twofold rotational symmetry
Quick Summary
In crystallography, twofold rotational symmetry refers to an imaginary line through a crystal around which it can be rotated by 180 degrees and appear unchanged. This symmetry element is one of several, including rotation axes, rotoinversion axes, a center of symmetry, and mirror planes, that describe the external shape of a crystal and reflect its internal atomic arrangement. Crystals are classified into 32 crystal classes based on these symmetry elements, which are further grouped into six crystal systems. A twofold rotation axis is a fundamental element in understanding the geometric properties and classification of crystalline solids.12
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