The
R23Cu
7Mg
4 (
R = Ca, Eu) intermetallics, studied by single-crystal X-ray diffraction, were found to be isostructural with the Yb
23Cu
7Mg
4 prototype (
hP68,
k4h2fca, space group
P6
3/mmc), forming a small group inside the bigger 23:7:4 family, otherwise adopting the
hP68-Pr
23Ir
7Mg
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,
R10CuMg
3 (parent type:
hP28-
kh2ca, SG 194) and
R13Cu
6Mg (parent type:
hR60-
b6a2, SG 160). The ratio of these slabs in the studied structure is 2:2 per unit cell, corresponding to the simple equation, 2 ×
R10CuMg
3 + 2 ×
R13Cu
6Mg = 2 ×
R23Cu
7Mg
4. This description assimilates the studied compounds to the {Ca, Eu, Yb}
4CuMg ones, where the same slabs (of
p3
m1 layer symmetry) are stacked in a different way/ratio and constitutes a further step towards a structural generalization of
R-rich ternary intermetallics.
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