Search

The hydrogen bond in K3H(SO4)2 was investigated using a neutron single crystal diffraction technique at room temperature and 2.3 K. The title compound crystallizes in the monoclinc space group A12/...

Impedance measurements were performed on single crystals of (NH4)3H(SO4)2 in the temperature range of 295–441 K. Anisotropy of protonic conductivity was found in the monoclinic room-temperature phase (space group C2/c) as well as in the rhombohedral high-temperature phase (space group R 3 ¯ m ). Above the high-temperature phase transition at 413 K, conductivity in the hexagonal (001) plane yields a value of ~ 10− 2 S cm− 1, which is typical for superionic conductors. A verification of a sample quality after the impedance measurements, using X-ray powder diffraction, showed minor additional peaks of (NH4)2(SO4). This suggests a beginning of thermal decomposition at high temperature. The possible pathways of proton conduction were discussed according to crystal structure analyses by single-crystal neutron diffraction method.

A new polarized neutron single crystal diffractometer POLI (Polarization Investigator) has been developed at the Maier-Leibnitz Zentrum, Garching, Germany. After reviewing existing devices, spherical neutron polarimetry has been implemented on POLI as a main experimental technique using a third-generation cryogenic polarization analysis device (Cryopad) built in cooperation between RWTH University and Institut Laue-Langevin. In this report we describe the realization and present the performance of the new Cryopad on POLI. Some improvements in the construction as well as details regarding calibrations of Cryopad and its practical use are discussed. The reliable operation of the new Cryopad on POLI is also demonstrated.

The highly disordered crystal structure of triammonium hydrogen disulfate, (NH4)3H(SO4)2, in the high-temperature phase I was studied using single-crystal neutron diffraction. It is known that the O atom involved in hydrogen bonding between neighbouring SO4 tetrahedra is disordered and takes a split-atom position, building a two-dimensional hydrogen-bond network in the (001) plane. The H atoms in these SO4–H—SO4 hydrogen bonds are disordered and hence refined with a split-atom model. Moreover, from the much larger anisotropic mean-square displacements of ammonium protons the NH_4^+ groups were refined with a reasonable split-atom model, and their motional behaviour was also analysed by rigid-body treatment. Finally, careful consideration was given to show possible supplementary proton migration between the ammonium protons and those of the hydrogen bonds in this high-temperature phase.

The temperature variation of the Co2SiO4structural parameters has been investigated by means of both neutron powder and single-crystal diffraction. Measurements were performed in a broad temperature range from 5 to 500 K. There is clear evidence of an anomalous thermal expansion related to the magnetic phase transition at 50 K. Below this temperature an abrupt elongation and contraction were observed along thecandbaxes (space groupPnma), respectively, whereas the lattice parameteraremains constant in the range between 5 and 50 K. The resulting volume of the unit cell is contracted below 50 K. This phenomenon is attributed to magnetostriction effects. A model based on the specific heat capacity data was applied for the quantitative description of the anomalous thermal expansion behaviour. On the other hand, no anomalies can be observed in the temperature dependency of the interatomic distances and angles.

The device “MuPAD” for zero-field spherical neutron polarimetry (SNP) has recently been successfully used for experiments in SANS geometry at the very cold neutron beam line MIRA at the research reactor FRM II in Munich and at the cold triple axis spectrometer TASP at Paul Scherrer Institut (PSI). At the hot neutron beam line SR 9 of FRM II, the construction of the polarised neutron single -crystal diffractometer POLI-HEiDi is under way. One of the major applications of this new instrument will be the investigation of complex magnetic structures by means of SNP. As MuPAD has originally been designed for use with thermal and cold neutrons, we carried out a first feasibility experiment with MuPAD at the existing diffractometer HEiDi in order to verify its performance when operated with short wavelength neutrons. In addition, we tested the combination of MuPAD and 3He spin filters that wherewere used as polariser and analyser. Here we present the setup of the experiment and first successful measurements of the complete polarisation tensor of the (3 3 3) Bragg reflection of a Si single-crystal. In addition, we discuss the particularities of using two 3He filter cells simultaneously. A method to perform the corrections of the data, taking into account the time dependence of the polarising efficiency in both cells, is proposed.

