Structure correlation and dynamics in crystals – a tribute to Hans-Beat Bürgi

Structure correlation, dynamics and disorder in crystals have been major research topics throughout Hans-Beat Bürgi's career that is celebrated in this special issue.

Crystal structures

A personal view on crystal structures is offered: past and present, ordered and disordered, static and dynamic, and problems solved and open.

Rearrangements in the multiverse

This essay outlines of the binding interactions between Hans-Beat Bürgi and Roald Hoffmann, and a potential rearrangement of their molecules is considered.

Collaboration with Hans-Beat Bürgi: crystallography at Bern and Lausanne

Memories of the scientific collaboration over several decades of the crystallographers at the Universities of Bern and Lausanne are shared.

Hans-Beat Bürgi in Strasbourg – the story of an encounter

Collaborative work at the University of Strasbourg in the 1970s is remembered.

Remarks on X-ray constrained/restrained wavefunction fitting

Some comments concerning the meaning and significance of X-ray constrained/restrained wavefunctions are given.

Bone hierarchical structure: spatial variation across length scales

A brief review of bone hierarchical structure is given. Recent findings that have resulted from X-ray imaging are highlighted.

X-ray constrained wavefunctions based on Hirshfeld atoms. I. Method and review

Methods for reconstructing `experimental model wavefunctions' from X-ray data are reviewed and clarified, and the key problem of deciding when to terminate the wavefunction reconstruction procedure is defined as 'the halting problem'. The role of Hans-Beat Bürgi in all of this is described.

Dynamic simulation of orientational disorder in organic crystals: methyl groups, tri­fluoro­methyl groups and whole mol­ecules

Advanced simulation methods are used to portray the structural, static and dynamic aspects of the rotations of methyl and tri­fluoro­methyl groups, as well as entire mol­ecules, in their crystals at equilibrium.

Obtaining diffuse scattering patterns from computer simulations – a retrospective

The paper describes how the calculation of diffuse scattering from atomistic model crystals has developed over the last approximately 50 years.

Mesocrystalline structure and mechanical properties of biogenic calcite from sea urchin spine

Diffuse scattering and inelastic scattering were probed by X-rays and compared for inorganically formed natural calcite and biogenic calcite from the sea urchin spine. The differences in the mechanical properties between the `strong' biogenic and `brittle' abiotic material are attributed to a mesoscopic architecture of the spine.

Temperature-dependent crystal structure investi­gation of 4f hybridized thermoelectric clathrate Ba_8–xCe_xAu_ySi_46–y

Multi-temperature synchrotron powder diffraction data are used to characterize the crystal structure of thermoelectric clathrate Ba_7.74 (9)Ce_0.26 (9)Au_5.42 (3)Si_40.58 (3).

Metastable disordered phase in flash-frozen Prussian Blue analogues

Reported here is a new metastable phase in flash-frozen disordered Prussian blue analogues characterised by the appearance of diffuse scattering clouds and reduction of the cubic symmetry to a tetragonal or lower space group.

Low-frequency lattice vibrations from atomic displacement parameters of α-FOX-7, a high energy density material

The normal mode analysis of variable-tem­per­ature anisotropic atomic displacement parameters (ADPs) of the α-phase of 1,1-di­amino-2,2-di­nitro­ethyl­ene (DADNE or FOX-7) is reported.

Interplay of thermal diffuse scattering and correlated compositional disorder in KCl_1–xBr_x

Single-crystal diffuse scattering measurements are used to study the interplay of short-range anion order and anharmonic lattice dynamics in the series KCl_1–xBr_x.

Preliminary observations of the interplay of radiation damage with spin crossover

X-ray radiation damage increases unit-cell volume, apparent anisotropic displacement parameters and shifts the spin state equilibrium to lower temperatures. It may therefore serve as another tool to tune a spin transition sequence.

X-ray constrained wavefunctions based on Hirshfeld atoms. II. Reproducibility of electron densities in crystals of α-oxalic acid dihydrate

The electron density in α-oxalic acid obtained from an experimental wavefunction analysis on multiple experiments is analysed and the improvements are compared to previous work from twenty years ago.

Dynamics and disorder: on the stability of pyrazinamide polymorphs

The enantiotropic relationship between the four polymorphs of pyrazinamide is analyzed by means of accurate X-ray diffraction measurements, normal-mode refinement and periodic DFT calculations.