On the H- and A-type structure of La-2[Si2O7]

By thermal decomposition of La3F3[Si3O9] at 700 degreesC in a CsCl flux sin gle crystals of a new form of La-2[Si2O7] have been found which is called H type (triclinic, P (1) over bar; a=681.13(4), b=686.64(4), c=1250.23(8)pm, alpha = 82.529(7), beta = 88.027(6) gamma = 88.959(6)degrees; V-m = 87.223 (9) cm(3)/ mel, D-x = 5.113(8) g/cm(3), Z = 4) continuing Felsche's nomencl ature. It crystallizes isotypically to the triclinic K-2[Cr2O7] in a struct ure closely related to that of A-La-2[Si2O7] (tetragonal, P4(1); a=683.83(7 ), c=2473.6(4)pm; V-m = 87.072(9) cm(3)/mol, D-x = 5.122(8) g/cm(3), Z = 8) . For comparison, the latter has been refined from single crystal data, too . Both the structures can be described as sequence of layers of each of two crystallographically different [Si2O7](6-) anions always built up of two c orner-linked [SiO4] tetrahedra in eclipsed conformation with non-linear Si- O-Si bridges (measured angle (Si-O-Si) = 128-132 degrees) piled up in [001] direction and aligned almost parallel to the c axis. They differ only in l ayer sequence: Whereas the double tetrahedra of the disilicate units are ti lted alternating to the left and in view direction ([010]; stacking sequenc e: AB) in H-La-2[Si2O7], after layer B there follow due to the 4(1) screw a xis layers with anions tilted to the right and tilted against view directio n ([010]; stacking sequence: ABA'B') in A-La-2[Si2O7]. The extremely irregu lar coordination polyhedra around each of the four crystallographically ind ependent La3+ cations in both forms (H and A type) consist of eight to ten oxygen atoms in spacing intervals of 239 to 330 pm. The possibility of more or less ordered intermediate forms will be discussed.