X-ray powder diffraction, differential scanning calorimetry (DSC)/thermogravimetry (TG) and single-crystal neutron diffraction methods were used to investigate triammonium hydrogen disulfate (NH4)3H(SO4)2 (TAHS) in the temperature range between 293 and 493 K. The temperature-dependent X-ray powder diffraction measurements show a clear hysteresis of the I \leftrightarrow II phase transition of TAHS with transition temperatures of T up = 412.9 (1) K on heating and of T down = 402.6 (1) K on cooling. From the existence of hysteresis and from the jump-like changes of the lattice parameters, the I \leftrightarrow II phase transition of TAHS is considered to be first order. With DSC/TG measurements we confirmed that there is only one phase transition between 293 and 493 K. Through careful investigation on single crystals of TAHS using neutron diffraction, the correct space group (C2/c) of room-temperature TAHS-II phase was confirmed. Crystal structure analysis by single-crystal neutron diffraction showed a strongly elongated displacement ellipsoid of the proton which lies in the middle of the (SO4)H(SO4) dimer with \bar 1 local symmetry. The protons of the NH4 groups also show strongly enlarged anisotropic mean-square displacements. These findings are interpreted in terms of a characteristic proton disorder in the TAHS-II phase.

Data obtained from single crystal X-ray and neutron diffraction experiments have been combined with results from thermogravimetry in order to derive an improved structure model for potassium tris(oxalato)chromate hydrate. The degree of hydration for this compound has been reinvestigated: Earlier work assumed a trihydrate stoichiometry and had to accept an unusually short K···O distance of 2.3 Å. Our neutron data reveal the position of the hydrogen atoms in the water molecules; they prove that abnormally short separations between a cation and atoms of a water molecule can only occur between sites of mutually exclusive occupancy and hence remain without chemical relevance. Closest K···O distances in our revised hydration model amount to 2.6 Å, in good agreement with expectation. Both diffraction experiments and thermogravimetric data agree with the stoichiometry K3[Cr(ox)3] · 2.7 H2O for the title compound.

Single crystal neutron diffraction was used to characterize the structural evolution of electrochemically oxidized La2NiO4+δ crystals as a function of temperature. In order to better describe the disorder of the apical and interstitial oxygen atoms we used the maximum entropy method. There is structural evidence not to attribute a peroxide character between the apical and interstitial oxygen atoms situated in exactly ( 1 4 1 4 1 4 ) of the F4/mmm unit cell. We have also reinvestigated the structural phase transition of stoichiometric La2NiO4 crystals at 74 K. The symmetry changes from Bmab to Pccn and remains orthorhombic down to 5 K.

The neutron diffraction study of the crystal structure of C(CH 3 )(OH)(PO 3 H 2 ) 2 ·;H 2 O carried out at room temperature led to a least squares refinement based on 949 reflections with R ( F 2 ) = 0.0471 and R w ( F 2 ) = 0.0473. This compound crystallizes with the monoclinic P2 1 / c (n°;14) space group and a = 6.983(3)A, b = 17.56(3)A, c = 7.109(9)A, g = 108.5(1)°;, V = 826.4A 3 , and Z = 4. This investigation gives precise details about the hydrogen atom positions, the hydrogen bonds, and the configuration of the hydroxyethylidene bisphosphonic molecule which is then compared to that of the rubidium salt previously studied also by neutron diffraction. 2

A displacive, 2nd order structural phase transition at Ts=395 K from space group I`4 2 m below Ts to I 4/m c m above Ts has been discovered in the two-dimensional spin dimer compound SrCu2(BO3)2. The temperature evolution of the structure in both phases has been studied by X-ray diffraction and Raman scattering, supplemented by differential scanning calorimetry and SQUID magnetometry. The implications of this transition and of the observed phonon anomalies in Raman scattering for spin-phonon and interlayer coupling in this quantum spin system will be discussed., Comment: 13pages, 13 figures

In this paper we present a high temperature heating device, working under defined environmental conditions, for a Siemens D500 Bragg–Brentano powder diffractometer. The powder sample is prepared in a flat mould on a metal block consisting either of copper or of platinum depending on the temperature range selected for investigations. Although the heating cell can be used separately under ambient conditions up to sample temperatures of 1000 °C, it is possible to work under defined environmental conditions in the temperature range between 20 and 200 °C and up to a water vapour pressure of 1000 mbar. For that purpose a special cover for the in situ control of temperature and water vapour pressure has been constructed. It is important to note that the three sample conditions (sample temperature, gas temperature, and gas humidity) can be adjusted separately by the user. Current studies have shown that the described X-ray heating device is a powerful tool to study dehydration reactions in the frame of fundamental research as well as to understand industrially relevant processes concerning dehydration reactions and their mechanisms.

