Synthesis and Structure Determination of AgScP 2 Se 6 , AgErP 2 Se 6 and AgTmP 2 Se 6

AgScP 2 Se 6 , AgErP 2 Se 6 and AgTmP 2 Se 6 were obtained by high temperature reaction of stoichiometric amounts of the elements in evacuated silica ampoules. They crystallize in the space group P $/bar{3}$ 1 c , Z = 2 with the lattice constants a = 6.463(1) Å, c = 13.349(1) Å (AgScP 2 Se 6 ), a = 6.578(1) Å, c = 13.410(2) Å (AgErP 2 Se 6 ), and a = 6.567(1) Å, c = 13.422(1) Å (AgTmP 2 Se 6 ). The crystal structures of AgScP 2 Se 6 and AgErP 2 Se 6 were refined from merohedrally twinned crystals. No twinning was observed for AgTmP 2 Se 6 . The crystal structures are isotypic and can be described as a variant of the CdI 2 structure type. Thus, selenium atoms are hexagonally close packed. The octahedral voids of every second layer are occupied by Ag, M 3+ ( M = Sc, Er, Tm), and P 2 dumbbells in an ordered fashion. The crystal structures are isotypic to AgScP 2 S 6 and AgInP 2 S 6 .