Kahlenberg, V.; Manninger, T.; Perfler, L.; Többens, D. M.: One-pot occurrence of two polymorphs of Rb2Sc[Si4O10]F and their structural, spectroscopic and computational characterization. Journal of Solid State Chemistry 220 (2014), p. 79-90
10.1016/j.jssc.2014.08.002

Abstract:
Single-crystal growth experiments in the system RbF-Sc2O3-SiO2 resulted in the simultaneous crystallization of two polymorphs of Rb2ScSi4O10F within the same run. Basic crystallographic data of the two forms at ambient conditions are as follows: phase I: space group I 4/m, a = 11.2619(3)Å, c = 8.3053(4)Å, V = 1053.36(6)Å3, Z = 4; phase II: space group P 21/m, a = 11.5085(6)Å, b = 8.3805(3)Å, c = 11.6750(6)Å, β = 111.147(6)°, V = 1050.19(9)Å3, Z = 4. The structures were determined by direct methods and refined to residuals of R(|F|) = 0.0231 (phase I) for 516 and 0.0249 (phase II) for 2050 independent observed reflections with I > 2σ(I). Phase I showed twinning by merohedry which was accounted for during the refinement process. Both compounds are based on multiple chains of [SiO4]-tetrahedra forming closed columns or tubes. The multiplicity of the unbranched fundamental chains is two (for phase I) and four (for phase II), respectively. The periodicity of the multiple chains has a value of four in both modifications which is also reflected in similar translation periods along the chain directions (phase I: t[001] = 8.30Å; phase II: t[010] = 8.38Å). Alternatively, both polymorphs can be described as mixed octahedral-tetrahedral frameworks, which can be classified according to their polyhedral microensembles. A topological analysis of both networks is presented. Structural investigations have been supplemented by micro-Raman spectroscopy. The interpretation of the spectroscopic data, including the allocation of the bands to certain vibrational species as well as considerations concerning the stabilities of both modifications, have been aided by DFT-calculations.