The crystal structures of CeO1.68 (C–Ce2O3+δ), Ce7O12, and Ce11O20 with ordered oxygen vacancy distributions have been determined by refinement of single crystal neutron diffraction data collected at ambient temperature. CeO1.68 is cubic, of space group Ia3 with a=11.111(2) A; one of the two oxygen positions is partially occupied with a probability of 37(3)%. For Ce7O12 the rhombohedral space group R3 with a=6.785(1) A, α=99.42(1)° was confirmed. Ce11O20 is isomorphous to Tb11O20 with the triclinic space group P1. The lattice parameters are a=6.757(3) A, b=10.260(5) A, c=6.732(3) A, α=90.04(4)°, β=99.80(4)°, and γ=96.22(4)°. For Ce11O20 there are no 12[1, 1, 1]F oxygen vacancy pairs, i.e., pairs corresponding to half of the space diagonal of the fluorite lattice along [1, 1, 1]F. The shortest vacancy pair type in that structure is 12[1, 1, 1]F, which is also present in the two other phases. High temperature neutron diffraction measurements showed that Ce7O12 is stable up to 1072 K. Then it transforms into a fluorite type structure with disordered oxygen vacancies.

The structure of the spin-Peierls compound ${\mathrm{CuGeO}}_{3}$ has been analyzed by time-of-flight neutron powder diffraction as a function of external pressure. The structural changes allow us to explain the strong pressure dependence of the magnetic susceptibility most likely related to the pronounced pressure-induced increase of the spin-Peierls transition temperature. The structural changes induced by Si substitution are found to be opposite to those observed upon applying external pressure.

In a laboratory X-ray powder diffraction study, the evaluation of the patterns of three Bragg–Brentano powder diffractometers with different monochromator geometries has been undertaken. For the measurements on each diffractometer, the standard reference material SRM 640 (silicon) and the corundum samples SRM 674a and SRM 1976 have been used. In each case, the peak profiles were fitted with a split Pearson VII function and the FWHM (full width at half-maximum) parameters and exponentmwere determined for the left (lower 2θ) and the right (higher 2θ) sides of the Bragg peaks. It was found that there is a strong dependence of both the FWHM and the exponentmon the diffraction angle for the two configurations that included monochromators, whereas nearly constant values ofmwere found for the geometrically simplest diffractometer working without a monochromator. Finally, the two components of the CuKα doublet show systematically different peak profiles. There is a clear difference not only concerning the FWHM, which becomes more obvious at higher 2θ values, but also in the course ofmwith respect to the diffraction angle for the left and the right tails of the powder reflections. This is the main reason for the difficulties inKα2stripping and also in single-line-profile analysis when using theKα doublet. Therefore, it is not surprising that this phenomenon, which can be explained by Heisenberg's uncertainty principle, does affect the reliability (represented by standardRvalues) of structure refinement by the Rietveld method.

Bragg intensities from neutron diffraction data for single crystals were used to determine the rotational potential at room temperature. The rotational potential is evaluated in terms of mixed rotator functions . The expansion coefficients , i.e. potential parameters, are obtained directly from experiment. The potential exhibits well developed minima, absolute ones at the Euler angle set and relative ones at . They correspond to the two orientations, i.e. pentagonal and hexagonal facing, of the molecules in their low-temperature arrangement. The overall potential barrier height of the absolute minimum is 400 K and the difference between the two types of minima is 270 K.

Gaseous 3He neutron spin filters (NSF) are simple transmission devices, with predictable characteristics, that do not require any special optical adjustments on a neutron beam. Since polarized nuclei of 3He possess very high spin-dependent neutron absorption efficiency over a wide range of energies, the 3He NSF can be used as a broadband neutron polarizer or analyzer, with the possibility to optimize its efficiency for nearly all neutron wavelengths. In 2005 the facility for large-scale production of highly polarized 3He gas, HELIOS, was put into operation at FRM-II.

