245 research outputs found
The ash heap of crystallography: restoring forgotten basic knowledge. Corrigendum
No full textOne imprecise and one incomplete statement in the article by Nespolo [J. Appl. Cryst. (2015), 48, 1290–1298] are corrected.</jats:p
The Everlasting Challenges of Mathematical and Theoretical Crystallography
No full textThe IUCr Commission on Mathematical and Theoretical Crystallography (MaThCryst) was established in 2005 after a three-year informal existence as unofficial workgroup. The reasons for this initiative were essentially the need to draw new lymph in a field that was sliding away from our community, and to energize the education of young (and sometimes less young) crystal-lographers in the foundations that too often are overlooked in the University curricula. The task seemed extremely challenging , but despite some scepticism we had to face at first, our work has been successful from the beginning. Today we count more than fifty events (workshops, satellite conferences, schools) during our fifteen years of existence. In relation with this intense activity , we have published four special issues and the present one is our fifth achievement. It collects papers submitted by the participants to the several events we have organized in the summer of 2019. Umayahara and Nespolo present a historical perspective of the barely known pioneering work of Japanese crystallographers on modular structures, with a revised classification based on groupoid approach which generalizes the original description (ar-ticle number 1900045). Nespolo and co-workers present a potentially case of a mis-attribution of X-ray diffraction data that could represent a trap for the less-experienced crystallographer (article number 1900063). Stoeger and Rotter discuss a complex case of polytypism and twinning in a crystal with simple chemistry and demonstrate how this sort to problems need a crystallographic knowledge well above what can be obtained in a normal training aiming at a routine approach (article number 1900164). Stoeger and co-workers present a different case of polytypism in a chemically more complex structure, where the coexistence of two different polytypes leads to residual peaks in the electron density (arti-cle number 1900182). Kurlin challenges the problem of quantifying the similarity between crystal structures, in search for an algorithm capable of discriminating nearly identical structures with Prof. M. Nespol
Iron Metabolism. From Molecular Mechanisms to Clinical Consequences .
No full textInternational audienceAuthor(s) of this paper may load this reprint on their own web site or institutional repository provided that this cover page is retained. Republication of this article or its storage in electronic databases other than as specified above is not permitted without prior permission in writing from the IUCr. For further information see http://journals.iucr.org/services/authorrights.html Acta Cryst. (2017). D73, 952–954 Nespolo · Iron metabolis
Scaling phenomena driven by inhomogeneous conditions at first-order quantum transitions
No full textWe investigate the effects of smooth inhomogeneities at first-order quantum transitions (FOQTs), such as those arising in the presence of a space-dependent external field, which smooths out the discontinuities of the low-energy properties at the transition. We argue that a universal scaling behavior emerges in the space transition region close to the point in which the external field takes the value for which the homogeneous system undergoes the FOQT. We verify the general theory in two model systems. We consider the quantum Ising chain in the ferromagnetic phase and the q-state Potts chain for q = 10, investigating the scaling behavior which arises in the presence of an additional inhomogeneous parallel and transverse magnetic field, respectively. Numerical results are in full agreement with the general theory
Finite-Size Scaling at First-Order Quantum Transitions
No full textWe study finite-size effects at first-order quantum transitions (FOQTs). We show that the low-energy properties show a finite-size scaling (FSS) behavior, the relevant scaling variable being the ratio of the energy associated with the perturbation driving the transition and the finite-size energy gap at the FOQT point. The size dependence of the scaling variable is therefore essentially determined by the size dependence of the gap at the transition, which in turn depends on the boundary conditions. Our results have broad validity and, in particular, apply to any FOQT characterized by the degeneracy and crossing of the two lowest-energy states in the infinite-volume limit. In this case, a phenomenological two-level theory provides exact expressions for the scaling functions. Numerical results for the quantum Ising chain in transverse and parallel magnetic fields support the FSS Ansatzes
Finite-size scaling at the first-order quantum transitions of quantum Potts chains
No full textWe investigate finite-size effects at first-order quantum transitions. For this purpose we consider the onedimensional
q-state quantum Potts chain, in particular with q = 10, which undergoes a first-order transition,
separating the quantum disordered and ordered phases with a discontinuity in the energy density of the ground
state. In agreement with the general theory, around the transition the low-energy properties show finite-size
scaling with respect to appropriate scaling variables. Their size dependence is particularly sensitive to boundary
conditions, which is a specific feature of first-order quantum transitions. Finally, we also discuss the finite-size
behavior of the q-state Potts model (q 2) at the first-order transitions driven by a parallel magnetic field,
occurring in the ferromagnetic phase
Charge distribution as a tool to investigate structural details. III. Extension to description in terms of anion-centred polyhedra
No full textInternational audienceThe charge distribution (CHARDI) method is a self-consistent generalization of Pauling's concept of bond strength which does not make use of empirical parameters but exploits the experimental geometry of the coordination polyhedra building a crystal structure. In the two previous articles of this series [Nespolo et al. (1999). Acta Cryst. B55, 902-916; Nespolo et al. (2001). Acta Cryst. B57, 652-664], we have presented the features and advantages of this approach and its extension to distorted and heterovalent polyhedra and to hydrogen bonds. In this third article we generalize CHARDI to structures based on anion-centred polyhedra, which have drawn attention in recent years, and we show that computations based on both descriptions can be useful to obtain a deeper insight into the structural details, in particular for mixed-valence compounds where CHARDI is able to give precise indications on the statistical distribution of atoms with different oxidation number. A graph-theoretical description of the structures rationalizes and gives further support to the conclusions obtained via the CHARDI approach
Comment on the article `Protein crystal lattices are dynamic assemblies: the role of conformational entropy in the protein condensed phase'
No full textInternational audienc
Metallomics. A Primer of Integrated Biometal Sciences.
No full textelectronic reprint IUCr JournalsInternational audienc
Pyroxenes
No full textEntrée d'encyclopédieInternational audiencePyroxenes are single-chain silicates with the general formula XYSi2O6, where X and Y are either both divalent cations (mainly Ca, Fe, Mg), or mono-(Na, Li) and trivalent cations (Al, Fe) respectively. The stacking of chains gives rise to two types of cavities, labeled M1 (close to octahedral in shape, where the Y cations are located) and M2 (more irregular in shape, where the X cations are located). The relative rotation of the Si-centred tetrahedra about the bridging oxygen atom is responsible for the structural flexibility of pyroxenes and explains their polymorphism and their existence in a wide range of temperature and pressure environments. The different polymorphs of pyroxenes can be described as derived from the stacking of modules consisting of layers contained within pairs of (100) planes located on tetrahedral cations
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