Alloying induced superionic β-phase formation in Mg3Sb2 based Zintl compounds†

The off-centering phenomenon manifests as locally distorted configurations with broken symmetry in a crystal structure due to the displacement of constituent atoms from their ideal coordination centers within the lattice. In contrast to the anticipated formation of anionic solid solutions of Mg₃(Sb_1−x−yBi_xGe_y)₂, herein we report β-Mg₃(Sb, Bi)₂ based superionic phase formation (space group – Ia, 206) with off-centering of the dominant trigonal α-Mg₃(Sb, Bi)₂ phase and segregation of nanophase Mg₃Ge upon equiatomic (Bi, Ge) alloying. The discordant nature of Ge is unveiled within the layered α-Mg₃(Sb, Bi)₂ structure and is assessed employing (3 + 1) dimensional superspace to reveal an off-centering (dz) along the z direction for the constituent atoms in the range of ±0–0.02 Å. The (Bi, Ge) alloying results in favourable tuning of the desired p-type conduction for attaining higher power factors by band engineering and synergistic reduction of lattice thermal conductivity. The stable superionic polymorph co-existing in an anionic solid solution of Mg₃(Sb, Bi)₂ provides a renewed basis for understanding the crystal structure and its transformation in CaAl₂Si₂-type Zintl compounds.