A new hot neutrons single-crystal diffractometer for magnetic structure investigations with polarised neutrons is under construction now at FRM-II near Munich. The problems by implementing the polarised neutron option on the existing instrument HEiDi lead to the decision to build a parallel instrument especially designed for this purpose, called Polarisation Investigator-HEiDi (POLI-HEiDi). Both options, zero field for spherical polarisation analysis in complex magnetic structures and high magnetic field for flipping ratio measurements to study spin density distributions, should be available at the new instrument. We present the proposed design and construction of POLI-HEiDi. As neutron polarisation method, 3He spin filter cells (NSFC's) are planned to be used. Dedicated filter cell design as well as the results of the first experiments with a 3He NSFC, performed recently at the HEiDi diffractometer by means of compact magnetostatic cavities, are discussed. The novel multifunctional compact device Helium Rotating Polariser (HERO) is presented.

HEiDi is one of the two single crystal diffractometers at the neutron source Heinz Maier-Leibnitz (FRM II). The instrument is placed at the beam tube SR9b in the experimental hall. It was developed in collaboration between the RWTH Aachen (Institut fur Kristallographie) and the TU Munchen (FRM II) to cover a broad range of scientific cases in the area of structural research on single crystals.

The crystal structure of the antiferroelectric phase of TlD2PO4, deuterated thallium dihydrogenphosphate, has been determined from single-crystal neutron diffraction data collected at room temperature. The para-antiferroelectric transition (T_c^d = 353 K) of TlD2PO4 is analysed from a structural point of view and compared with the phase transition of TlH2PO4 at TI = 357 K, already characterized. The distinct phase sequences observed in the two compounds when decreasing temperature from that of the high-temperature prototype phase (prototype phase/room-temperature phase/low-temperature phase) are discussed and associated with the different ordering of the two crystallographically inequivalent H (D) atoms existing in the prototype phase.

The structural phase transition in Gd 2− x Ce x CuO 4 ( x = 0, 0.12) characterized by the rotation of the CuO 4 -squares around the c axis has been studied by different diffraction techniques. The order parameter of this transition is found to increase continuously below 658(1) K for Gd 2 CuO 4 and below 707.2(7) K for Gd 1.88 Ce .12 CuO 4 with an unusual temperature dependence. The rotation of the CuO 4 -squares is coupled to a spontaneous strain along the c -axis. Structure analysis of the undistorted high temperature T′-phase in Gd 2 CuO 4 indicates persisting local distortions.

The structure of the antiferroelectric phase in TlD2PO4 is presented. The phase diagrams of both TlH2PO4 and TD2PO4 are discussed from the point of view of a structural study by neutron diffraction on single crystals, carried out at different temperatures.

The structure of ${\mathrm{CuGeO}}_{3}$ has been analyzed by means of x-ray and neutron single-crystal diffraction as a function of temperature. There is a pronounced temperature dependency of the atomic positions in the high-temperature Pbmm phase. The observed deformation scheme on cooling from 295 to 20 K may be characterized by a rotation of the Cu[O(2)${]}_{2}$ ribbons around the c axis. It is related to the distortion below the spin-Peierls transition, and has an impact on the magnetic interaction parameter J. For the distorted structure in the spin-Peierls phase, based on a large set of superstructure reflections, we qualitatively confirm the proposed model (space group Bbcm) with a slightly different direction of the oxygen displacements. A computation of J reveals that its splitting in the spin-Peierls phase is dominated by the modulation of the Cu-O-Cu bond angle. \textcopyright{} 1996 The American Physical Society.

Bragg intensities from neutron and X-ray diffraction data of C 60 single crystals were used to determine the nuclear- and electron-density distributions of C 60 at room temperature. The anisotropic density distribution is reconstructed by the maximum-entropy method and evaluated in terms of symmetry-adapted spherical harmonics. From this analysis, the orientational probability density function f(ω) has been calculated and the rotational potential V(ω) that is experienced by a C 60 molecule in the cubic surrounding at 295K has been obtained. f(ω) shows strong deviations from the uniform orientational probability density function that would result from isotropic rotation. Accordingly, V(ω) exhibits well developed minima. The absolute potential minimum is found at an Euler-angle set ω 1 and a second set of minima at slightly higher energy at ω 2 . The potential difference between V(ω 1 ) and V(ω 2 ) is 313 K, whereas the overall rotational potential barrier height amounts to 522 K. ω 1 and ω 2 are comparable with the major and minor orientations that are adopted by the molecules in their low-temperature arrangement. The angles ω 1 and ω 2 are fixed by the intrinsic geometry of the Euler-angle space (α, β, γ) under the combined action of the cubic site and the icosahedral molecular point group.

