development X-ray crystallography. In recent years it has been discovered that it is
also possible to form quasi-periodic crystals (quasicrystals), which have a structure
that is ordered but not periodic. A quasicrystalline pattern can continuously fill all
available space, but it lacks translational symmetry. According to the classical
crystallographic restriction theorem, crystal lattices can only possess two-, three-,
four-, and sixfold rotational symmetries. The Bragg diffraction pattern of quasicrystals shows sharp peaks with other symmetry orders, e.g. fivefold rotational symmetry. Aperiodic tilings were discovered by mathematicians in the early 1960s, and,
some 20 years later, they were found to apply to the study of natural quasicrystals.
The discovery of these aperiodic forms in nature has produced a paradigm shift in the
fields of crystallography. In 1982 the materials’ scientist Dan Shechtman observed
that certain aluminium-manganese alloys produced the unusual diffractograms
which today are seen as diagnostic of quasicrystalline structures. In 2009 after an
extensive search, icosahedrite a naturally occurring mineral was shown to be a
quasicrystal. Dan Shechtman was awarded the Nobel Prize in Chemistry in 2011
for his research in this area (see Table 1).
6 Spectacular Growth of Structural Data
By 1920 the structures of 50 elements and compounds had been studied by this new
technique although many of the major contributors had been diverted to assist their
governments in war-related activities between 1914 and 1918. After the war the
activity soon picked up and by 1925 600 structures had been reported. Currently
more than one million structures have been determined, and the instrumental and
computing advances which have made this possible are discussed in subsequent
chapters. As early as 1930, it was recognised that the increase in structural information was going to create archival and accession problems for future generations.
R.W.G. Wyckoff made important contributions to addressing these issues and
providing mechanisms for efficiently disseminating structural information [66].
These developments in theoretical chemistry provided the intellectual framework
for chemistry over the last century and led to its exponential expansion into a wide
range of new areas. The growth of chemistry can be appreciated by the following
statistics. The Chemical Abstracts Service has estimated that it contains information
on 100 million organic and inorganic compounds, and the Cambridge Structural
Database (CSD) currently has crystallographic data on over a million inorganic and
organic compounds. Only 4,500 structures had been solved and documented
between 1923 and 1962 and formed the basis of the original database. Olga Kennard
was the driving force behind setting up the original database. It is both a repository
and a validated and curated resource for the three-dimensional structural data of
molecules generally containing at least carbon and hydrogen. This means that it
covers a very wide range of organic, metal-organic, organometallic and
co-ordination molecules submitted by crystallographers and chemists from around
the world. The database now contains more than a million entries. The specific
Early History of X-Ray Crystallography
25
also possible to form quasi-periodic crystals (quasicrystals), which have a structure
that is ordered but not periodic. A quasicrystalline pattern can continuously fill all
available space, but it lacks translational symmetry. According to the classical
crystallographic restriction theorem, crystal lattices can only possess two-, three-,
four-, and sixfold rotational symmetries. The Bragg diffraction pattern of quasicrystals shows sharp peaks with other symmetry orders, e.g. fivefold rotational symmetry. Aperiodic tilings were discovered by mathematicians in the early 1960s, and,
some 20 years later, they were found to apply to the study of natural quasicrystals.
The discovery of these aperiodic forms in nature has produced a paradigm shift in the
fields of crystallography. In 1982 the materials’ scientist Dan Shechtman observed
that certain aluminium-manganese alloys produced the unusual diffractograms
which today are seen as diagnostic of quasicrystalline structures. In 2009 after an
extensive search, icosahedrite a naturally occurring mineral was shown to be a
quasicrystal. Dan Shechtman was awarded the Nobel Prize in Chemistry in 2011
for his research in this area (see Table 1).
6 Spectacular Growth of Structural Data
By 1920 the structures of 50 elements and compounds had been studied by this new
technique although many of the major contributors had been diverted to assist their
governments in war-related activities between 1914 and 1918. After the war the
activity soon picked up and by 1925 600 structures had been reported. Currently
more than one million structures have been determined, and the instrumental and
computing advances which have made this possible are discussed in subsequent
chapters. As early as 1930, it was recognised that the increase in structural information was going to create archival and accession problems for future generations.
R.W.G. Wyckoff made important contributions to addressing these issues and
providing mechanisms for efficiently disseminating structural information [66].
These developments in theoretical chemistry provided the intellectual framework
for chemistry over the last century and led to its exponential expansion into a wide
range of new areas. The growth of chemistry can be appreciated by the following
statistics. The Chemical Abstracts Service has estimated that it contains information
on 100 million organic and inorganic compounds, and the Cambridge Structural
Database (CSD) currently has crystallographic data on over a million inorganic and
organic compounds. Only 4,500 structures had been solved and documented
between 1923 and 1962 and formed the basis of the original database. Olga Kennard
was the driving force behind setting up the original database. It is both a repository
and a validated and curated resource for the three-dimensional structural data of
molecules generally containing at least carbon and hydrogen. This means that it
covers a very wide range of organic, metal-organic, organometallic and
co-ordination molecules submitted by crystallographers and chemists from around
the world. The database now contains more than a million entries. The specific
Early History of X-Ray Crystallography
25