{Ca, Eu, Yb}[sub.23]Cu[sub.7]Mg[sub.4] as a Step towards the Structural Generalization of Rare Earth-Rich Intermetallics

• Published in: Crystals (Basel) 2024-01, Vol.14 (2)
• Main Authors: Solokha, Pavlo, Freccero, Riccardo, De Negri, Serena
• Format: Article
• Language: English
• Subjects: Chemical properties; Crystals; Intermetallic compounds; Rare earth metals; Structure
• ISSN: 2073-4352; 2073-4352
• DOI: 10.3390/cryst14020156

The R[sub.23] Cu[sub.7] Mg[sub.4] (R = Ca, Eu) intermetallics, studied by single-crystal X-ray diffraction, were found to be isostructural with the Yb[sub.23] Cu[sub.7] Mg[sub.4] prototype (hP68, k4h2fca, space group P6[sub.3] /mmc), forming a small group inside the bigger 23:7:4 family, otherwise adopting the hP68-Pr[sub.23] Ir[sub.7] Mg[sub.4] crystal structure. The observed structural peculiarity is connected with the divalent character of the R component and with a noticeable volume contraction, resulting in the clear clustering of title compounds inside the whole 23:7:4 family. The occurrence of fragments typical of similar compounds, particularly Cu-centered trigonal prisms and Mg-centered core–shell polyicosahedral clusters with R at vertices, induced the search of significant structural relationships. In this work, a description of the hexagonal crystal structure of the studied compounds is proposed as a linear intergrowth along the c-direction of the two types of slabs, R[sub.10] CuMg[sub.3] (parent type: hP28-kh2ca, SG 194) and R[sub.13] Cu[sub.6] Mg (parent type: hR60-b[sup.6] a[sup.2] , SG 160). The ratio of these slabs in the studied structure is 2:2 per unit cell, corresponding to the simple equation, 2 × R[sub.10] CuMg[sub.3] + 2 × R[sub.13] Cu[sub.6] Mg = 2 × R[sub.23] Cu[sub.7] Mg[sub.4] . This description assimilates the studied compounds to the {Ca, Eu, Yb}[sub.4] CuMg ones, where the same slabs (of p3m1 layer symmetry) are stacked in a different way/ratio and constitutes a further step towards a structural generalization of R-rich ternary intermetallics.