Structural features. To answer these questions, a successful isovalent substitution from Ag8SiSe6 to Ag8GeSe6 and Ag8SnSe6 is achieved as shown by the linear increase of the lattice volume (Supplemental Note 1, 2). These materials are superionic conductors and have been intensely investigated for their thermoelectric transport properties due to the low thermal conductivity. The different compositions have varying room-temperature structures, but ultimately undergo a phase transition into the same cubic structure that is characterized by its strongly disordered Ag + sublattice (Figure a and Supplemental Note 2). Structurally, there are a large number of Ag + sites that are tetrahedrally coordinated and have face-sharing connectivity. These sites have an average occupation of only 25 %. While the low-temperature phase of Ag8SiSe6 is cubic with an ordered Ag + sublattice, both Ag8GeSe6 and Ag8SnSe6 crystallize in an orthorhombic structure at room-temperature (Supplemental Note 2). In the orthorhombic phase, the Ag + sublattice consists of five distinct lattice sites, each either tetrahedrally or trigonally coordinated by Se 2-arranged in a corner-or edge-sharing fashion, respectively. Near the phase transition, the lattice volume increases strongly in a small temperature window before it settles into a volumetric thermal expansion coefficient ranging from 7.7 to 8.7 × 10 -5 K -1 , with an average of 8.2 ± 0.5 × 10 -5 K -1 for all compositions (Supplemental Note 2, 3).