Lasse Bjerg

Experimental determination of core electron deformation in diamond

Synchrotron powder X-ray diffraction data are used to determine the core electron deformation of diamond. Core shell contraction inherently linked to covalent bond formation is observed in close correspondence with theoretical predictions. Accordingly, a precise and physically sound reconstruction of the electron density in diamond necessitates the use of an extended multipolar model, which abandons the assumption of an inert core. The present investigation is facilitated by negligible model bias in the extraction of structure factors, which is accomplished by simultaneous multipolar and Rietveld refinement accurately determining an atomic displacement parameter (ADP) of 0.00181 (1) Å(2). The deconvolution of thermal motion is a critical step in experimental core electron polarization studies, and for diamond it is imperative to exploit the monatomic crystal structure by implementing Wilson plots in determination of the ADP. This empowers the electron-density analysis to precisely administer both the deconvolution of thermal motion and the employment of the extended multipolar model on an experimental basis.

Multi-temperature synchrotron PXRD and physical properties study of half-Heusler TiCoSb

Phase pure samples of the half-Heusler material TiCoSb were synthesised and investigated. Multi-temperature synchrotron powder X-ray diffraction (PXRD) data measured between 90 and 1000 K in atmospheric air confirm the phase purity, but they also reveal a decomposition reaction starting at around 750 K. This affects the high temperature properties since TiCoSb is semiconducting, whereas CoSb is metallic. Between 90 K and 300 K the linear thermal expansion coefficient is estimated to be 10.5 × 10(-6) K(-1), while it is 8.49 10(-6) K(-1) between 550 K and 1000 K. A fit of a Debye model to the Atomic Displacement Parameters obtained from Rietveld refinement of the PXRD data gives a Debye temperature of 395(4) K. The heat capacity was measured between 2 K and 300 K and a Debye temperature of 375(5) K was obtained from modelling of the data. Coming from low temperatures the electrical resistivity shows a metallic to semiconducting transition at 113 K. A relatively high Seebeck coefficient of ∼-250 μV K(-1) was found at 400 K, but the substantial thermal conductivity (∼10 W mK(-1) at 400 K) leads to a moderate thermoelectric figure of merit of 0.025 at 400 K.

Comparative study of X-ray charge-density data on CoSb3.

CoSb3 is an example of a highly challenging case for experimental charge-density analysis due to the heavy elements (suitability factor of ~0.01), the perfect crystallinity and the high symmetry of the compound. It is part of a family of host-guest structures that are potential candidates for use as high-performance thermoelectric materials. Obtaining and analysing accurate charge densities of the undoped host structure potentially can improve the understanding of the thermoelectric properties of this family of materials. In a previous study, analysis of the electron density gave a picture of covalent Co-Sb and Sb-Sb interactions together with relatively low atomic charges based on state-of-the-art experimental and theoretical data. In the current study, several experimental X-ray diffraction data sets collected on the empty CoSb3 framework are compared in order to probe the experimental requirements for obtaining data of high enough quality for charge-density analysis even in the case of very unsuitable crystals. Furthermore, the quality of the experimental structure factors is tested by comparison with theoretical structure factors obtained from periodic DFT calculations. The results clearly show that, in the current study, the data collected on high-intensity, high-energy synchrotron sources and very small crystals are superior to data collected at conventional sources, and in fact necessary for a meaningful charge-density study, primarily due to greatly diminished effects of extinction and absorption which are difficult to correct for with sufficient accuracy.

Atomic properties and chemical bonding in the pyrite and marcasite polymorphs of FeS2: a combined experimental and theoretical electron density study

The electron density distributions in both polymorphs of the promising photovoltaic material iron disulfide have been determined by multipole modelling against state-of-the-art synchrotron X-ray diffraction data collected at 10 K using minute single crystals with dimensions less than 10 μm. Charge density analysis of FeS2 pyrite and marcasite offers a unique opportunity to relate local atomic properties, such as 2-center chemical bonding, atomic charges and d-orbital populations, to polymorphism in extended crystal structures. In combination with results from periodic calculations on the compounds in the experimental geometries using WIEN2k, the study provides unambiguous answers to a number of unsolved issues regarding the nature of the bonding in FeS2. The Fe–S bonds exhibit all the virtues of polar covalent bonds, with only minor charge accumulation but significantly negative energy densities at the bond critical points. Compared to a non-interacting model, the density is found to be concentrated along the Fe–S interaction line in support of a partial covalent bonding description. The homopolar covalent S–S interaction is seemingly stronger in pyrite than in marcasite, determined not only from the shorter distance but also from all topological indicators. The study also clarifies that the atomic charges are significantly smaller than the estimation based on crystal-field theory of Fe2+, S−1. The experimentally derived Fe d-orbital populations are found to deviate from the commonly assumed full t2g set, empty eg set, and they fit exceptionally well with the theoretical individual atomic orbitals projected density of states showing a higher dxy participation in the valence band in marcasite compared with pyrite. Thus, the differences between the two polymorphic compounds are directly reflected in their valence density distributions and d-orbital populations.

