Jan Kroon (1937–2001)

Transition from the incommensurately modulated structure to the lock-in phase in Co-åkermanite

In situ transmission electron microscopy and diffraction observations characterize the incommensurate modulation in Ca₂CoSi₂O₇ at 300 K as dominated by an irregular arrangement and partial overlapping of octagonal nanodomains, whereas the commensurate lock-in phase at 100 K is described by a periodic arrangement with complete overlapping of the nanodomains. Within the metastable phase transition region remarkable changes in the microdomain morphology are observed.

Structure refinement of the layered composite crystal Sc₂B_1.1C_3.2 in a five-dimensional formalism

The crystal structure of a layered compound Sc₂B_1.1C_3.2 has been refined based on a commensurate composite crystal model and a five-dimensional formalism using single-crystal X-ray diffraction data.

Modulated structure of the pseudohexagonal InFe_1−x−4δTi_x+3δO_3+x/2 (x = 0.61) composite crystal

The structure of the pseudohexagonal InFe_1−x−4δTi_x+3δO_3+x/2 (x = 0.61) was refined on the basis of a four-dimensional superspace group. The compositely modulated structure consists of two orthorhombic subsystems mutually incommensurate in b.

Hexagonal Frank–Kasper phases interpreted as modulated crystals

The structures of the hexagonal Frank–Kasper phases F and K are interpreted as modulated crystal structures. They are basically composed of two close-packed layers with ordered atomic vacancies rotated by 90° to each other about their normal.

Intergrowth polytypoids as modulated structures: a superspace description of the Sr_n(Nb,Ti)_nO_3n + 2 compound series

A new structural characterization based on the superspace approach is presented for the Sr_n(Nb,Ti)_nO_3n + 2 compound series. A single superspace group and a common structural model with step-like occupation modulations describe the whole compound series.

Ferroelectricity and structure in the YMnO₃ family

Analysis of the YMnO₃ structure yields a ferroelectric Curie temperature of 1220 (100) K, in accord with the experimental value of 913 K. Similar analysis yields Curie temperatures for ErMnO₃, LuMnO₃, YbMnO₃, YInO₃, ScMnO₃, InMnO₃ and YGaO₃; the experimental and calculated T_c for the first four are in acceptable agreement, the last three are without experimental values but are predicted to be new ferroelectrics.

Parametrization of perovskite structures: an ab initio study

A revised version of the global parametrization method (GPM) of perovskite structures proposed by Thomas is presented. The improved GPM in combination with ab initio total energy calculations is used to predict changes in the structural distortion under increasing hydrostatic pressure for the magnesium silicate perovskite.

Structures of copper(II) and manganese(II) di(hydrogen malonate) dihydrate; effects of intensity profile truncation and background modelling on structure models

Problems with scan-angle-induced spectral truncation and problems with pseudo-I-centring hamper the structure analysis of the title compounds. The difficulties are enhanced by the select use of data with I > 3σ(I). Replacing the background–peak–background procedure by an evaluation based on a background model, all observations could be included in the structure analysis. This helped to reduce model inconsistencies.

Two fluoradene derivatives: pseudosymmetry, eccentric ellipsoids and a phase transition

Two highly pseudosymmetric structures that have Z′ = 2 have been investigated. The pseudosymmetry in each was assessed by analysing the atomic coordinates, by performing refinements in the smaller pseudocells and by making separate Wilson plots for the classes of reflections that are systematically strong and systematically weak.

Correction of the space group of [Ni(bipy)₂(ONO₂)₂]·2(pyrene)

A change in the space group of [Ni(bipy)₂(ONO₂)₂]·2(pyrene) from Pn to P2₁/n leads to a revised description of the structure.

Solid-state structures of group 1 and group 2 metal 1,5-naphthalenedi­sulfonates: systematic investigation of lamellar three-dimensional networks constructed by metal arenedi­sulfonate

Crystal structures of seven Group 1 and Group 2 metal 1,5-naphthalenedisulfonates demonstrate uniquely the coordination strength of Group 1 and Group 2 metal cations with large organic anions.

Structure determination of the [Fe(teec)₆](BF₄)₂ metal complex from laboratory and synchrotron X-ray powder diffraction data with grid-search techniques

The structure of the coordination compound [Fe(teec)₆](BF₄)₂ [teec = 1-(2-chloroethyl)tetrazole] has been determined with a direct-space grid-search technique based on Guinier-camera and high-resolution synchrotron powder diffraction data and refined with the latter. The methodology followed is discussed as well as the crystal structure characteristics in relation to the spin-crossover behaviour.

Basic forms of supramolecular self-assembly organized by parallel and antiparallel hydrogen bonds in the racemic crystal structures of six disubstituted and tris­ubstituted cyclo­pentane derivatives

Racemic crystals of six optically active molecules formed by a saturated five-membered ring bearing two vicinal (either cis or trans) hydrogen-bond donor/acceptor functions, and in three cases a tert-butyl substituent, were found to represent five closely related canonical forms of supramolecular close packing. In each crystal structure there are hydrogen-bonded homochiral or heterochiral chains of molecules assembled either in antiparallel or parallel modes stabilized by a second hydrogen bond.

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. III. Photoracemization of 1-cyano­ethyl cobaloxime complexes

Single-crystal neutron diffraction for two cobaloxime crystals, with the chiral 1-cyanoethyl group bonded to the cobalt atom, showed that the photo-inversion of the chiral group proceeds via rotation of the cyanoethyl radical after Co—C bond cleavage to permit the opposite face of the radical to recombine with the Co atom.

Crystal engineering in the gem-alkynol family: the key role of water in the structure of 2,3,5,6-tetra­bromo-trans-1,4-diethynyl-cyclohexa-2,5-diene-1,4-diol dihydrate determined by X-ray and neutron diffraction at 150 K

Crystallizing in space group P4₂/ncm, the structure reveals a complex set of supramolecular synthons, with the water molecule playing the crucial role. Cyclic tetrameric O2(w)—H⋯O units are linked by further O—H⋯O2(w) and C≡C—H⋯O2(w) bonds, while O2(w)—H⋯Br bonds provide links to cyclic tetramers of Br⋯Br interactions.

Experimental and theoretical charge density of DL-alanyl-me­thionine

The methionine S atom and polar interactions are analyzed in the experimental and theoretical electron distributions of the dipeptide DL-Ala-Met.

Structural complexity of a polar perhydrotri­phenylene inclusion compound brought to light by synchrotron radiation

Complete three-dimensional diffuse and satellite scattering of a heavily disordered molecular inclusion compound has been measured for the tips and the centre of the same crystal at room temperature and 120 K. Qualitative interpretations are proposed on the basis of the molecular form factors and of the disorder deduced from the average structure.

General rules for the packing of hydrogen-bonded crystals as derived from the analysis of squaric acid anions: aminoaromatic nitrogen base co-crystals

The crystal packings of four co-crystals formed between squaric acid and aminated aromatic nitrogen bases are analyzed and discussed in terms of the intermolecular hydrogen bonds formed. General rules for the packing of hydrogen-bonded crystals are proposed.

Ferroelectric–paraelectric phase transitions with no group–supergroup relation between the space groups of both phases?

The four compounds Sr₃(FeF₆)₂, β-NbO₂, TlBO₂ and CrOF₃ are confirmed as possible ferroelectrics, fulfilling the usual group–supergroup relations between the phase symmetries.

Oxford handbook of nucleic acid structure

Nucleic acids: structures, properties and functions