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)degC and 931(3)degC. Crystal growth was realised using a Bridgman technique and single crystalline grains of about 50-100 10^{-6}m were analyzed 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 superstructure is challenging. The compounds YbCu4.4 and YbCu4.25 fit in Cerny`s (J. Solid State Chem. 174 (2003) 125) building principle {(RECu5)n(RECu2)} where RE = Yb with n = 4 and 3. YbCu4.4 and YbCu4.25 base 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).