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

Off-centering phenomenon manifests as locally distorted configurations with broken symmetry in a crystal structure due to displacement of constituent atoms from their ideal coordination centers within the lattice. Defying the anticipated formation of anionic solid solutions of Mg3(Sb1−2xBixGex)2, herein we report Mg3(Sb, Bi)2 based monoclinic structure formation (Space Group - C2/m, 12) with off centering of dominant trigonal α-Mg3(Sb, Bi)2 phase and segregation of nanophase Mg3Ge upon equiatomic (Bi, Ge) alloying. A discordant nature of Ge is unveiled within the layered α-Mg3(Sb, Bi)2 structure and assessed employing (3+1) dimensional superspace to reveal an off-centering (dz) along the z direction for all the constituent atoms in the range of 0 – 0.2 Å at 300 K. The (Bi, Ge) alloying results in favourably tuning of the p-type conduction for attaining higher power factors by band engineering and synergistic reduction of lattice thermal conductivity. The stable monoclinic 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 as CaAl2Si2-type Zintl compounds.