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Four figures used for hexagonal crystals.
Also known as: hkil notation, Four index hexagonal notation
The four-index notation used for hexagonal and trigonal crystals, in which the third index is fixed by the first two.
Names a plane in a hexagonal or trigonal crystal using four indices rather than three.
Contexts: Laboratory
Fixes the third index as the negative sum of the first two, so it adds no information and exists only to make symmetry-equivalent planes look equivalent.
Contexts: Laboratory
Four figures written side by side in brackets, one more than the three used for every other crystal system.