The B1-cell subpopulation is diminished in patients with relapsing–remitting multiple sclerosis

B cell subsets in newly diagnosed untreated, relapsing-remitting multiple sclerosis (MS) patients were examined. The fraction of CD20(+) B cells was significantly increased in MS. Among subsets of B cells, MS patients had increased frequency of naïve cells, but reduced frequency of memory and B1 cells. The frequencies of B1 cells were inversely correlated with the time since last attack. B1 cells resembled the phenotype of either lymphocytes (CD11b(-) B1 cells) or monocytes (CD11b(+) B1 cells) and a small fraction of cells was CD3(+)CD20(+) by confocal microscopy.

Altered frequency of T regulatory cells is associated with disability status in relapsing- remitting multiple sclerosis patients

A perturbed immunoregulation may be part of the pathogenesis of relapsing-remitting multiple sclerosis (RRMS). In this study, we demonstrate a dichotomy within the frequency of Treg among newly diagnosed RRMS patients but not in healthy controls. A group of RRMS patients was characterized by a significantly lower percentage of Treg cells than that of their matched, healthy controls, and this was inversely correlated with their score on the expanded disability status scale (EDSS). Since the EDSS reflected severity of the last attack, this study demonstrates a correlation between low frequency of Treg and severity of clinical disease in RRMS.

Higher incidence of Epstein–Barr virus‐induced lymphocyte transformation in multiple sclerosis

Epstein–Barr virus (EBV) infection is associated with multiple sclerosis (MS), and EBV may transform lymphoblastoid cell lines more frequently in MS patients than controls, but it is not clear whether this reflects a higher viral load or an enhanced ability to reactivate EBV.

Synchrotron powder diffraction of silicon: high-quality structure factors and electron density.

Crystalline silicon is an ideal compound to test the current state of experimental structure factors and corresponding electron densities. High-quality structure factors have been measured on crystalline silicon with synchrotron powder X-ray diffraction. They are in excellent agreement with benchmark Pendellösung data having comparable accuracy and precision, but acquired in far less time and to a much higher resolution (sin θ/λ < 1.7 Å(-1)). The extended data range permits an experimental modelling of not only the valence electron density but also the core deformation in silicon, establishing an increase of the core density upon bond formation in crystalline silicon. Furthermore, a physically sound procedure for evaluating the standard deviation of powder-derived structure factors has been applied. Sampling statistics inherently account for contributions from photon counts as well as the limited number of diffracting particles, where especially the latter are particularly difficult to handle.

Guest host interaction and low energy host structure dynamics in tin clathrates

The two binary clathrates with vacancies (□) Rb8Sn44□2 and Cs8Sn44□2 have been examined using powder inelastic neutron scattering (INS). Rattling energies of Rb and Cs are found to be similar by both experiment and calculations, ℏωCs/ℏωRb|Exp.=0.98(1) and ℏωCs/ℏωRb|Calc.=1.0, despite the significant mass difference: mCs/mRb=1.6, which shows that guest-host interaction is non-negligible for the studied system. For Rb8Sn44□2, a low energy phonon mode is observed at ≈3.5 meV, below the phonon mode which in the literature is attributed to the guest atom. The 3.5 meV mode is interpreted to have significant spectral weight of Sn host atoms based on temperature dependence and comparison with published theoretical phonon calculations. The record of low thermal conductivity of the tin clathrates can be attributed to the host structure dynamics rather than the guest atom rattling.

Risk-Factor Trajectories Preceding Diabetic Polyneuropathy: ADDITION-Denmark

OBJECTIVE To study cardiometabolic risk-factor trajectories (in terms of levels and changes over time) preceding diabetic polyneuropathy (DPN) 13 years after a screen-detected diagnosis of type 2 diabetes. RESEARCH DESIGN AND METHODS We clinically diagnosed DPN in a nested case-control study of 452 people in the Danish arm of the Anglo-Danish-Dutch Study of Intensive Treatment in People with Screen-Detected Diabetes in Primary Care (ADDITION). By linear regression models, we estimated preceding risk-factor trajectories during 13 years. Risk of DPN was estimated by multivariate logistic regression models of each individual’s risk-factor trajectory intercept and slope adjusting for sex, age, diabetes duration, height, and trial randomization group. RESULTS Higher baseline levels of HbA1c (odds ratio [OR] 1.76 [95% CI 1.37; 2.27] and OR 1.68 [95% CI 1.33; 2.12] per 1% and 10 mmol/mol, respectively) and steeper increases in HbA1c over time (OR 1.66 [95% CI 1.21; 2.28] and OR 1.59 [95% CI 1.19; 2.12] per 1% and 10 mmol/mol increase during 10 years, respectively) were associated with DPN. Higher baseline levels of weight, waist circumference, and BMI were associated with DPN (OR 1.20 [95% CI 1.10; 1.31] per 5 kg, OR 1.27 [95% CI 1.13; 1.43] per 5 cm, and OR 1.24 [95% CI 1.12; 1.38] per 2 kg/m2, respectively). CONCLUSIONS Both higher levels and slopes of HbA1c trajectories were associated with DPN after 13 years. Our findings indicate that the rate of HbA1c increase affects the development of DPN over and above the effect of the HbA1c level. Furthermore, this study supports obesity as a risk factor for DPN.