The corresponding crystal structures are shown in Fig. with ball-and-stick and tetrahedral representations, the latter featuring corner-sharing irregular tetrahedra (vide infra). The atoms occupy the Wyckoff positions (3c) ½, 0, 0 and (3d) ½, 0, ½. Differences are observed in the volumes (total and atom-averaged) and the interatomic distances, which increase due to the large increase of the atomic radius from B to Al: 0.83 Å versus 1.83 Å; r(P)=1.10 Å. Looking at the angles related to the constituent tetrahedra, they differ significantly from the regular tetrahedral type (109.47°), thus showing the specificity of the qtz topology. From Table , which shows the energy per formula unit (FU), the qtz structures are cohesive and have higher energies than the zincblende ones, known as the ground state structures at normal conditions. In terms of volume per FU, smaller volumes characterize the qtz phases suggesting possible transitions at high pressures (vide infra).