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Two different modifications of the composition NaTbSi 2 O 6  were synthesized within the class of quaternary alkali-metal rare-earth metal(III) oxosilicates. Monoclinic NaTbSi 2 O 6  (A-type structure: space group  P 2 1 /c (no. 14);  a  = 542.57(4),  b  = 1376.54(9),  c  = 762.83(5) pm,  β  = 110.086(3)°,  V m   = 80.557(9) cm 3 /mol,  Z  = 4) crystallizes isotypically with NaYSi 2 O 6  und LiScSi 2 O 6 . In this  ino -oxosilicate, the [SiO 4 ] 4–  tetrahedra are vertex-shared and linked to form infinite zigzag chains  ∞  1 {[SiO 2/2  v O 2/1  t ] 2– } along [001]. Both, the Na +  and the Tb 3+  cations are coordinated by seven O 2–  anions each. The [TbO 7 ] 11–  polyhedra get connected via  trans -oriented edges to build up  ∞  1 {[TbO 4/2  e O 3/1  t ] 7– } chains running along [001], which alternate with the  ∞  1 {[SiO 2/2  v O 2/1  t ] 2– } chains. In contrast, triclinic NaTbSi 2 O 6  with a novel D-type structure (space group:  P -1 (no. 2),  a  = 549.41(4),  b  = 938.65(6),  c  = 972.30(7) pm,  α  = 117.263(2),  β  = 97.239(2),  γ  = 99.676(2)°,  V m   = 85.798(9) cm 3 /mol,  Z  = 3) belongs to the  cyclo -oxosilicates according to Na 3 Tb 3 [Si 6 O 18 ] with  Z  = 1. The arrangement of the resulting corrugated six-membered [Si 6 O 18 ] 12–  rings of vertex-shared [SiO 4 ] 4–  tetrahedra is similar to a hexagonal closest packing parallel to (100). The two types of distorted [TbO 6 ] 9–  octahedra are linked along [010] via common edges to form undulated infinite  ∞  1 {[TbO 4/2  v O 2/1  t ] 5– } chains, which become alternatingly piled with the  cyclo -[Si 6 O 18 ] 12–  units and the incorporated Na +  cations along [010]. While the square-pyramidally coordinated (Na2) +  cations are situated between the isolated  cyclo -oxohexasilicate anions, the sixfold coordinated (Na1) +  cations can be found inside the rings. They are therefore surrounded by the six bridging O 2–  anions of the  cyclo -[Si 6 O 18 ] 12–  units in a  quasi -planar hexagonal coordination sphere.

The new oxosilicate NaTbSi 2 O 6 : A comparison of its A- and D-type structure