Crystal growth of copper-rich ytterbium compounds: The predicted giant unit cell structures YbCu4.4 and YbCu4.25

Two new phases YbCu4.4 and YbCu4.25 are found as a result of careful phase diagram investigations. Between the congruent and peritectic formation of YbCu4.5 and YbCu3.5, respectively, the phases YbCu4.4 and YbCu4.25 are formed peritectically at 934(2) deg C and 931(3) deg C. Crystal growth was realised using a Bridgman technique and single crystalline grains of about 50-100 mum were analysed by electron diffraction and single crystal X-ray diffraction. Due to the only slight differences in both compositions and formation temperatures the growth of larger single crystals of a defined phase is challenging. The compounds YbCu4.4 and YbCu4.25 fit in Cernys [J Solid State Chem 2003;174:125] building principle {(RECu5)n*(RECu2)} where RE = Yb with n = 4 and 3. YbCu4.4 and YbCu4.25 are based on AuBe5/MgCu2-type substructures and contain approximately 4570 and 2780 atoms per unit cell. The new phases close the gap in the series of known copper-rich rare-earth compounds for n = 1, 2 (DyCu3.5, DyCu4.0) and n = 5 (YbCu4.5, DyCu4.5).