Zeitschrift für Kristallographie: Discussions

Discussion of unusual hydrogen atom display in solid acids
Any crystallographer knows that due to the lowest scattering factor localization of hydrogen atoms using X-ray diffraction data is a non-trivial task, especially if the hydrogen atoms are disordered. Normally, it would be considered very difficult to identify electron density related to disordered hydrogen atoms, particularly, in the presence of heavy atoms such as Cs or Rb.

Boris V. Merinov
Studying crystal structures of superprotonic phases of alkali metal hydrogen sulfates and selenates, a very unusual phenomenon has been revealed … download the full article.

This article is published in Issue 9 (2010).

And what’s your opinion?
You can either kristin [dot] berber-nerlinger [at] oldenbourg [dot] de (subject: Comment) (give your comment) which will exclusively appear on this page or send your contribution for publication in Zeitschrift für Kristallographie to the kristin [dot] berber-nerlinger [at] oldenbourg [dot] de (subject: Comment) (Editorial Office).

Chirality, a never-ending source of confusion

There are three common misunderstandings regarding the actual meaning of the term chiral. Firstly, we often associate chirality with objects although abstract things such as symmetry operations also can be chiral. Secondly, chiral does not mean enantiomerically pure and finally, an atom cannot be chiral.

 Kenneth Wärnmark (above), Ola F. Wendt (below) et al.
In this Discussion a few points of confusion concerning chirality are clarified.
The term chirodescriptive is proposed for such space groups that lack inversion centres, reflection planes, glide planes or rotary-inversion axes and thus can contain enantiopure, chiral objects.

It is pointed out that chiral compounds can (and often do) crystallise in non-chirodescriptive space group when they occur as racemates and in such cases there is no incompatibility between chirality and mirror planes or centres of inversions. We also propose... 
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This article is published in Issue 12 (2009).

And what’s your opinion?You can either berber [at] oldenbourg [dot] de (give your comment) which will exclusively appear on this page or send your contribution for publication in Zeitschrift für Kristallographie to the Editorial Office: kristin [dot] berber-nerlinger [at] oldenbourg [dot] de.

On the concept of long range order in solids

The discovery of quasicrystals rendered the definition of crystalline state as a periodic solid too narrow. Consequently, a discussion on how to better define the notion of crystalline order has been going on for more than ten years, and it is still open.

Ernesto Estevez-Rams (see photo) and Rosario González-Férez
The paper focus on the ongoing discussion on how to better define the notion of crystallinity. We suggest that the use of algorithmic complexity captures, from a mathematical point of view... download the full article

This article is published in Issue 3 (2009).

And what’s your opinion?
You can either berber [at] oldenbourg [dot] de (give your comment) which will exclusively appear on this page or send your contribution for publication in Zeitschrift für Kristallographie to the Editorial Office: kristin [dot] berber-nerlinger [at] oldenbourg [dot] de.  

Massimo Nespolo (see photo), Bernd Souvignier and Daniel B. Litvin
The definition of "noncrystallographic symmetry" given in Vol. B of the International Tables for Crystallography actually corresponds to the concept of "local symmetry". A new definition is proposed...download the full article
This article is published in Issue 9 (2008). 

  And what’s your opinion?

You can either berber [at] oldenbourg [dot] de (give your comment) which will exclusively appear on this page or send your contribution for publication in Zeitschrift für Kristallographie to the Editorial Office: kristin [dot] berber-nerlinger [at] oldenbourg [dot] de. 

What is a co-crystal?

The term “co-crystal” was first used in 1967 to describe a multi-component crystal and is being discussed ever since ...

  Julio Zukerman-Schpector and Edward R. T. Tiekink
What might very well be classified as a revolution in chemical crystallography is the emergence of crystal engineering as a discipline and the pivotal role X-ray crystallography plays in this. Although dating back to the 1950’s, the field has exploded in the last decade or so, and 

now is at the forefront of contemporary chemical and cognisant research. One of the areas in crystal engineering of greatest potential impact relates to the pharmaceutical industry – drug formulation and intellectual property – where issues relating to crystallisation, polymorphism, etc. are paramount. download the full article 

This article is published in Issue 3 (2008).

  What is a co-crystal – and does it matter?

Andrew Parkin (see photo), Christopher J. Gilmore and Chick C. Wilson
The debate about the definition of the term co-crystal is a distraction from the quality of molecular solid-state science being published.
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This article is published in Issue 6 (2008). 

And what’s your opinion?

You can either berber [at] oldenbourg [dot] de (give your comment) which will exclusively appear on this page or send your contribution for publication in Zeitschrift für Kristallographie to the Editorial Office: kristin [dot] berber-nerlinger [at] oldenbourg [dot] de.

What is a crystal?

What a strange question, you may think, because there is a nice definition in the International Tables for Crystallography.

Walter Steurer
Introductory remarks to an ongoing discussion
What is a crystal? What a strange question, you may think, because there is a nice definition in the International Tables for Crystallography, Vol. A.
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Shelomo I. Ben-Abraham
What is a crystal?
The Commission on Aperiodic Crystals adopted the following definition of a crystal:"…by 'crystal' we mean any solid having an essentially discrete diffraction diagram, …
This definition has, in my opinion, two flaws.

• 1: It explicitly depends on diffraction, hence on a specific method, albeit important and widespread
• 2: More importantly, and in part as a consequence of (1), it is not general enough.

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Ted Janssen
No need for reconsidering the definition!
The definition is a bit vague but points out to a spectrum with essentially Bragg peaks. Of course, it was well known that there are other notions of order. There is a zoo of mathematical models with largely varying types of order.
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Marjorie Senechal
Thoughts on "crystal"
While the research program implied by the survey is important, the definition isn't!I think of the furor last summer over Pluto's demotion from planetary status. The astronomer’s debate centered on a new definition of "planet". Who cares? Whatever its label, Pluto still circles the sun, and planetary scientists still study it.
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Michael Baake and Dirk Frettlöh
Is there a crystal clear definition?
No one seems to be satisfied with the present fuzzy definition, since

• it is neither one in the mathematical sense
• nor is it reasonable to use a scheme where a quasicrystal is called a crystal but not necessarily vice versa.

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Ron Lifshitz
What is a crystal?
Almost 25 years have passed since Shechtman discovered quasicrystals, and 15 years since the Commission on Aperiodic Crystals of the IUCr put forth a provisional definition of the term crystal to mean “any solid having an essentially discrete diffraction diagram.” Are we ready now to propose a permanent definition for crystal to be used by all? I argue that time has come to put a sense of order in all the confusion.
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Helmuth Zimmermann
About crystals and their definition
A look back into history tells us that crystallography was developed without a definition of a crystal, or more precise: the definition has grown with the knowledge about an only vaguely described object. This is painful for mathematicians but not for physicists. Physicists can show the material they deal with, it exists as soon as they have shown, named and described it, mathematicians have to define it: it does not exist as long as it is not defined.
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These articles are published in Issue 6 (2007).

Shelomo I. Ben-Abraham
What is a crystal? – follow-up
Having started the discussion on “What is a crystal?” and having carefully read all contributions to it, I feel obliged to summarize and draw some (interim) conclusions. The “lattice + motif” definition served well two purposes. On one hand, it was a definition in the strict mathematical sense. On the other hand, at the same time it served as a statement of the subject of study in an introductory course to students of physics, chemistry, materials and the like. That cannot be said of the current “definition”. Unfortunately, at this time it seems to be impractical to come up with an adequate substitute.
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This article is published in Issue 10 (2007).

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