ERSC 2P21 Lecture Notes - Lecture 12: Crystal System, Improper Rotation, Angle Of Rotation
19 Dec 2018
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Document Summary
External morphology, or crystal form, is the outward expression of the internal arrangement of atoms. External morphology, or crystal form, can be described by a set of symmetry operations. First, some basics: a repeated motif creates symmetrical order. In a crystal, the motif can be an atom or group of atoms in 3 dimensions. Unit cell: the smallest unit of a structure or that can be pattern to infinitely create entire structure repeated the structure. We can also recognize and systematically describe crystal form. Point symmetry: external morphology is defined by point symmetry. It expresses how a motif (e. g. , a crystal face!) repeats around a point: rotation axes, mirror planes, centers of symmetry, rotoinversion axes. 4 operations of point symmetry: rotation about a rotation axis, reflection across a mirror plane. Element rotation axis mirror plane centre of symmetry. Just as in 2d wallpaper, the object (or group of objects) being repeated is called the motif.
