Formation of F- and P-Type Icosahedral Quasicrystals in the Zn–Mg–Ho Alloy System^*

Two types of icosahedral quasicrystals, F- and P-types, formed at 563°C in the Zn₆₀Mg_40-xHo_x alloy depending on Ho content. In the composition range of 6≦x≦10, F-type icosahedral quasicrystal formed as a major phase in these alloys. The intensity of superlattice reflections which occurred due to F-type ordering became weaker in the alloys with lower Ho content. At x=5, P-type icosahedral quasicrystal formed, coexisting with MgZn₂-type Laves phase at 563°C and 503°C. Some electron diffraction patterns of the P-type icosahedral quasicrystal exhibited diffuse scattering intensity at positions corresponding to F-type superlattice reflections. By annealing at 403°C, a mixture of the P-type icosahedral quasicrystal and the MgZn₂-type phase changed to a mixture of the F-type quasicrystal and a decagonal quasicrystal. The former state was reversibly obtained by additional annealing at 503°C. These experimental results support the interpretation that the three types of quasicrystals form as a thermodynamically stable state in the Zn–Mg–Ho alloy system.