Les structures cristallines de TlH 2 PO 4 et TlD 2 PO 4 ont ete determinees a 373 K par diffraction de neutrons sur des echantillons monocristallins. L'acquisition de donnees est faite avec un diffractometre 4-cercles, en utilisant la longueur d'onde λ = 0.830 A. A cette temperature les deux composes ont la meme symetrie orthorhombique decrite dans le groupe d'espace Pbcn. Cette phase haute temperature apparait comme la phase mere du diagramme de phase obtenu pour des temperatures decroissantes pour chacun des deux composes TDP et DTDP.

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.

In order to explain the observed 35Cl-NQR anomaly of the title compound, a neutron diffraction study on single crystals was carried out at 295 K and 158 K. No significant structural changes concerning phase transitions or molecular disorder between the two experiments were observed. The space group is P21/c = C2h 5 with 8 molecules per unit cell. The asymmetric unit contains 2 molecules. The cell parameters were determined to a = 1264(4) pm, b = 825(2) pm, c = 1503(3) pm, ß = 14.3(3)° at 295 K and a = 1243(2)pm, b = 815(1) pm, c = 1494(2) pm, ß = 114.3(1)° at 158 K. The molecules show rigid-body behaviour. No significant changes of the molecular geometry were observed. A strong variation of the temperature factors as function of the temperature was detected. An anharmonicity of the librational potential of the molecules was found by evaluation of the thermal parameters as function of temperature. For verifying the existence of dynamic processes which lead to a bleaching out of the 35Cl-NQR lines difference, Fourier syntheses were calculated. A reorientation or an order-disorder process as suggested by 35Cl-NQR spectroscopy above 230 K could not be confirmed within the error of the experiments. The degree of deuteration of the samples was determined by refinement of the occupation factors of the deuterium sites.

Die Titelverbindung ist das erste beschriebene Hydrat eines ternaren Alkalihydroxids. Ihre Struktur ist verwandt mit der der hexagonalen Perowskite und zugleich durch ein komplexes System von Wasserstoffbrucken gepragt. Dieses war in einer vorangegangenen Rontgenstrukturanalyse bei Raumtemperatur an der isotypen nicht deuterierten Substanz nur unzureichend charakterisiert worden. Daher wurde nun eine vollstandige Strukturbestimmung an einem perdeuterierten Einkristall mit Tieftemperatur-Neutronenbeugungsdaten durchgefuhrt (Pca21, Z = 4; a = 13,86, b = 6,070, c = 12,41 A bei 120 K; 1 655 beobachtete unabhangige Reflexe, R = 0,027). Hydrates of Weak and Strong Bases. IX. Hydrogen Bonding in CsNa2(OD)3 · 6D2O: A Single-Crystal Neutron-Diffraction Study at 120 K The title compound is the first hydrate of a ternary alkali hydroxide reported. Its structure is related to the hexagonal perovskites and at the same time shaped by a complex system of hydrogen bonds. In a previous X-ray structure analysis at room temperature of the isotypic undeuterated material, these had been characterized only insufficiently. Therefore a complete structure determination has now been performed on a perdeuterated single crystal using low-temperature neutron-diffraction data (Pca21, Z = 4; a = 13.86, b = 6.070, c = 12.41 A at 120 K; 1 655 observed independent reflections, R = 0.027).

We present the results of neutron-diffraction experiments on a series of La2−xSrxCuO4+δ single crystals at temperatures between 540 K and 22 K. There is an intrinsic misfit between the size of the LaO9 polyhedra and the CuO6 octahedra, which seems to cause the structural transition from the high-temperature tetragonal (HTT) to the low-temperature orthorhombic (LTO) phase. Within this misfit model it is possible to explain almost the entire phenomenology of the HTT-LTO transition. The electronic structure is related to this phase transition via the length of the CuO bonds. One of the effects of the transition on the structure is to lengthen these CuO bonds and hence to alter the electronic band structure. The introduction of additional charge by doping leads to a decrease of the equilibrium CuO distances, and hence to a reduction of the misfit. In this way the charge-carrier concentration determines the structural transition. We always observe strongly anisotropic thermal parameters, which confirm the instability of the La2−xSrxCuO4 structure against a rotation of the CuO6 octahedra around any axis. In the case of an undoped crystal (x = 0.0), we find a good agreement between the experimental mean square displacements and the results of a lattice dynamics model which is based on the known phonon dispersion curves. In a doped (x = 0.13) and superconducting crystal, the anharmonic effects related to the structural transition lead to significant changes in the temperature dependence of the thermal parameters.

