Accurate hydrogen parameters for the amino acid L-leucine

The structure of the primary amino acid L-leucine has been determined for the first time by neutron diffraction. This was made possible by the use of modern neutron Laue diffraction to overcome the previously prohibitive effects of crystal size and quality. The packing of the structure into hydrophobic and hydrophilic layers is explained by the intermolecular interaction energies calculated using the PIXEL method. Variable-temperature data collections confirmed the absence of phase transitions between 120 and 300 K in the single-crystal form.</jats:p

Neutron and high-pressure X-ray diffraction study of hydrogen-bonded ferroelectric rubidium hydrogen sulfate

The pressure- and temperature-dependent phase transitions in the ferroelectric material rubidium hydrogen sulfate (RbHSO4) are investigated by a combination of neutron Laue diffraction and high-pressure X-ray diffraction. The observation of disordered O-atom positions in the hydrogen sulfate anions is in agreement with previous spectroscopic measurements in the literature. Contrary to the mechanism observed in other hydrogen-bonded ferroelectric materials, H-atom positions are well defined and ordered in the paraelectric phase. Under applied pressure RbHSO4undergoes a ferroelectric transition before transforming to a third, high-pressure phase. The symmetry of this phase is revised to the centrosymmetric space groupP21/c, resulting in the suppression of ferroelectricity at high pressure.</jats:p

Quenched chirality in RbNiCl3_3

The critical behaviour of stacked-triangular antiferromagnets has been intensely studied since Kawamura predicted new universality classes for triangular and helical antiferromagnets. The new universality classes are linked to an additional discrete degree of freedom, chirality, which is not present on rectangular lattices, nor in ferromagnets. However, the theoretical as well as experimental situation is discussed controversially, and generic scaling without universality has been proposed as an alternative scenario. Here we present a careful investigation of the zero-field critical behaviour of RbNiCl$_3$, a stacked-triangular Heisenberg antiferromagnet with very small Ising anisotropy. From linear birefringence experiments we determine the specific heat exponent $\alpha$ as well as the critical amplitude ratio $A^+/A^-$. Our high-resolution measurements point to a single second order phase transition with standard Heisenberg critical behaviour, contrary to all theoretical predictions. From a supplementary neutron diffraction study we can exclude a structural phase transition at T$_N$. We discuss our results in the context of other available experimental results on RbNiCl$_3$ and related compounds. We arrive at a simple intuitive explanation which may be relevant for other discrepancies observed in the critical behaviour of stacked-triangular antiferromagnets. In RbNiCl$_3$ the ordering of the chirality is suppressed by strong spin fluctuations, yielding to a different phase diagram, as compared to e.g.\@ CsNiCl$_3$, where the Ising anisotropy prevents these fluctuations

Use of a miniature diamond-anvil cell in a joint X-ray and neutron high-pressure study on copper sulfate pentahydrate

Single-crystal X-ray and neutron diffraction data are usually collected using separate samples. This is a disadvantage when the sample is studied at high pressure because it is very difficult to achieve exactly the same pressure in two separate experiments, especially if the neutron data are collected using Laue methods where precise absolute values of the unit-cell dimensions cannot be measured to check how close the pressures are. In this study, diffraction data have been collected under the same conditions on the same sample of copper(II) sulfate pentahydrate, using a conventional laboratory diffractometer and source for the X-ray measurements and the Koala single-crystal Laue diffractometer at the ANSTO facility for the neutron measurements. The sample, of dimensions 0.40 × 0.22 × 0.20 mm(3) and held at a pressure of 0.71 GPa, was contained in a miniature Merrill–Bassett diamond-anvil cell. The highly penetrating diffracted neutron beams passing through the metal body of the miniature cell as well as through the diamonds yielded data suitable for structure refinement, and compensated for the low completeness of the X-ray measurements, which was only 24% on account of the triclinic symmetry of the sample and the shading of reciprocal space by the cell. The two data-sets were combined in a single ‘XN’ structure refinement in which all atoms, including H atoms, were refined with anisotropic displacement parameters. The precision of the structural parameters was improved by a factor of up to 50% in the XN refinement compared with refinements using the X-ray or neutron data separately

