George A. Jeffrey (1915–2000)

How precise are measurements of unit-cell dimensions from single crystals?

The best measurements of cell dimensions from (small) single crystals and powders on the most favourable specimens have a precision of some parts in 10⁵, which seems to be the maximum attainable with current, and older, techniques. There has been little real improvement over a period of some 60 years. `Routine' measurements, on less favourable specimens, give precisions of some parts in 10⁴ or even 10³. Some current assessments of the standard uncertainties of cell dimensions are fictional at best.

Towards a unified description of the AMOB₂O₅ (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

The structure of RbNbOB₂O₅ is re-refined as an incommensurate modulated structure using the program JANA98.

Bond-valence parameters for ammonium–anion interactions

A database search for ammonium ion coordination by anions is used to determine bond-valence parameters. The applicability of these parameters is discussed and exemplified.

Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb₂CoWO₆

Rietveld refinement of the modulated structure of lead cobalt tungstate. A displacive model involving significant atomic shifts for Pb atoms and quite a complex mixing of tilt and deformation of the oxygen octahedra is proposed.

Structure of uranium(VI) oxide dihydrate, UO₃·2H₂O; synthetic meta-schoepite (UO₂)₄O(OH)₆·5H₂O

The structure of meta-schoepite, UO₃·2H₂O, is constructed from layers of stoichiometry (UO₂)₄O(OH)₆ separated by water molecules. The material is structurally related to schoepite, UO₃·2.25H₂O, by the removal and redistribution of molecules in the water layers.

Study of NH₄⁺ in the zeolite phillipsite by combined synchrotron powder diffraction and IR spectroscopy

The Rietveld refinement on synchrotron data was combined with IR data to study three natural hydrated NH₄⁺-exchanged phillipsites with different Si/Al ratios. They all display the typical monoclinic P2₁/m cell with NH₄⁺, which occupy sites I and II where it acts as a donor in hydrogen bonding with O atoms of the framework and water molecule.

Electron density and atomic displacements in KTaO₃

The electron density and atomic displacement pattern in the virtual ferroelectric KTaO₃ studied with accurate X-ray diffraction data gives a comprehensive description of the atomic interactions in this crystal.

Structural phase transitions of HfV₂ at low temperatures

A high-resolution synchrotron X-ray diffraction study on a HfV₂ Laves C15 phase conducted at low temperatures has illustrated the structural and lattice changes as a function of temperature and definitely revealed the crystal structure of a tetragonal I4₁amd phase for the first time.

Analysis of LiKSO₄ crystals in the temperature range from 573 to 943 K

Structure solution and refinement details are given for the multi-domain high-temperature (708 T 943 K) phase of LiKSO.

Ab initio structure determination and Rietveld refinement of a high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate from in situ temperature-resolved powder diffraction data

The crystal structures of a new high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate have been solved and refined from in situ temperature-resolved synchrotron powder diffraction data.

A new relation between bond valence and bond distance

A new empirical relationship between bond distances and bond valences is derived from the hypothesis that bond valence is proportional to the average electron density between bonding atoms.

Identification of two allotwins of mica polytypes in reciprocal space through the minimal rhombus unit

Two rare examples of allotwins are identified in an oxybiotite mica, using the X-ray precession method and geometrical analysis of the minimal unit in reciprocal space. 1M and 2M₁ polytypes are attached by rotations of either 120 or 60°, respectively.

Aplanarity of CO₃ groups: a theoretical investigation

Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO groups in some carbonates is a ground-state property.

A new stacking motif: complex alloy structures interpreted as modulated structures

The structures of λ-Al₄Mn, κ-Al₁₇₇Cr₄₉Ni and ∊-Al₄Cr, which have been described in terms of the aggregation of clusters until now, are interpreted as modulated structures. The observed stacking motif of atoms can be found in many phases related to icosahedral and decagonal quasicrystals.

Structures and phase transitions of B-Ta₂O₅ and Z-Ta₂O₅: two high-pressure forms of Ta₂O₅

The crystal structures of B-Ta₂O₅ and Z-Ta₂O₅, two high-pressure forms of Ta₂O₅, have been determined and refined from X-ray powder data. The structures are closely related and an intergrowth between them was indicated by the HRTEM study.

