Synthesis and Crystal Structure of Na 2 Ba 9 Si 20 O 50 —An Intermediate Phase Along the Join Na 2 Si 2 O 5 ‐ Ba Si 2 O 5

Single crystals of Na 2 Ba 9 Si 20 O 50 were obtained from solid state reactions performed along the join Na 2 Si 2 O 5 ‐ BaSi 2 O 5 . The crystal structure has been determined from a data set collected at ambient temperatures and subsequently refined to a residual of R (|F|) = 0.0328 for 2211 independent reflections. The compound belongs to the group of phyllosilicates and adopts the monoclinic space group C 2/ m with the following lattice parameters: a  =   39.111(3) Å, b  =   7.6566(6) Å, c  =   8.2055(6) Å, β = 97.319(6)°, V  = 2437.2(3) Å 3 , Z  = 2. Furthermore, weak one‐dimensional diffuse streaks running parallel to a * as well as a very small number of low intensity reflections at b */3, indicating the presence of a superstructure, were observed. Basic buiding units are silicate layers parallel to (40‐1) which can be obtained from the condensation of single chains with a periodicity of four running along [010]. The sheets can be partitioned into two kinds of consecutive strips containing (i) a sequence of four‐ and eight‐membered rings and (ii) a four‐ring wide “zig‐zag shaped” unit consisting of exclusively six‐membered rings. The sodium and barium cations—distributed among six crystallographically independent positions—are sandwiched between subsequent layers and are linked to seven to nine nearest oxygen neighbors. The structure of Na 2 Ba 9 Si 20 O 50 is closely related to that of K 2 Ba 5 Si 12 O 30 and K 2 Ba 7 Si 16 O 40 , respectively. There are strong arguments that the previously claimed phase Na 4 Ba 8 Si 20 O 50 is actually misinterpreted Na 2 Ba 9 Si 20 O 50 and that the composition of the intermediate phase along the join Na 2 Si 2 O 5 – Ba Si 2 O 5 is slightly different from that described in the literature.