The Gist of Science Reporter: February 2015

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.

Early Research

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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