The Gist of Science Reporter: February 2015
The Story of Crystals
Crystals are pleasing to the eye. Their beautiful shapes and
symmetries have always evoked a sense of amazement and wonder since antiquity.
Crystalline materials are all around us and find applications in almost all
walks of life. More than 90% of the naturally occurring solids are crystalline.
Minerals, rocks, sand, snowflakes, ice, clay, gems, jewels, metals, carbon, and
salts all have crystalline structures. This is because regular arrangements of
atoms results in lowest energy and hence greatest stabilization.
The word crystal is believed to have originated two thousand
years ago when the Greeks described the quartz crystals by the word krystallos,
meaning ice due to their resemblance to frozen water. However, for scientists
the aesthetics of the crystalline world has been more of a motivation for
understanding the crystal structure and its various manifestations. However, the
first scientific investigation into the regularity and symmetry of crystals
began only in the 17th century. We can consider atoms as points and imagine
points arranged in space in an orderly manner. Such an array in which each point
has surroundings exactly identical to the surrounding of any other point in that
array is called a lattice. In such a lattice if we attach an atom or a group of
atoms, called the basis, then a crystal is formed. The smallest building block
of a crystal so formed is then understood as the unit cell of that crystal. Just
as a wall is built by systematic repetition of bricks, a crystal can be looked
upon as a systematic repetition of unit cells in three dimensions.
In 1845, Auguste Bravais, a French physicist, showed that
there can be only 14 such unique lattices, which are named after him as Bravais
lattices, i.e., any crystal in nature can be understood in terms of only these
14 lattices. All crystalline materials recognized until now (excluding
quasicrystals) fit in one of these arrangements. Bravais published a memoire
about Crystallography in 1847.
Diffraction - A Key Concept
In the nineteenth century, optical microscopes were
discovered and there were many attempts to understand the cause of symmetry in
crystals using them. Metals were etched and examined under microscopes to reveal
their microstructural features. However, all these investigations could not
reveal anything smaller than the wavelength of light (4000-7000 A). The reason
behind this was understood on the basis of the phenomenon of diffraction.
Light is found to be diffracted by obstacles like thin slits
and sharp edges. When a light wave encounters such an obstacle, the secondary
waves interfere to produce regions of maximum and minimum intensity, called the
diffraction pattern. Coloured rings around a street light in frosty weather and
the pattern seen when a distant source of light is seen through a crack between
two fingers are examples of diffraction pattern.
An interesting development took place on 8th November 1895,
when W.C. Rontgen, the German physics professor, discovered accidently a new
type of radiation which he named ‘X-rays’, due to its unknown nature and
conspicuous properties. He found that X-rays travel in straight lines like
visible light but do not show other properties of light such as reflection,
refraction, diffraction or polarization.
Rontgen was awarded the first Nobel Prize in Physics for this
work in 1901. He was also appointed to the chair of experimental physics at
Munich University in 1900. The theoretical physics group at this University was
headed by A. Sommerfeld. Diffraction patterns produced by various obstacles were
found to contain the knowledge of both the diffracting obstacle and the
diffracted wave. Thus diffraction emerged as a key concept that promised to
bring to fore connections between two lengths in nature: the length of the
probing wave and the length (size) of the probed obstacle. We see next how this
concept united X-rays with crystals.
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