The atomic displacement parameters of individual ions in SrO have been determined from the Rietveld analysis of high-resolution powder neutron diffraction data. As the neutron velocity is smaller than the sound velocity in SrO, the intensity data was not corrected for the effect of thermal diffuse scattering. The room temperature value of overall isotropic thermal displacement parameters wasB=0.57(2) Å2; which corresponds to Debye temperature, Θ of 242(4) K. The results are compared with experimental and theoretical estimates.

Capacitance-dilatometer measurements of the anisotropic thermal expansion on a superconducting single crystal of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (x\ensuremath{\sim}0.13) show large anomalies at the structural transition from the high-temperature tetragonal to the low-temperature orthorhombic phase followed by smaller anomalies of the opposite sign at the superconducting phase transition. Neutron-diffraction experiments on the same crystal indicate that at ${\mathit{T}}_{\mathit{c}}$ the tilt of the ${\mathrm{CuO}}_{6}$ octahedra related to the structural phase transition is frozen in.

The crystal structure of the orthorhombic high-temperature phase II (708–949 K) of KLiSO4 was analyzed by single-crystal neutron diffraction. The ambiguity concerning the symmetry of this phase is solved and a disordered structure is established in the space-group Pcmn. Hence, KLiSO4 II is confirmed to be isostructural to the highest-temperature modification I of RbLiSO4.The investigated KLiSO4 crystals were pseudohexagonally twinned with three ferroelastic domain states according to the phase transition from P63 (phase III at room temperature) to Pcmn (phase II).

Synthetic Co2SiO4 crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50K. We have studied the magnetic properties of Co2SiO4 by means of both nonpolarized and polarized neutron diffraction as well as X-ray magnetic circular dichroism. Measurements with non-polarized neutrons were made at 2.5 K whereas polarized neutron diffraction experiments were done at 70 K and 150 K in an external magnetic field of 7 T parallel to the b axis. Symmetry analysis of the Co2SiO4 magnetic structure proves that it corresponds to the magnetic (Shubnikov) group Pnma which allows the antiferromagnetic configuration (Gx,Cy,Az) for the 4a site with inversion symmetry (Co1 position) and (0,Cy,0) for the 4c site with mirror symmetry m (Co2 position). The polarized neutron study of the magnetization induced by an applied field indicates a delocalization of the magnetic moment from Co towards neighbouring O due to the superexchange coupling. This is also in agreement with X-ray magnetic circular dichroism measurements performed at the Co K-edge in the temperature range 7K–150K in an external magnetic field of 6T applied close to the b axis of Co2SiO4. An important orbital contribution to the total magnetic moment of Co2+ in Co2SiO4 predicted by non-polarized neutron diffraction was independently proven by means of the X-ray magnetic circular dichroism and polarized neutron diffraction studies which indicate an orbital to spin magnetic moment ratio μL/μs ≈ 0.3.

Synthetic Co2SiO4 crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50 K. We have investigated the temperature variation of the Co2SiO4 magnetic structure by means of non-polarized and polarized neutron diffraction for single crystals. Measurements with non-polarized neutrons were made at 2.5 K (below T N), whereas polarized neutron diffraction experiments were carried out at 70 and 150 K (above T N) in an external magnetic field of 7 T parallel to the b axis. Additional accurate non-polarized powder diffraction studies were performed in a broad temperature range from 5 to 500 K with small temperature increments. Detailed symmetry analysis of the Co2SiO4 magnetic structure shows that it corresponds to the magnetic (Shubnikov) group Pnma, which allows the antiferromagnetic configuration (Gx , Cy , Az ) for the 4a site with inversion symmetry \bar{1} (Co1 position) and (0,Cy ,0) for the 4c site with mirror symmetry m (Co2 position). The temperature dependence of the Co1 and Co2 magnetic moments obtained from neutron diffraction experiments was fitted in a modified molecular-field model. The polarized neutron study of the magnetization induced by an applied field shows a non-negligible amount of magnetic moment on the oxygen positions, indicating a delocalization of the magnetic moment from Co towards neighbouring O owing to superexchange coupling. The relative strength of the exchange interactions is discussed based on the non-polarized and polarized neutron data.

