Last week, we talked about Kepler’s observations on the lattice structure of snowflakes, Steno’s law, and Rene Just Huay’s fundamental unit hypothesis.
Crystal Classification
In 1830, a few decades after Huay’s fundamental unit hypothesis, Johann F. C. Hessel used geometry to derive the possible unit structures of crystals. He knew that crystals could only have certain types of rotation, two-fold, three-fold, four-fold, and six-fold. That means, starting from the center of the unit, drawing lines that split the crystal into two, three, four, or six equal sections. Think of it like a circle; starting from the center, the number of lines to the edge of the circle you draw while making sectors of the same shape is the number of rotational symmetry. Because lattices have a limited number of rotational axes, even without a microscope, Hessel was able to describe all possible fundamental unit symmetries. Auguste Bravais described many of these symmetries more specifically as types of lattice structures.
X-ray Crystallography
In December of 1895, Wilhelm Conrad Röntgen published a groundbreaking paper on the discovery of an “X”-ray that could pierce through thick objects. While this was extremely important for the medical field and finding lesions in patients, it also led to the birth of modern crystallography. Nearly three decades later, a German chemist named Max von Laue discovered that crystals could diffract, or change the direction of X-rays. Even more importantly, every crystal would diffract the same X-ray in a different direction. That meant you could qualitatively measure the structure of any crystal, and be able to tell each one apart. This crucial discovery would eventually allow scientists to do all the brilliant and important things with crystals we need for modern life.
Next week’s post will go over 1900s and early 2000s X-ray crystallography ideas like Bragg’s law, Electron and Neutron Diffraction, and cryo-electron Microscopy.