Quasicrystal Formation, Composition, and Stability in the Yb–Au–Sn and Yb–Au–Al Systems: A Comparative Study

Yb-based icosahedral quasicrystals (iQCs) can exhibit intermediate-valence behavior. Here, we revisit the Yb–Au–Sn and Yb–Au–Al systems, which host iQCs with average Yb valences of approximately 2.18 and 2.8, respectively. Particular emphasis was placed on the formation conditions, compositions, thermodynamic stability, and relationship to related periodic approximant crystals (ACs). i-YbAuSn, Yb13.6Au64.8Sn21.6, can only be obtained by solution growth from melts containing 4–6 at. % Yb and melts incongruently at ∼510 °C to form the 2/1 AC Yb14.2Au63.5Sn22.3. In contrast, i-YbAuAl, Yb15.8Au53.5Al30.7, is accessible only as a metastable phase by arc melting Yb-rich compositions (∼17 at. % Yb). The phase remains kinetically stable up to ∼400 °C. The 1/1 AC YbAuAl melts congruently and occurs in two structural variants: Yb13.6Au56.2Al30.2, featuring an (Au,Al)4 tetrahedron at the center of the Tsai-cluster building unit, and Yb14.0Au54.7Al31.11, in which the central tetrahedron is partially (∼10%) replaced by a single Yb atom. It is proposed that thermodynamically stable Yb-based iQCs (such as i-YbAuSn, i-YbAgIn, and i-YbCd) are characterized by a low Yb average valence (close to 2). In contrast, iQCs in which Yb exhibits a substantially higher average valence (>2.4), such as i-YbAuAl, appear to be metastable.