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

Abstract

Abstract A new synthetic crystalline mixed alkali disilicate of composition NaKSi 2 O 5 has been prepared by re-crystallization of a glass of corresponding composition. The compound is monoclinic, space group P2 1 /n ( a =7.3005(8) A, b =17.389(2) A, c =12.353(1) A, β =91.14(1)°, V =1567.9 A 3 , Z =12, D calc =2.52 g cm −3 ). The crystal used for the data collection showed twinning by pseudo-merohedry according to 2 [001] , a feature we took account of in the refinements. The structure was solved by direct methods and refined to a residual of R 1 =0.070 (170 parameters). The compound belongs to the group of tetrahedral single layer silicates. Individual sheets are parallel to (001). The stepped layers can be described as being built by the condensation of unbranched dreier double chains. The double chains in turn consist of two unbranched dreier single chains connected via common corners and exclusively containing tertiary ( Q 3 ) tetrahedra. The stacking of the layers consisting of four-, six- and eight-membered rings results in a three-dimensional structure in which the alkali cations reside in the voids between neighboring sheets. The sodium and potassium atoms show an ordered distribution among the six symmetrically independent alkali sites. The coordination numbers for Na and K vary between 4 to 5 and 6 to 7, respectively.