbitrary three-beam cases occur at the same ψ position. The modified twobeam Bethe solution of the fundamental equations of dynamical theory in a four-beam case shows, that the ψ -scan profiles depend on two first-order triplet-phase terms and two second-order quartet-phase terms and a second-order phase-independent extinction term. Only those multiplet phase invariants occur, which contain the phase due to the primary scattering vector coinciding with the ψ -rotation axis. If in a systematic, symmetric four-beam case the ψ -rotation axis coincides with a symmetry direction of the structure carrying a two-fold rotation axis 2 or screw axis 2 1 , special relations between the multiplet phase invariants exist. Then the two first-order triplet-phase terms which experimentally govern the ψ -scan profiles, in agreement with the theoretical evaluations, have the same value. In this case, determination of this triplet phase becomes feasible analogous to a three-beam ψ -scan experiment. All other four-beam cases cannot be used for phase determination in practice.

The importance of synchrotron radiation for the evaluation of stress fields in polycrystalline specimens is outlined. As a special example we investigated residual stress-fields σ xx ( z ) and σ yy ( z ) which vary significantly below the surface of the polycrystalline specimens within the penetration depth τ of the X-rays. The individual crystals are elastically anisotropic. However on a macroscopic scale inside the irradiated volume the specimens are supposed to be isotropic and homogeneous. Special effects related to plastic deformations are neglected. It is shown how properly normalized strain curves ε ψψ are obtained from the diffraction experiments. From the ε -data a ‘universal curve’ is calculated which is independent of the reflecting lattice planes and the wavelength and which should be the same for experiments performed in the Ω - and in the Ψ -mode. It was possible to demonstrate that strongly non-linear ε (sin 2 ψ )-curves observed for the (531) reflection of a cold-rolled nickel plate are related to strong stress gradients. The experiments yield σ xx ( τ ) and σ yy ( τ ) which are closely related to the Laplace transforms of the corresponding curves in z -space.

1-Hydroxypyridinium Trichloroacetate, C 7 H 6 Cl 3 NO 3 , the hydrogen-bonded 1:1 adduct of pyridine-1-oxide and trichloroacetic acid, crystallizes in space group P 2 1 / c with (at 120 K) a = 11.649(3), b = 8.936(2), c = 10.610(3) Å, β = 110.35(2)°. Its crystal structure has been redetermined by low-temperature neutron diffraction (120.0(5)K) and has been refined to R ( F ) = 0.028 for 2567 unique neutron data ( F o ≥ 3 σ , (sin Θ / λ ) max = 0.70 Å −1 ) and 183 variables. The hydrogen bridge that links the N oxide and the acid moiety is very short (O … O = 2.430(2) Å), almost linear (H – O … H = 174.9(1)°), and asymmetric (NO – H = 1.148(2) Å, CO … H = 1.284(2) Å). A rigid-body fit to the thermal parameters of the pyridine ring gave r.m.s. components of 0.076(2) Å for bond-stretching, 0.117(2) Å for in-plane and 0.150(2) Å for out-of-plane bending modes of C–H. The barrier to rotation of the CCl 3 group is estimated to be 21 kJ/mol at 120 K and 14 kJ/mol at room temperature.

Mixtures of Fe crystallites and Cu crystallites with diameters of about 6 – 14 nm, have been prepared by inert gas condensation and subsequent compaction of the crystallites. Small angle X-ray scattering experiments using synchrotron radiation have been carried out to study the microstructure of the compacts on a nanometer scale. By variation of the atomic scattering factors near the K absorption edge of Fe, partial intensities have been obtained which provide separate information about the environment of the Fe or the Cu atoms, respectively. The partial intensities are interpreted in terms of scattering contributions of particles with different sizes. The resulting size distribution functions not only indicate the presence of Fe and Cu crystallites but also give evidence for correlations between the crystallites. These correlations between Fe and Cu crystals are interpreted in terms of a mixture of the chemically different crystallites on a nanometer scale.

High-resolution neutron-diffraction techniques have been used to determine the crystal structure of deuterated squaric acid, D 2 C 4 O 4 , in its ordered phase at 298 K. Comparison with H 2 C 4 O 4 at the same temperature reveals the effects of deuteration alone on the crystal structure. The main changes are seen in the geometry of the O – H(D)…O bonds: the overall O … O bondlength increases by ∼ 0.020 Å on deuteration, and the O – D bond in D 2 C 4 O 4 is ∼ 0.015 Å shorter than O – H in H 2 C 4 O 4 . These changes probably account for much of the increase in the H(D)-ordering temperature, T c , in this system, from 371 K in H 2 C 4 O 4 to 516 K in D 2 C 4 O 4 .

