According to the Hermann Mauguin notation, the'2'refers to the two-fold axis while the'm'refers to the presence of a single mirror plane.
22.
Sarabauite is of the 2 / m crystal class; it has a 2 fold rotation axis perpendicular to a mirror plane.
23.
It only has two symmetry operations : identity ( E ) and a mirror plane that goes through the three atoms.
24.
Its crystal system is monoclinic with possible crystal classes of twofold symmetry, mirror plane symmetry, and twofold with a mirror plane.
25.
Its crystal system is monoclinic with possible crystal classes of twofold symmetry, mirror plane symmetry, and twofold with a mirror plane.
26.
Consequently, it is the only member of that group to possess mirror planes that do not pass through any of the faces.
27.
It should be noted that this structure lacks a horizontal mirror plane and thus inversion symmetry, which is essential to realize Weyl semimetal.
28.
Conversely ? 2 and ? 4 are symmetric with respect to the C 2 axis and antisymmetric with respect to the ? mirror plane.
29.
? 1 and ? 3 are symmetric with respect to the mirror plane as the sign of the p-orbital lobes is preserved under the symmetry transformation.
30.
There should also not be a mirror plane or twofold axis perpendicular to the polar axis, because they will make both directions of the axis equivalent.
