Single crystal structure investigation of twinned NaKSi2O5 - a novel single layer silicate

A new synthetic crystalline mixed alkali disilicate of composition NaKSi2O5 has been prepared by re-crystallization of a glass of corresponding compos ition. The compound is monoclinic, space group P2(1)/n (a = 7.3005(8) Angst rom, b = 17.389(2) Angstrom, c = 12.353(1) Angstrom, beta = 91.14(1)degrees , V = 1567.9 Angstrom (3), Z = 12, D-calc = 2.52 g cm(-3)). The crystal use d for the data collection showed twinning by pseudo-merohedry according to 2([001]), a feature we took account of in the refinements. The structure wa s solved by direct methods and refined to a residual of R-1 = 0.070 (170 pa rameters). The compound belongs to the group of tetrahedral single layer si licates. Individual sheets are parallel to (001). The stepped layers can be described as being built by the condensation of unbranched dreier double c hains. The double chains in turn consist of two unbranched dreier single ch ains connected via common corners and exclusively containing tertiary (Q(3) ) tetrahedra. The stacking of the layers consisting of four-, six- and eigh t-membered rings results in a three-dimensional structure in which the alka li cations reside in the voids between neighboring sheets. The sodium and p otassium atoms show an ordered distribution among the six symmetrically ind ependent alkali sites. The coordination numbers for Na and K vary between 4 to 5 and 6 to 7, respectively. (C) 2001 Editions scientifiques et medicale s Elsevier SAS. All rights reserved.