Single-crystal X-ray diffraction study of Cs2Er[Si6O14]F and Cs2 Er[Si4O10]F

Single-crystal growth experiments in the system CsF-Er2O3-SiO2 resulted in the simultaneous crystallization of two chemically related compounds within the same run: Cs2Er[Si6O14]F (phase I) and Cs2Er[Si4O10]F (phase II). They represent the first examples for cesium erbium silicates containing fluorine. Basic crystallographic data are - phase I: space group Cmca, a = 17.2556(6) angstrom, b = 24.6565(7) angstrom, c = 14.4735(5) angstrom, V = 6157.9(3) angstrom(3), Z = 16; phase II: space group Pnma, a = 22.3748(7) angstrom, b = 8.8390(2) angstrom, c = 11.9710(4) angstrom, V = 2367.5(1) angstrom(3), Z = 8. The structures were determined by direct methods and refined to residuals of R(vertical bar F vertical bar) = 0.0229 for 2920 (phase I) and 0.0231 for 2314 (phase II) independent observed reflections with I > 2 sigma(I). The structure of phase I represents a previously unknown structure type with a three dimensional tetrahedral framework consisting of Q(3) and Q(4) groups in the ratio 2:1. Basic building units of the network are unbranched sechser single-chains running parallel to [001]. The network can be conveniently built up from the condensation of tetrahedral layers parallel to (010) or (100), respectively. The crystal structure of phase II can be classified as a tubular or columnar chain silicate indicating that the backbones of the structure are multiple chains of silicate tetrahedra. This structure is isotypic to a Cs2Y[Si4O10]F, a compound that has been characterized previously. Alternatively, both compounds can be described as mixed octahedral-tetrahedral frameworks, which can be classified according to their polyhedral microensembles. A topological analysis of both nets is presented.