a complex intermetallic
Most metal crystals are simple: a few atoms per unit cell, a pattern you can draw on the back of an envelope — copper has 4 atoms in its cell, iron has 2. Now imagine a metallic crystal whose single repeating unit contains not a handful but hundreds, or even more than a thousand, atoms — a unit cell so large it dwarfs an ordinary protein. A complex intermetallic (also called a complex metallic alloy, CMA) is exactly that: an ordered compound of two or more metals with a giant, intricate unit cell and structure that looks almost bewilderingly rich.
What organises all those atoms is not chaos but hierarchy. The atoms group into well-defined clusters — often nested shells around a centre, like an icosahedron inside a dodecahedron inside a larger cage (a 'Mackay' or 'Bergman' cluster) — and these clusters, rather than single atoms, are the real building units that pack to fill the cell. So a complex intermetallic is best read at two levels: the cluster (tens of atoms) and the arrangement of clusters (the giant cell). Beta-aluminium-magnesium (beta-Al-Mg) has over 1000 atoms per cell; the cubic compound NaCd2 famously has more than 1100; and the Taylor phase and many aluminium-transition-metal compounds have hundreds. These are the periodic cousins of quasicrystals — indeed many are 'approximants', crystals whose local cluster order is nearly icosahedral, mimicking a quasicrystal but repeating on a (large) lattice.
Complex intermetallics matter both scientifically and practically. Scientifically they are the missing link that helped explain quasicrystals: study the approximants and you learn the cluster chemistry the quasicrystal is built from. Practically their unusual structure gives unusual properties — low friction, low thermal conductivity, hardness, and sometimes catalytic activity — because the giant cell scatters electrons and phonons in ways a simple metal cannot. They are honestly hard to solve (a thousand-atom structure taxes even modern crystallography) and were long dismissed as messy, but they are now recognised as a rich frontier of structure science, sitting between the simple metals and the aperiodic quasicrystals.
The compound NaCd2 defeated crystallographers for decades: its cubic unit cell holds over 1100 atoms, organised into nested icosahedral and other clusters rather than a simple repeating motif. Solving it meant reading the structure as clusters-of-clusters, not atom-by-atom — the trademark difficulty, and reward, of a complex intermetallic.
A complex intermetallic: a giant unit cell (hundreds to 1000+ atoms) organised as nested clusters — many are quasicrystal approximants.
'Complex' here means a large, intricate but still PERIODIC cell — not disordered. The atoms are precisely placed; there are just very many of them per repeat, usually organised as clusters rather than single atoms.