A8, grey-selenium

Structural typeA8
Common namegrey-selenium
Definitiondistortion of sc towards 2-fold coordination
Pearson symbolhP3
Space group152 P3121 D36
Chemical formulaA
Bondingcovalent within the polymer
Atomic positions A (3a)  xi  0  1/3
Coordination2+4 (6ap), see the figure below
PDB filesA8 at ξ=1/3 as sc, A8 at ξ=1/2 at ideal packing 
Parametersc/a, 0<ξ<1/2 (see the figure below for precise range, other ξ can be reduced to ξ<1/4)
Superstructuresgraphite layers in eclipsing stacking for ξ=0,
sr for ξ=1/3,
in particular sc for c/a=sqrt(3/2),
180 P6222 symmetry for ξ=1/2
Substances VI elements