the rock-salt structure

The rock-salt structure is the arrangement of ordinary table salt, sodium chloride, and it is the prototype every materials student learns first. Picture a three-dimensional checkerboard: sodium and chloride ions alternate in every direction, each one surrounded by six of the opposite kind. Its clean simplicity makes it the reference point for six-coordinate ionic solids.

Precisely, it is two interpenetrating face-centred-cubic lattices, one of chloride and one of sodium, offset from each other by half a cell edge, (1/2, 0, 0). Equivalently, and more useful for counting holes, it is an FCC array of chloride ions with sodium filling every one of the octahedral interstitial sites. Each ion has coordination number 6, and the conventional cubic cell contains 4 formula units of NaCl. The large radius ratio band from 0.414 to 0.732 favours exactly this sixfold coordination.

Rock salt is astonishingly common. Beyond the alkali halides, it is the structure of magnesium oxide, iron(II) oxide, nickel oxide, lead sulfide (galena), and hard carbides and nitrides such as titanium carbide. As a structure prototype it anchors a whole family: name a compound as rock-salt-type and you have specified its coordination, its cell contents, and how to count its ions in one word.