Structural Science

 

Acta Cryst. (2000). B56, 747-749  [ doi:10.1107/S0108768100008466 ]

U_equiv: its past, present and future

D. Watkin

Synopsis: Neither of the common definitions of U_equiv yield a parameter of any great use, and its standard uncertainty is even more suspect. The need for a better `one-parameter' measure of the quality of a.d.p.'s is demonstrated.

Online October 2000

 

Acta Cryst. (2000). B56, 750-765  [ doi:10.1107/S0108768100006480 ]

Architecture of crystal structures from square planes

J. Hauck and K. Mika

Synopsis: The ordering of A and B atoms in b.c.c., f.c.c. or p.c. A_xB_y alloys and related compounds is restricted to a small number of thermodynamically stable structures. The structural units of these structures are assembled in different ratios for a variety of attractive or repulsive interactions.

Online October 2000

 

Acta Cryst. (2000). B56, 766-772  [ doi:10.1107/S0108768100006893 ]

Refinement of framework disorder in dehydrated CaA zeolite from single-crystal synchrotron data

F. Porcher, M. Souhassou, H. Graafsma, A. Puig-Molina, Y. Dusausoy and C. Lecomte

Synopsis: The crystal structure of dehydrated CaA zeolite, solved using single-crystal synchrotron data, shows a disordered distribution of framework oxygen atoms and cations.

Online October 2000

 

Acta Cryst. (2000). B56, 773-779  [ doi:10.1107/S0108768100006704 ]

X-ray diffraction study of K₃NdSi₇O₁₇: a new framework silicate with a linear Si-O-Si bond

S. M. Haile and B. J. Wuensch

Synopsis: Hydrothermal investigations in the high-silica region of the K₂O-Nd₂O₃-SiO₂ system, carried out in a search for novel fast-ion conductors (FICs), yielded the new compound, K₃NdSi₇O₁₇. Its structure is unusual in that it contains a symmetry-constrained Si-O-Si bond angle of 180°.

Online October 2000

 

Acta Cryst. (2000). B56, 780-784  [ doi:10.1107/S0108768100003657 ]

Twinning and defects in N-Nb₂O₅

F. E. Rohrer and A.-K. Larsson

Synopsis: High-resolution electron-microscopy images of N-Nb₂O₅ reveal two different kinds of coherent twinning. The striking similarity between the observed electron-diffraction patterns for multiply twinned N-Nb₂O₅ and that reported for M-Nb₂O₅ can be understood by considering the similar block arrangements in the twin borders of N-Nb₂O₅ and the disordered M-Nb₂O₅.

Online October 2000

 

Acta Cryst. (2000). B56, 785-792  [ doi:10.1107/S0108768100006285 ]

Synchrotron X-ray analysis of RbTiOAsO₄

V. A. Streltsov, J. Nordborg née Almgren and J. Albertsson

Synopsis: An accurate synchrotron X-ray diffraction measurement of a single crystal of the RbTiOAsO₄ compound shows that there is a substantial (dynamic and/or static) disorder of the Rb atoms over additional sites related by pseudosymmetry at room temperature.

Online October 2000

 

Acta Cryst. (2000). B56, 793-804  [ doi:10.1107/S0108768100007849 ]

Systematic prediction of new ferroelectrics in space group P3

S. C. Abrahams

Synopsis: A total of 21 structures reported in the Inorganic Crystal Structure Database under space group P3 satisfy the structural criteria for ferroelectricity. One of the remaining structures is a previously discovered ferroelectric, 19 are probably centrosymmetric and seven are polar but not ferroelectric.

Online October 2000

 

Acta Cryst. (2000). B56, 805-810  [ doi:10.1107/S0108768100005383 ]

Crystal structure and charge distribution of YbFeMnO₄

M. Nespolo, M. Isobe, J. Iida and N. Kimizuka

Synopsis: The X-ray single-crystal structure and charge distribution analysis of synthetic YbFeMnO₄ are reported. A vacancy of approximately 4% is present at the Yb octahedral site.

Online October 2000

 

Acta Cryst. (2000). B56, 811-821  [ doi:10.1107/S0108768100006716 ]

Two-dimensional incommensurately modulated structure of (Sr_0.13Ca_0.87)₂CoSi₂O₇ crystals

B. Bagautdinov, K. Hagiya, K. Kusaka, M. Ohmasa and K. Iishi

Synopsis: The structural details of (Sr_0.13Ca_0.87)₂CoSi₂O₇, including the displacement of atoms, rotation and distortion of the structural units, and the partial ordering of the Sr and Ca atoms accompanied with the modulation have been determined. The structural discrepancies in (Sr_0.13Ca_0.87)₂CoSi₂O₇ and in Ca₂CoSi₂O₇ are discussed to find the effects of the partial substitution of Ca by Sr.

Online October 2000

 

Acta Cryst. (2000). B56, 822-827  [ doi:10.1107/S0108768100003773 ]

Isomorphism and pseudosymmetry in 2,6-dichloro- and 2,6-dibromobenzonitrile

D. Britton, W. E. Noland and M. J. Pinnow

Synopsis: 2,6-Dichloro- and 2,6-dibromobenzonitrile are isomorphous, with two half-molecules in the asymmetric unit of a monoclinic unit cell. The molecules pack in layers with a fourfold pseudosymmetry. The pseudosymmetry appears to be the consequence of halogen-halogen intermolecular interactions.

Online October 2000

 

Acta Cryst. (2000). B56, 828-832  [ doi:10.1107/S0108768100008727 ]

Two-dimensional packing and pseudosymmetry

D. Britton

Synopsis: It is possible to understand, or at least rationalize, the occurrence of pseudosymmetry elements in some crystals as a consequence of a higher symmetry in a two-dimensional layer being obscured by the three-dimensional packing of the layers.