Synthesis, structure and magnetic properties of tetra­kis-μ-carboxylato-bis­(do­decylnicotinato)dicopper(II) complexes; crystal and molecular structure of the decyl carboxylate derivative

Dodecylnicotinate bis-adducts of binuclear copper carboxylates, of the general formula Cu₂(O₂CC_{n − 1}H_{2n − 1})₄(C₅H₄NCOOC₁₂H₂₅)₂, were synthesized for n = 10, 12, 14, 16, 18 and 20 and their crystal structure, thermal behavior and magnetic properties studied. The molecular structure of the decyl derivative has been determined from single-crystal X-ray diffraction data.

Frequency of Z′ values in organic and organo­metallic crystal structures

The frequencies of Z′ values (formula units per asymmetric crystal unit) are documented for organic and organometallic crystal structures, using the Cambridge Structural Database.

Determining the crystal structure of twinned 2-methylpyrazine

The structure of merohedrally twinned 2-methylpyrazine at 123 and 168 K has been solved using SHELXL97.

Crystal-to-crystal photodimerization of ­trans-cinnamamides

Single-crystal photoirradiations of trans-cinnamamide and the 4-methyl and 4-chloro derivatives have been carried out to observe the transformation from a pair of monomers related by a center of symmetry into an α-type dimer. The lower photoreactivity of trans-cinnamamide than those of the other two is not only due to a longer C⋯C distance between the nearest neighboring C=C double bonds, but also due to partial breakdown of the hydrogen-bond network in the process of photodimerization.

X-ray studies on crystalline complexes involving amino acids and peptides. XXXV. Invariance and variability in amino acid aggregation in the complexes of maleic acid with L-histidine and L-lysine

The histidine complex provides an example of the retention of the basic features of amino acid aggregation in different environments and in the presence of different molecules, while the pattern observed in the lysine complex serves as a note of caution and demonstrates how difficult it is to predict crystal structures even though broad generalizations are useful.

A test of crystal structure prediction of small organic molecules

The results are reported of a blind test of crystal structure prediction of four organic molecules using the principal computer programs currently available. Successful predictions for all four molecules were obtained using calculated lattice energy, but no program was successful in all cases.

Non-centrosymmetric racemates: space-group frequencies and conformational similarities between crystallographically independent molecules

The distribution of crystalline racemates with more than one molecule in the asymmetric unit among the common space groups has been determined. The conformational similarities between crystallographically independent enantiomers in 114 non-centrosymmetric racemates were quantified using the r.m.s. deviation for a molecular superposition. The r.m.s. distribution curve shows that the conformations of the crystallographically independent molecules are in general very similar.

Structural relationships in crystals accommodating different stereoisomers of 2-amino-3-methylpentanoic acid

The spontaneous resolution of the racemate DL-allo-isoleucine upon crystallization is rationalized on the basis of an analysis of interatomic distances in the 1:1 DL-isoleucine:DL-allo-isoleucine complex. An unfavorable C⋯C contact prevents the two enantiomers from forming a stable crystal structure with the same hydrogen-bond arrangement as in 1:1 DL-isoleucine: DL-allo-isoleucine and related amino acid racemates.

Electron density and electrostatic properties of two peptide molecules: tyrosyl-glycyl-glycine monohydrate and glycyl-aspartic acid dihydrate

The electron density and the electrostatic properties of the title peptide molecules have been determined from high-resolution diffraction data collected at 123 K. The topological characteristics of the (3,−1) critical points of C=O, C—N and C—C bonds are compared in 17 peptide links.

Single crystal structure and molecular dynamics analysis of a myo-inositol derivative

The dynamical disorder found in the crystal of a myo-inositol derivative is studied using molecular dynamics calculations.

P1 or P? Corrigendum

The space group of bis((phenyl-O,N,N-azoxy)oxy)methane, previously revised from P1 to C2, is further revised to Fdd2.

Structure predictions allowing more than one molecule in the asymmetric unit. Erratum

Erratum to Acta Cryst. (2000). B56, 535–542.