lattice constant
/ LAT-iss KON-stuhnt /
Think of a brick wall, where every brick is the same size. Once you know the length of a single brick, you know how far it is from one brick's edge to the next identical edge, and how the whole wall scales. A crystal has the same kind of fixed repeat distance, and its value — the size of one repeating step — is the lattice constant.
The lattice constant is the length of the edge of a unit cell — the fixed distance over which the crystal's pattern repeats. A simple cubic crystal is described by a single lattice constant, usually written a; less symmetric crystals need more numbers (different edge lengths and angles) to pin the cell down fully. These distances are tiny, typically a few tenths of a nanometre, a few atom-widths, and they are measured with great precision by diffraction, which turns the angles of scattered X-rays into exact spacings.
The lattice constant matters because it is one of a crystal's most basic, measurable numbers, and small changes in it carry big consequences: it sets a material's density, shifts slightly with temperature as a crystal expands, and must be matched carefully when growing one crystalline layer on another for lasers and chips. The honest caveat is that the lattice constant is not perfectly fixed — it breathes with temperature and pressure, strains under stress, and shifts when impurities are added — so a quoted value always comes attached to specific conditions.
Silicon's lattice constant is about 0.543 nanometres. Chip makers must grow other materials whose lattice constants nearly match this, or the mismatch builds up strain and defects that ruin the device.
Layers grown on silicon must match its 0.543 nm repeat, or defects form.
A single number a fully describes only the most symmetric cells, like cubic ones. Lower-symmetry crystals need up to six parameters — three edge lengths and three angles — together called the lattice parameters.