Unit Cell Hell

I experienced a wake-up call recently when a student dropped by to ask about unit cells. Wow, I realized, I know nothing about crystal structures. Analyzing simple cubic on the fly is easy enough, but the close-packed structures require quite a bit of mental gymnastics. Working mostly on the lab side of things, I don’t often think about this topic (although some nice activities with solids as their focus have been developed).

While the visualization skills needed to understand unit cells inside and out can turn students off, they’re a classic example of how chemists use microscopic structure and properties to explain and predict macroscopic phenomena. Stripping away the messy details, there are relatively few properties of unit cells that general chemists care about:

  • Packing fraction (also interesting from a physical and mathematical point of view)
  • Density
  • Hole geometry and count
  • Dimensions and atomic/molecular radii

This video is a great introduction to the most important crystal structures from a materials science point of view. The best thing a student can do, in my opinion, is use a physical model to build up the structures herself—this is particularly true for the close-packed structures, which to me have a kind of magical allure. How can there be two ways to pack hard spheres as closely as possible? The answer becomes apparent after you’ve stacked up two planes of close-packed spheres…

Two layers of close-packed spheres stacked one on top of the other. Note the two types of pockets for the next layer!

Two layers of close-packed spheres. Note the two types of pockets for the next layer!

The next layer of spheres will sit down in the triangular “pockets” between the red spheres, but there are two inequivalent types of pockets: those above tetrahedral holes and those above octahedral holes. Because of the size of the spheres, both types of pockets cannot simultaneously be occupied. Ergo, there are two inequivalent close-packed structures! Continue reading →


Demo This!: Smash Glow Crystals

It’s time for a nod to one of my favorite chemistry-themed YouTube channels, NurdRage. NurdRage’s channel is basically a laundry list of awesome, little-known chemistry experiments…with a creepy yet soothing voice-changer voice to boot. One of his recent videos is especially cool: he synthesizes blue triboluminescent crystals from copper thiocyanate, pyridine, and PPh3, then grinds them up. The experiment is based on a recent JCE article from Marchetti et al. The reaction itself is straightfoward to perform: mix everything up, heat to dissolve, wait for cystallization, and wash! The copper salt, pyridine, and triphenylphosphine react to form the coordination complex (SCN)Cu(py)2(PPh3). Grinding of the crystals produces a stunning blue light.

NurdRage’s explanation of triboluminescence (the direct transduction of mechanical energy to light) is clear and concise—check it out below! Long story short, charge separation occurs in the crystal upon mechanical agitation, and recombination of the charges produces a blue light.