A7, alpha-arsenic, rhombohedral graphite, and beta-oxygen

Structural typeA7
Common nameA7 – alpha-arsenic,
rhombohedral graphite (r-graphite)
DefinitionA7 – distortion of sc towards 3-fold coordination,
r-graphite – transitional structure between dia and graphite,
beta-oxygen – fcc of diatomic molecules in parallel orientation
Prototypeα-As, r-gra, β-O2
Pearson symbolhR2
Space group166 R-3m D3d5
Chemical formulaA
BondingA7 and r-graphite – covalent in layer
Atomic positions A (6c)  0  0  zeta
CoordinationA7 – 3+3 (6ap), r-graphite – 3, beta-oxygen – 1 (see the figure below)
PDB filesA7
Parametersc/a, ζ<1/4 (see the figure below for precise ranges, other zeta can be reduced to ζ<1/4)
Special valuesplain graphite sheets in abc stacking for zeta=1/4,
close packing (rcp) for zeta=1/2-1/sqrt(6) and c/a=3+sqrt(6) with z=10 and δ=2*sqrt(6)-4≈.90
Superstructuressr for ζ=1/4, in particular, sc for c/a=sqrt(6) (high pressure metallic form of V elements),
dia for ζ=1/8 and c/a=sqrt(6),
diatomic sr for ζ=0, in particular, diatomic fcc for c/a=sqrt(6)
SubstancesA7 – semimetals of V elements (see the table below),
beta-oxygen – molecular crystals of diatomic molecules

In close-packed layers notations this structure can be considered as stacked layers in the following positions: A at ±ζ, B at 1/3±ζ, and C at 2/3±ζ. For 0<ζ<1/6 we have AABBCC stacking and for 1/6<ζ<1/4 the stacking is ABCABC (the period is 6 layers). Hence rcp is not a local maximum in packing sliding AA layers apart enhances the packing.

Nat P>150 GPa but it is metastable (cubic-gauche is stable)
Pat P>5GPa