Thermodynamic Stability of the Icosahedral Quasicrystal i-ScZn: A Reassessment of the Sc–Zn Phase Diagram

Abstract The thermodynamic stability of quasicrystals (QCs) and their relationship to compositionally adjacent phases in intermetallic phase diagrams remain incompletely understood. We examined the icosahedral QC i-ScZn (Sc12Zn88), which is embedded in the Sc–Zn system between the cubic 1/1 approximant crystal (AC) phase ScZn6 (Sc14.3Zn85.7) and the ScZn12 phase (Sc7.7Zn92.3) adopting the tetragonal ThMn12 structure. Single crystalline (or single grain) samples of ScZn6, Sc12Zn88, and ScZn12 were obtained by solution growth from ScxZn100–x melts under appropriate composition and temperature conditions. Variable-temperature powder X-ray diffraction and thermal analysis experiments reveal that as-cast Sc12Zn88 undergoes annealing-induced decomposition into Sc12+xZn88–x (x ≈ 1) and ScZn12, indicating the existence of a temperature-dependent homogeneity range of the i-ScZn phase. Upon heating, i-ScZn decomposes peritectically at approximately 625 °C into ScZn6 and liquid. Interpretation of the phase equilibria is complicated by pronounced kinetic undercooling of approximately 30 °C associated with the crystallization of ScZn12, which decomposes on heating at 505 °C into i-ScZn and liquid, but crystallizes only upon cooling to 475 °C. The present results support prior suggestions of an enthalpy-driven stabilization of i-ScZn, while establishing Sc12+xZn88–x (x ≈ 1) as the thermodynamically stable QC composition at low temperatures.