Pressure-induced suppression of structural distortions manifested in group–subgroup relationships

Based on widely accepted rules of thumb [1, 2] of high-pressure crystal chemistry, elements at elevated pressure increase their coordination number and behave like the elements below them in the periodic table at lower pressures. It was noted already in the highly acclaimed structural chemistry book by Alexander Frank Wells [3] that the crystal structures of gray arsenic (Strukturbericht A7, space group R¯3m) and black phosphorus (bP, Strukturbericht A17, space group Cmce) may be regarded as the distorted simple cubic (sc, space group Pm¯3m) structure (Fig. 1). The ideal sc structure is very rare in nature, and relatively unstable because of its low packing efficiency and low coordination number. Further high-pressure studies demonstrated that bP transforms first to the A7 phase and then, passing through a pseudo-simple-cubic structure, to the sc phase [4, 5]. A similar pressure-induced phase transition from the A7 to the sc phase is observed in elemental arsenic. Recent reports show that polymeric cg-N (space group I21 3) and bp-N (Cmce, bP-type structure) phases of nitrogen can also be regarded as distorted sc atomic arrangements [6]. Furthermore, recently discovered crystalline arsenic nitride AsN (space group P21 3) [7] can be considered as a highly distorted rock salt (Strukturbericht B1, space group Fm¯3m) structure [8].