Synthetic Co2SiO4 has an olivine structure with isolated SiO4 groups (space group Pnma) and shows magnetic ordering below 50 K. Single-crystal neutron diffraction was applied to determine precise crystal structure parameters at low temperatures. No structural phase transition was revealed in the temperature range 2.5–300 K. Lattice parameters were determined by high-resolution X-ray powder diffraction between 15 and 300 K. There is a clear evidence of an anomalous thermal expansion related to the magnetic phase transition which can be attributed to magnetostriction.

The ND3 group in Ni(ND3)6Br2 is orientationally disordered at room temperature: the D3 triangle gives rise to a density distribution with maxima on the corners of a square. We explain this observation as the consequence of a coupled rotational-translational motion of the molecule in a two-dimensional anharmonic single particle potential with tetragonal symmetry. Potential parameters are calculated from experimental data by a least squares procedure. From the total crystal potential of the D3 group we conclude that a combination of center of mass motion and rotation which is possible essentially without an energy barrier, is the basic process observed in quasielastic neutron scattering observations of the jump diffusion.

The ND3 group is in the high temperature phase of the title compound dynamically disordered. Fourier maps derived from the analysis of neutron single crystal Bragg data with phases based on a split atom and a Frenkel model show uniquely that each D3 group has four density maxima occupying the corners of a square. Neither one nor three dimensional models of purely rotational disorder can describe the observations adequately.

The structure of La2NiO4.13 has been studied as a function of temperature (20–673 K) with single crystal neutron diffraction. In order to achieve a high resolution, data sets were measured up to 1.05 A −1 . Structure refinements were carried out in this space group F4/mmm. Due to the incorporated oxygen on interstitial lattice sites the apical oxygen becomes disordered and the NiO6 octahedra are partly tilted around the [110] direction. The disorder is still present at temperatures above the high temperature orthorhombic/tetragonal phase transition of stoichiometric La2NiO4.00 at 670 K.

A combined X-ray and neutron diffraction study on powder samples and single crystals revealed the following sequence of polymorphic phases: Fdd 2 ( I ) ← 147 K → I 4 2 d ( II )—357 ± 7 K → P 2 1 / a ( III ) ← 380 ± 3 K → P 2 1 / m ( IV ) . The low temperature phases I and II are iso-structural to those of KDP. The H-ordering in RDP is almost complete at Tc − 1 K, which favours a first-order character of this ferroelectric phase transition.

The crystal structure of TlH2PO4 (TDP) has been studied at low temperature. The lattice parameters were derived from high-resolution X-ray powder diffraction in the temperature range between 8 and 300 K. A detailed crystal structure analysis of the antiferroelectric low-temperature phase TDP-III has been performed based on neutron diffraction data measured at 210 K on a twinned crystal consisting of two domain states. The structure model in the triclinic space group P\bar 1 is characterized by a complete ordering of all the H atoms in the asymmetric O—H...O hydrogen bonds. The phase transition from the ferroelastic TDP-II to the antiferroelectric TDP-III phase at 229.5 ± 0.5 K is only slightly of first order and shows no detectable hysteresis effects. Its mechanism is driven by the hydrogen ordering between the partially ordered TDP-II state and the completely ordered TDP-III state. The polymorphism of TDP and the fully deuterated TlD2PO4 (DTDP) is presented in the form of group–subgroup relations between the different space groups.

For the first time, the difference between the thermal parameters of metal and nonmetal ions has been positively established by high-resolution neutron diffraction measurements on a powder sample of TiN. Measurements were made on a double- as well as triple-axis neutron spectrometer. The data were analysed by the Rietveld method. The refined occupancy factor indicated a chemical composition of TiN0.95 (1). The values of the individual temperature factors were BTi = 0.209 (13) A2 and BN = 0.271 (7) A2. The results of the present investigation are compared with the values obtained by X-ray and neutron diffraction techniques.

