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

TL;DR

This study reports the discovery and crystal growth of two new copper-rich ytterbium phases, YbCu4.4 and YbCu4.25, with giant unit cells, expanding the known series of rare earth copper compounds.

Contribution

It introduces the synthesis and structural analysis of two novel phases, filling gaps in the series of copper-rich rare earth compounds and demonstrating their complex superstructures.

Findings

YbCu4.4 and YbCu4.25 are formed peritectically at ~930°C.

Single crystals of these phases contain approximately 4570 and 2780 atoms per unit cell.

The phases fit into Cerny's building principle for rare earth copper compounds.

Abstract

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).