AlP is observed at V/FU = 33.84 Å (V/V 0 = 0.81 for zb-AlP). The corresponding pressure calculated using the experimental B 0 and B 0 ʹ values of zb-AlP (Table ) is equal to 28 GPa, i.e. qtz AlP formation could be expected at higher pressures. However, at room temperature, the phase transformation of zb-AlP into a high-pressure phase with NiAs-type structure, which is energetically favorable over the whole pressure range, is observed already at about 10 GPa . Thus, the formation of qtz AlP at sufficiently high (> 28 GPa) pressures can only be possible as a result of alternative metastable behavior under far-from-equilibrium conditions (shock compression, etc.).