Neutron and high resolution X-ray diffraction investigations on perfect single crystals of RbH2PO4 (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric - ferroelectric phase transition, support a disorder - order character of [PO4H2]--groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time-space average of orthorhombic symmetry. According to the group - subgroup relation between the tetragonal space group I42d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at TC = 147 K. From the high resolution X-ray data it is concluded that below TC the ferroelastic strain in the (a,b)-plane leads to micro-angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature TC is decreased. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric spectroscopy experiments are carried out in the ferroelectric phase of rubidium dihydrogen phosphate $({\mathrm{RbH}}_{2}{\mathrm{PO}}_{4}),$ within the frequency range $0.1 \mathrm{Hz}lfl1 \mathrm{MHz}.$ A strong dielectric time decay (ageing) dominates after thermal equilibration. However, spectra taken after long dwelling times reveal a significant dispersion below the fundamental piezoelectric resonance frequency ${f}_{\mathrm{res}}.$ The permittivity of the well-aged sample is found to decrease linearly with the logarithm of the frequency. Not detectable above the paraelectric-ferroelectric phase transition temperature ${T}_{c},$ the dispersion is observed both in the range ${T}_{c}gTg{T}_{f}\ensuremath{\approx}117 \mathrm{K}$ of the anomalously high domain wall contribution and in the low-temperature range $Tl{T}_{f}$ corresponding to the frozen-in domain-wall response. The result indicates that the concept of weak pinning in the context of the theory of elastic interfaces in disordered media may be applicable to describe the interaction between the ferroelectric/ferroelastic domain walls in ${\mathrm{RbH}}_{2}{\mathrm{PO}}_{4}$ and randomly distributed impurities.

In this work, we present a structure investigation on K3H(SO4)2 by single crystal neutron diffraction. Letovicite with a chemical composition (NH4)3H(SO4)2 belongs to a large family of M3(H,D)(XO4)2 compounds, where M = K+, Rb+, (NH4)+, Cs+, Tl+ and X = Se6+ and S6+. This compound crystallizes in the monoclinic space group A2/a with a = 9.789(7) Å, b = 5.6815(9) Å, c = 14.703(2) Å and β = 103.03(4)0at 300K. At 2.3K, the lattice parameters are a = 9.687(20) Å, b = 5.648(13) Å, c = 14.613(9) Å and β = 103.23(14)0. Data at 2.3K were measured up to (sinθ/λ) = 0.807Å-1 with the single crystal neutron diffractormeter HEiDi at the FRM-II, Germany. H/D shows a dynamic disorder at high temperature, which can be related to very high proton conductivity. In letovicite, two types of disorder related with hydrogen atoms are reported [1]. Although letovicite shows various phase transitions owing to the proton ordering at low temperature, K3H(SO4)2, without the possibility of an orientational disorder of NH4+, undergoes no phase transition at low temperature. At room temperature, the title compound is isostructural to lectovicite, and has an inversion center in the middle of the SO4-H-SO4 dimer. The bond length, 2.483(3) Å, and bond angle, 1800, support the hypothesis that the disordered proton shows a double-well potential, if the distance between the oxygen atoms of the hydrogen bond Ro-o are longer than a critical bond length rc(2.47 Å for protons and 2.40 Å for deuterons) [2]. However, it is not easy to determine if the hydrogen bond is a low-barrier hydrogen bond (LBHB) or centered hydrogen bond (centered HB). Based on an analysis of the anisotropic parameters, the bond lengths and elongation of the hydrogen atom toward the two oxygen atoms by neutron single crystal diffraction experiments at 300K and 2.3K, it seems that the hydrogen bond in the title compound can be classified as a centered hydrogen bond or intermediate form between a cigar-like shape and the disk-like shape [3].

We have determined the anisotropic Debye-Waller parameters of ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ with high precision using neutron diffraction on single crystals. The mean-square displacements are explained well by the contribution due to phonons and disorder induced by the mixing of La and Sr ions; the latter was simulated by relaxing the structure locally. Our analysis puts rather low upper boundaries on the amplitude of possible hidden local structural distortions; in particular, there is no evidence for in-plane charge inhomogeneities related to stripe correlations. Further, we conclude that the tilt transition in ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ is essentially displacive and continuous in nature.