On the Initial Verification of the Crystal Structures of the Argyrodite Family

The crystal structures of the phases belonging to the scientifically and technologically important family of argyrodites of the general chemical formula Am+(12–n–x)/mBn+X2–6–xY–x (where A = Li+; Cu+, Ag+, Cd2+, Hg2+…; B = Ga3+, Si4+, Ge4+, Sn4+, P5+, As5+…; X = O2–; S2–, Se2–, Te2–; Y = Cl–, Br–, I–; 0 ≤ x ≤ 1) are characterized by high mobility and disorder of the cationic sublattice (substructure) A. This feature of argyrodites makes them promising solid-state ionic conductors, but significantly complicates the X-ray analysis of argyrodite phases due to the inability to reliably predict the positions of A ions in the unit cell of the crystal structure of a particular argyrodite sample and the site occupancy factors of these positions. The analysis of the published structures of argyrodites has revealed the fact that the positions of typical B cations and their X ligands in the tetrahedral environment [BX4] are fully occupied by the corresponding ions, which makes it possible to perform an initial verification of the structural models of the argyrodite phases (already published in the scientific literature or newly obtained) within the framework of the bond valence model (BVM) – by calculating the bond valence sums for the B cations from the interatomic distances dBX in the coordination tetrahedra [BX4]. In addition to the initial verification of structural models of argyrodite phases, the use of the BVM with the numerical parameters recommended in this work allows researchers to create reliable starting partial structural models of argyrodite phases for their further refinement by X-ray diffraction analysis, as well as to stabilize the process of refining the crystal structure by imposing soft restraints on the deviation of the calculated interatomic distances dBX from the predicted distances in the [BX4] tetrahedra.