APPENDIX A-3
Convention for Assigning MillerIndices for Hexagonal Crystals
Crystal systems that have hexagonal symmetry will not have the same set of indices for planes or directions as other (e.g., cubic or essentially cubic) systems, even though some are crystallographically equivalent. This problem is circumvented by utilizing a four-index Miller-Bravais coordinate system as shown in Figure A3.1. The three axes a1, a2, and a3 all lie within the basal plane and are at 120° angles to one another, while the z-axis (sometimes referred to as c-axis in hexagonal close-packed [HCP]) is oriented perpendicular (90°) to the basal plane.
Indices for Directions
Directional indices are obtained as for cubic systems as [uvw] (or [hkl] in some references) ...
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