Online October 2000

 

Acta Cryst. (2000). B56, 833-848  [ doi:10.1107/S0108768100004274 ]

Packing modes of (R,R)-tartaric acid esters and amides

U. Rychlewska and B. Warzajtis

Synopsis: The packing of primary tartramides is driven by NHO=C hydrogen bonds and supplemented by strong OHO=C and weak NHOH bonds, while in methylated derivatives OHOHO=C patterns dominate. Problems with the packing of methyl groups are overcome by changes in conformation (esters) or by co-crystallization with solvent water molecules (methylamides and dimethylamides).

Online October 2000

 

Acta Cryst. (2000). B56, 849-856  [ doi:10.1107/S0108768100003694 ]

Intramolecular hydrogen bonds: common motifs, probabilities of formation and implications for supramolecular organization

C. Bilton, F. H. Allen, G. P. Shields and J. A. K. Howard

Synopsis: The probabilities of occurrence of the 50 most common intramolecular hydrogen-bonded (N/O-HO/N) ring motifs in the Cambridge Structural Database are presented and their implications discussed.

Online October 2000

 

Acta Cryst. (2000). B56, 857-871  [ doi:10.1107/S0108768100007230 ]

Graph-set and packing analysis of hydrogen-bonded networks in polyamide structures in the Cambridge Structural Database

W. D. S. Motherwell, G. P. Shields and F. H. Allen

Synopsis: The hydrogen-bond networks and crystal packing arrangements of 81 secondary di- and polyamides in the Cambridge Structural Database are investigated. Graph-set analysis is used to identify and classify network motifs, which are rationalized in terms of the relative dispositions of the amide functional groups; symmetry relationships within and between hydrogen-bonded chains, ladders and sheets in the crystal packing are analysed.

Online October 2000

 

Acta Cryst. (2000). B56, 872-881  [ doi:10.1107/S0108768100005905 ]

Conformational transformation coupled with the order-disorder phase transition in 2-methyl-1,3-cyclohexanedione crystals

A. Katrusiak

Synopsis: The ring conformation transforms between sofa and dynamically disordered half-chair in the crystals of enolized 2-methyl-1,3-cyclohexanedione, C₇H₁₀O₂. The transformation takes place at the continuous phase transition at T_c = 244  K.

Online October 2000

 

Acta Cryst. (2000). B56, 882-892  [ doi:10.1107/S0108768100005437 ]

N-(6-Amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl) derivatives of glycine, valine, serine, threonine and methionine: interplay of molecular, molecular-electronic and supramolecular structures

J. N. Low, M. D. López, P. Arranz Mascarós, J. Cobo Domingo, M. L. Godino, R. López Garzón, M. D. Gutiérrez, M. Melguizo, G. Ferguson and C. Glidewell

Synopsis: The title compounds are all characterized by very short intermolecular O-HO hydrogen bonds and C-N-O fragments in which the C-N and N-O bond lengths are almost equal. These observations are rationalized by means of database analysis and computational modelling.

Online October 2000

 

Acta Cryst. (2000). B56, 893-905  [ doi:10.1107/S0108768100007114 ]

Conformational preferences and supramolecular aggregation in 2-nitrophenylthiolates: disulfides and thiosulfonates

C. Glidewell, J. N. Low and J. L. Wardell

Synopsis: The interplay between molecular conformation and supramolecular aggregation has been investigated in six 2-nitrophenyl disulfides and a 2-nitrophenyl thiosulfonate ester. Hydrogen bonds of the C-HO type are responsible for observed conformations remote from the global energy minimum.

Online October 2000

 

Acta Cryst. (2000). B56, 906-914  [ doi:10.1107/S0108768100007941 ]

Ferroelastic structures of n-pentyl-, n- hexyl- and n-nonylammonium dihydrogenphosphate crystals

J. Fábry, V. Petrícek, J. Kroupa and I. Císarová

Synopsis: The title compounds are ferroelastic. There are H atoms in the structures, involved in asymmetric hydrogen bridges, which hop between donor and acceptor O atoms during the ferroelastic switching. The structure of n-pentylammonium dihydrogenphosphate (C5ADP) belongs to the same structure type as C3ADP and differs from C2ADP, C4ADP and C6ADP-C10ADP.

Online October 2000

 

Acta Cryst. (2000). B56, 915-917  [ doi:10.1107/S0108768100006595 ]

Orthorhombic -Zn-Mg-Dy phase related to a Frank-Kasper type decagonal quasicrystal

E. Abe and A. P. Tsai

Synopsis: A model structure of a novel Zn-Mg-Dy compound is proposed based on electron diffraction and high-resolution transmission electron microscopy studies. Local atomic configurations in the structure are shown to be closely related to those in the Frank-Kasper type decagonal quasicrystal and in the Zn₇Mg₄ crystalline compound.

Online October 2000

 

Acta Cryst. (2000). B56, 918-919  [ doi:10.1107/S0108768100009733 ]

X-ray structure of barium titanate - missed opportunities

K. S. Chandrasekaran, S. K. Mohanlal, R. Saravanan and S. Israel

Synopsis: The dispersion effects have been calculated for the structure factors of barium titanate. It is stressed that the Friedel pairs should be treated independently in the refinements.

Online October 2000

 

Acta Cryst. (2000). B56, 920  [ doi:10.1107/S0108768100010016 ]

Phosphorus 2000. Chemistry, biochemistry & technology

Online October 2000

 

Acta Cryst. (2000). B56, 920  [ doi:10.1107/S0108768100012337 ]

Direct phasing in crystallography

Online October 2000

 

Acta Cryst. (2000). B56, 920  [ doi:10.1107/S0108768100012349 ]

Diffraction des rayonnements

Online October 2000