| Summary: | The <i>R</i><sub>23</sub>Cu<sub>7</sub>Mg<sub>4</sub> (<i>R</i> = Ca, Eu) intermetallics, studied by single-crystal X-ray diffraction, were found to be isostructural with the Yb<sub>23</sub>Cu<sub>7</sub>Mg<sub>4</sub> prototype (<i>hP</i>68, <i>k<sup>4</sup>h<sup>2</sup>fca</i>, space group <i>P</i>6<sub>3</sub><i>/mmc</i>), forming a small group inside the bigger 23:7:4 family, otherwise adopting the <i>hP</i>68-Pr<sub>23</sub>Ir<sub>7</sub>Mg<sub>4</sub> crystal structure. The observed structural peculiarity is connected with the divalent character of the <i>R</i> 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 <i>R</i> 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 <i>c</i>-direction of the two types of slabs, <i>R</i><sub>10</sub>CuMg<sub>3</sub> (parent type: <i>hP</i>28-<i>kh<sup>2</sup>ca</i>, SG 194) and <i>R</i><sub>13</sub>Cu<sub>6</sub>Mg (parent type: <i>hR</i>60-<i>b</i><sup>6</sup><i>a</i><sup>2</sup>, SG 160). The ratio of these slabs in the studied structure is 2:2 per unit cell, corresponding to the simple equation, 2 × <i>R</i><sub>10</sub>CuMg<sub>3</sub> + 2 × <i>R</i><sub>13</sub>Cu<sub>6</sub>Mg = 2 × <i>R</i><sub>23</sub>Cu<sub>7</sub>Mg<sub>4</sub>. This description assimilates the studied compounds to the {Ca, Eu, Yb}<sub>4</sub>CuMg ones, where the same slabs (of <i>p</i>3<i>m</i>1 layer symmetry) are stacked in a different way/ratio and constitutes a further step towards a structural generalization of <i>R</i>-rich ternary intermetallics.
|
|---|