The crystal structure has been determined for NH 4 H 2 PO 4 and ND 4 D 2 PO 4 at atmospheric pressure, and for NH 4 H 2 PO 4 at a hydrostatic pressure of 8.9 kbar. In each case the temperature was set to just above the antiferroelectric-to-paraelectric phase transition. The relationship between the transition temperature and the O – H – O and O – D – O bond dimensions is discussed in comparison to the isomorphous but ferroelectrically ordering KH 2 PO 4 and KD 2 PO 4 .

The critical behaviour of the high temperature phase transition ( T c = 365 K) of betaine phosphate was investigated by two different X-ray methods: the double crystal technique and the energy dispersive Laue method. We measured the temperature dependence of the order parameter, the thermal expansion, and the intensity of the critical scattering. The phase transition can be characterized as a second order transition which is very close to a tricritical one. The critical exponents correspond to those of the 3-dimensional Ising model within the accuracy of our measurements which is limited by the sample quality.

Pr 2 Ti 3 O 8 Cl 2 was prepared by chemical transport and investigated by X-ray diffraction and electron microscopical techniques. The single crystal structure investigation shows that Pr 2 Ti 3 O 8 Cl 2 is isotypic with Nd 2 Ti 3 O 8 Cl 2 : space group Pbcn (No. 60) with a = 14.6542(47) Å, b = 11.6161(52) Å, c = 5.0433(10) Å (diffractometer data), Z = 4. Least-squares refinement based on 1076 independent reflections led to an R value of 4.56%. Main building units are TiO 6 -octahedra and Pr(Cl 3 O)O 4 square antiprisms. Efforts to obtain Ln 2 Ti 3 O 8 Cl 2 (Ln = La,Ce) by chemical transport reaction failed. Electron diffraction and high resolution transmission electron microscopy images of Pr 2 Ti 3 O 8 Cl 2 along the [001] direction were obtained. They could be interpreted by comparing them with images calculated on the basis of the multi slice method.

Single crystals of α -methyl alaninium dihydrogen monophosphate ( α -MADP) and α -methyl alaninium dihydrogen monoarsenate ( α -MADA) were grown from aqueous solutions by a controlled isothermal evaporation method at 315 K. The chemical compositions of both compounds resulted from an automatic elementary analysis. They are consistent with the empirical formula (CH 3 ) 2 C(NH 2 )COOH · H 3 PO 4 ( α -MADP) and (CH 3 ) 2 C(NH 2 )COOH · H 3 AsO 4 ( α -MADA). At room temperature the crystals belong to the triclinic space group P [unk] with cell parameters: a = 10.086 Å, b = 7.641 Å, c = 5.871 Å, α = 97.7°, β = 108.0°, γ = 81.0° ( α -MADP) and a = 10.164 Å, b = 7.830 Å, c = 5.935 Å, α = 97.2°, β = 107.7°, γ = 81.9° ( α MADA). The magnitudes and the directions of the 3 principal refractive indices were measured at 589 nm and 295 K. Caloric measurements revealed phase transitions at 215 K ( α -MADP) and 178 K ( α -MADA), which could be detected also by dielectric methods. The principal values of the dielectric constant of α -MADP and α -MADA were determined for different temperatures below and above the phase transition. Dielectric hysteresis measurements with fields up to 20 kV/cm gave no indication of a ferroelectric, antiferroelectric or nonlinear behaviour.

ZrNi, ZrCo and their hydrides doped with 57 Fe are investigated using Mössbauer spectroscopy. Hydrogenation of ZrNi results first in ZrNiH and then in ZrNiH 3 , whereas only ZrCoH 3 is produced in the case of ZrCo. At low hydrogen concentrations hydrogen atoms enter mainly 4 c 1 interstitial sites, but 8 f 1 , 8 g 2 and 4 c 2 sites are also partially occupied. At high hydrogen concentrations, only 8 f 1 and 4 c 2 sites are occupied by hydrogen because the repulsive interaction between hydrogen is strong. ZrCoH is not observed. The hydrogen occupancy in ZrCoH 3 is similar to that in ZrNiH 3 .