Distinction of disorder, classical and quantum vibrational contributions to atomic mean-square amplitudes in dielectric pentachloronitrobenzene

The solid-state molecular disorder of pentachloronitrobenzene (PCNB) and its role in causing anomalous dielectric properties are investigated. Normal coordinate analysis (NCA) of atomic mean-square displacement parameters (ADPs) is employed to distinguish disorder contributions from classical and quantum-mechanical vibrational contributions. The analysis relies on multitemperature (5-295 K) single-crystal neutron-diffraction data. Vibrational frequencies extracted from the temperature dependence of the ADPs are in good agreement with THz spectroscopic data. Aspects of the static disorder revealed by this work, primarily tilting and displacement of the molecules, are compared with corresponding results from previous, much more in-depth and time-consuming Monte Carlo simulations; their salient findings are reproduced by this work, demonstrating that the faster NCA approach provides reliable constraints for the interpretation of diffuse scattering. The dielectric properties of PCNB can thus be rationalized by an interpretation of the temperature-dependent ADPs in terms of thermal motion and molecular disorder. The use of atomic displacement parameters in the NCA approach is nonetheless hostage to reliable neutron data. The success of this study demonstrates that state-of-the-art single-crystal Laue neutron diffraction affords sufficiently fast the accurate data for this type of study. In general terms, the validation of this work opens up the field for numerous studies of solid-state molecular disorder in organic materials.Comment: Now published in Physical Review

Magnetic interactions in thiazyl-based magnets: The role of the charge and spin densities

The crystal structure of the organic radical p-O2NC6F4CNSSN was determined at 20 K through a single-crystal neutron-diffraction experiment. It crystallises in the tetragonal space group P41212, unchanged from a previous single-crystal X-ray diffraction experiment at 220 K although there are some changes in molecular geometry and intermolecular contacts arising from the contraction of the unit cell. Polarized neutron diffraction at 1.5 K revealed that the spin distribution is predominantly localised on the N and S atoms of the heterocyclic ring with a small negative spin density on the heterocyclic C atom. Spin populations determined using a multipolar analysis were -0.06, +0.25 and +0.28 on the C, N and S sites, respectively. These spin populations are in excellent agreement with both ab-initio DFT calculations (spin populations on the C, N and S sites of -0.07, 0.22 and 0.31, respectively) and cw-EPR studies which estimated the spin population on the N site as 0.24. The DFT calculated spin density revealed less than 1% spin delocalisation onto the perfluoroaryl ring, several orders of magnitude lower than the density on the heterocyclic ring. cw-ENDOR studies at both X-band (9 GHz) and Q-band (34 GHz) frequencies probed the spin populations at the two chemically distinct F atoms. These spin populations on the F atoms ortho and meta to the dithiadiazolyl ring are of magnitude 10-3 and 10-4 respectively. Additional high-resolution single-crystal X-ray diffraction studies at 100 K analysed within the atoms-in-molecules (AIM) framework gave detailed information on the charge density distributio

Accurate H-atom parameters for the two polymorphs of L-histidine at 5, 105 and 295 K

The crystal structure of the monoclinic polymorph of the primary amino acid l-histidine has been determined for the first time by single-crystal neutron diffraction, while that of the orthorhombic polymorph has been reinvestigated with an untwinned crystal, improving the experimental precision and accuracy. For each polymorph, neutron diffraction data were collected at 5, 105 and 295 K. Single-crystal X-ray diffraction experiments were also performed at the same temperatures. The two polymorphs, whose crystal packing is interpreted by intermolecular interaction energies calculated using the Pixel method, show differences in the energy and geometry of the hydrogen bond formed along the c direction. Taking advantage of the X-ray diffraction data collected at 5 K, the precision and accuracy of the new Hirshfeld atom refinement method implemented in NoSpherA2 were probed choosing various settings of the functionals and basis sets, together with the use of explicit clusters of molecules and enhanced rigid-body restraints for H atoms. Equivalent atomic coordinates and anisotropic displacement parameters were compared and found to agree well with those obtained from the corresponding neutron structural models