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1999 iucr xviii
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1993 iucr xvi
1990 iucr xv
1987 iucr xiv
1984 iucr xiii
1981 iucr xii
1978 iucr xi
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1969 iucr viii
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1954 iucr iii
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people

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agre
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crick
curl
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huber
karle
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kendrew
klug
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laue
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levitt
lipscomb
mackinnon
michel
novoselov
pauling
perutz
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roentgen
shechtman
shull
skou
smalley
steitz
sumner
thomson
walker
warshel
watson
wilkins
yonath

resources

commissions

aperiodic crystals
biological macromolecules
quantum crystallography
crystal growth and characterization of materials
crystallographic computing
crystallographic nomenclature
crystallographic teaching
crystallography in art and cultural heritage
crystallography of materials
electron crystallography
high pressure
inorganic and mineral structures
international tables
journals
magnetic structures
mathematical and theoretical crystallography
neutron scattering
NMR crystallography
powder diffraction
small-angle scattering
structural chemistry
synchrotron radiation
xafs

outreach

openlabs

calendar
OpenLab Costa Rica
IUCr-IUPAP-ICTP OpenLab Senegal
Bruker OpenLab Cameroon
Rigaku OpenLab Bolivia
Bruker OpenLab Albania
Bruker OpenLab Uruguay 2
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Agilent OpenLab Turkey
Bruker OpenLab Indonesia
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Agilent OpenLab Argentina
Bruker OpenLab Pakistan

- Letter from the President
- Across the sciences
- Helical arrangement of cellulose fibrils in wood
- Experimental techniques and glass ceramics
- The powder diffraction handicap
- Twinning in chemical crystallography
- The Ewald Prize
- Cryo preservation and decay
- 30 years of Rietveld refinement
- Micro structure and texture of real materials
- Macromolecular electron microscopy
- X-ray and neutron complementarity
- Industrial analysis on-line
- Interfaces, thin films, multilayers
- Macromolecular phasing
- Phase transitions
- Applications of line broadening
- Metalloproteins, electron transport and EXAFS
- 50th anniversary of the biomolecular structure lab
- Tenerife school
- Croatian-Slovenian crystallography
- Crystal growth and materials in Brazil
- Surface structure
- European Crystallography Prize
- Schechtman receives 1999 Wolf Prize
- Awards and appointments
- CRUSH: the rigid unit mode program
- Oscail - single crystal and powder diffraction
- XD - a multipole refinement and analysis of charge densities
- CRYSTALS-32

With almost 400 *ab initio* structure determinations reported, powder diffraction is healthier than ever. In the last 10 years, a proliferation of new methods has extended our capability to extract detailed structural information from powder patterns. Two recent papers in *Acta Cryst*. B illustrate the strength of current methods.

The classical approach, Patterson and direct methods applied to the extracted “|F_{obs}|”, accounts for three-quarters of powder diffraction studies, but the most active area of research involves locating molecular fragments in known crystalline cells. Methods that use either a systematic grid search or a Monte Carlo calculation, simulated annealing and genetic algorithms have been applied successfully to nearly 50 powder patterns. A good example of such an application is described by Tremayne *et al*. [“2,4,6-Triisopropylbenzenesulfonamide: Monte Carlo structure solution from X-ray powder diffraction data for a molecular system containing four independent asymmetric rotors”, *Acta Cryst*. **B55** (1999), 1068-1074], who used the OCTOPUS program. The difficulty of application depends upon the number of degrees of freedom that a structure has, and increases with each independent torsion angle. Analogous single-crystal methods use Patterson and/or direct methods to position rigid models (e.g., PATSEE or DIRDIF programs). Because of reflection overlap – the powder diffraction handicap – the use of direct-space data is required to determine the position and conformation of a model.

The solution of a structure with unknown cell parameters using *ab initio* packing calculations is much more difficult. There are few successful applications of this approach, which requires previous knowledge of a quasi-complete molecule. In their paper, Karfunkel *et al*. [“Local similarity in organic crystals and the non-uniqueness of X-ray powder patterns”, *Acta Cryst*. **B55** (1999), 1075-1089] elucidate the crystal structure of some diketopyrrolopyrrole derivatives with surprisingly similar powder patterns, and introduce two new concepts for molecular solids, “local similarity” and “boundary-preserving isometry”. The degree of difficulty of indexing powder patterns is related to the complexity/resolution ratio. Crystallinity may strongly affect resolution. Poorly resolved powder patterns increase the ambiguity and decrease the accuracy of a structural model. The ability of distinctly different models to fit a powder pattern equally well is a recurring problem.

Owing to its one-dimensional nature, powder diffraction is less reliable than single-crystal analysis. Both of these articles address this problem. The high *R* factor in the Monte Carlo study (*R _{F} *= 0.10 for 517 reflections, 143 refined parameters, 120 geometrical restraints) suggests a problem with the model (a “local similarity” problem?) or the data. How wrong can structures determined from powder data be? The same question may be addressed to all underdetermined structural problems, including proteins, some of which are also being examined by powder diffraction!

The one-dimensional nature of powder diffraction will never be overcome. Consequently, a single-crystal study is always preferable to a powder study, when possible. The failure to obtain single crystals of many materials is the driving force behind the powder diffractionists’ quest for innovation. New methods such as those described in these articles (or at least those that are affordable, see http://sdpd.univ-lemans.fr/iniref/progmeth.html) may well stimulate crystallography as a whole.

Armel Le Bail, U. du Maine, Le Mans, France
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