Phase Width and Site Preferences in the EuMgxGa4−x Series

A series of EuMg_xGa_4?x compounds were synthesized using high temperature, solid‐state methods and characterized by both powder and single crystal X‐ray diffraction. All compounds crystallize in the tetragonal BaAl₄‐type structure (space group I4/mmm, Z = 2, Pearson symbol tI10) with full occupancy of Ga at the apical atom (4e) site and mixed‐occupancy of Mg and Ga at the basal atom (4d) site. Six compositions were analyzed by single crystal X‐ray diffraction: EuMg_0.21(1)Ga_3.79(1), EuMg_0.91(1)Ga_3.09(1), EuMg_1.22(1)Ga_2.78(1), EuMg_1.78(1)Ga_2.22(1), EuMg_1.84(1)Ga_2.16(1), and EuMg_1.94(1)Ga_2.06(1). As the larger Mg atoms increasingly replace Ga atoms at the basal site in EuMg_xGa_4?x, the a‐axis lengths at first decrease and then increase, while the c‐axis lengths increase monotonically along the series. The phase width of the BaAl₄‐type EuMg_xGa_4?x series is identified to be 0 ≤ x ≤ 1.94(1), a range which corresponds to 12.06(1)‐14 valence electrons per formula unit, and can be understood by their electronic structures using density of states (DOS) curves calculated by tight‐binding calculations. Mg substitution for Ga at the basal site is consistent with the site preferences for mixed metals on the three‐dimensional framework of the BaAl₄‐structure based on both electronegativities and sizes, and provides the rationale for the unusual behavior in lattice parameters. The observed site preference was also rationalized by total electronic energies calculated for two different coloring schemes.