Role of vibrational entropy in the stabilization of the high-temperature phases of iron

The phonon dispersions of the bcc and fcc phases of pure iron ({\alpha}-Fe, {\gamma}-Fe and {\delta}-Fe) at ambient pressure were investigated close to the respective phase transition temperatures. In the open bcc structure the transverse phonons along T1 [{\xi}{\xi}0] and T1 [{\xi}{\xi}2{\xi}] are of particularly low energy. The eigenvectors of these phonons correspond to displacements needed for the transformation to the fcc {\gamma}-phase. Especially these phonons, but also all other phonons soften considerably with increasing temperature. Comparing thermodynamic properties of the fcc and the two bcc phases it is shown that the high temperature bcc phase is stabilized predominantly by vibrational entropy, whereas for the stabilization of the fcc phase electronic entropy provides an equal contribution.Comment: to appear in Physical Review

Dynamics of C-phycocyanin in various deuterated trehalose/water environments measured by quasielastic and elastic neutron scattering

The molecular understanding of protein stabilization by the disaccharide trehalose in extreme temperature or hydration conditions is still debated. In the present study, we investigated the role of trehalose on the dynamics of the protein C-phycocyanin (C-PC) by neutron scattering. To single out the motions of C-PC hydrogen (H) atoms in various trehalose/water environments, measurements were performed in deuterated trehalose and heavy water (D2O). We report that trehalose decreases the internal C-PC dynamics, as shown by a reduced diffusion coefficient of protein H atoms. By fitting the Elastic Incoherent Structure Factor—which gives access to the “geometry” of the internal proton motions—with the model of diffusion inside a sphere, we found that the presence of trehalose induces a significantly higher proportion of immobile C-PC hydrogens. We investigated, by elastic neutron scattering, the mean square displacements (MSDs) of deuterated trehalose/D2O-embedded C-PC as a function of temperature in the range of 40–318 K. Between 40 and ∼225 K, harmonic MSDs of C-PC are slightly smaller in samples containing trehalose. Above a transition temperature of ∼225 K, we observed anharmonic motions in all trehalose/water-coated C-PC samples. In the hydrated samples, MSDs are not significantly changed by addition of 15% trehalose but are slightly reduced by 30% trehalose. In opposition, no dynamical transition was detected in dry trehalose-embedded C-PC, whose hydrogen motions remain harmonic up to 318 K. These results suggest that a role of trehalose would be to stabilize proteins by inhibiting some fluctuations at the origin of protein unfolding and denaturation

Neutron macromolecular crystallography at the FRM IIThe neutron single crystal diffractometer BIODIFF

The research reactor Heinz Maier-Leibnitz (FRM II) is a modern high flux neutron source which feeds at the present 27 state of the art instruments. The newly build neutron single crystal diffractometer BIODIFF is especially designed to collect data from crystals with large unit cells. The main field of application is the structure analysis of proteins, especially the determination of hydrogen atom positions. BIODIFF is a joint project of the Forschungszentrum Jülich (FZJ/JCNS) and the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II). Typical scientific questions addressed are the determination of protonation states of amino acid side chains and the characterization of the hydrogen bonding network between the protein and an inhibitor or substrate. BIODIFF is designed as a monochromatic instrument. By using a highly orientated pyrolytic graphite monochromator (PG002) the diffractometer is able to operate in the wavelength range of 2.4 Å to about 5.6 Å. Contaminations of higher order wavelengths are removed by a neutron velocity selector. To cover a large solid angle and thus to minimize the data collection time the main detector of BIODIFF consists of a neutron imaging plate system in a cylindrical geometry. A Li/ZnS scintillator CCD camera is available for additional detection abilities. The main advantage of BIODIFF is the possibility to adapt the wavelength to the size of the unit cell of the sample crystal while operating with a clean monochromatic beam that keeps the background level low. BIODFF is equipped with a standard Oxford Cryosystem “Cryostream 700+” which allows measurements in the temperature regime from 90K up to 500K

Diffraction-based determination of single-crystal elastic constants of polycrystalline titanium alloys

Single-crystal elastic constants have been derived by lattice strain measurements using neutron diffraction on polycrystalline Ti-6Al-4V, Ti-6Al-2Sn-4Zr-6Mo and Ti-3Al-8V-6Cr-4Zr-4Mo alloy samples. A variety of model approximations for the grain-to-grain interactions, namely approaches by Voigt, Reuss, Hill, Kroener, de Wit and Matthies, including texture weightings, have been applied and compared. A load-transfer approach for multiphase alloys was also implemented and the results are compared with single-phase data. For the materials under investigation, the results for multiphase alloys agree well with the results for single-phase materials in the corresponding phases. In this respect, all eight elastic constants in the dual-phase Ti-6Al-2Sn-4Zr-6Mo alloy have been derived for the first time

Powder diffraction computed tomography: A combined synchrotron and neutron study

Diffraction and imaging using x-rays and neutrons are widely utilized in different fields of engineering, biology, chemistry and/or materials science. The additional information gained from the diffraction signal by x-ray diffraction and computed tomography (XRD-CT) can give this method a distinct advantage in materials science applications compared to classical tomography. Its active development over the last decade revealed structural details in a non-destructive way with unprecedented sensitivity. In the current contribution an attempt to adopt the well-established XRD-CT technique for neutron diffraction computed tomography (ND-CT) is reported. A specially designed \u27phantom\u27, an object displaying adaptable contrast sufficient for both XRD-CT and ND-CT, was used for method validation. The feasibility of ND-CT is demonstrated, and it is also shown that the ND-CT technique is capable to provide a non-destructive view into the interior of the \u27phantom\u27 delivering structural information consistent with a reference XRD-CT experiment

A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth borohydrides LiBH 4 and Mg(BH 4 ) 2 and the mixture LiBH 4 + Mg(BH 4 ) 2

Quasielastic neutron scattering was used to investigate the low energy transfer dynamics of the complex borohydrides Mg(BH₄)₂ in the α- and β-modifications, LiBH₄ in the low and high temperature crystal structure, and an 1 : 1 molar mixture of LiBH₄ + α-Mg(BH₄)₂. All investigated compounds show a rich dynamic behaviour below an energy range of ΔE = 10 meV with the superposition of rotational dynamics of the constituent [BH₄]¯ anions and low lying lattice modes. For Mg(BH₄)₂, the rotational diffusion of the [BH₄] units was found to be much more activated in the metastable β-polymorph compared to the α-phase, and the low lying lattice modes are even softer in the former crystal structure. In Mg(BH₄)₂, the structural phase transition is mainly governed by the lattice dynamics, while alkaline LiBH₄ exhibits a transition of the [BH₄] rotations around the phase transition temperature. Ball milled LiBH₄ + α-Mg(BH₄)₂ remains a physical mixture of the parent compounds and each component retains its characteristic dynamic signature up to the melting temperature

Comments by the Guest Editor

n/