Melilite-like modulation and temperature-dependent evolution in the framework structure of K2Sc[Si2O6]F

Abstract

The crystal structure of synthetic K2Sc[Si2O6]F has been solved and refined as an incommensurately modulated structure in (3 + 2)-dimensional superspace. This paper describes the tetragonal structure in the superspace groupP42/mnm(α,α,0)000s(−α,α,0)0000 [a= 8.9878 (1),c= 8.2694 (2) Å,V= 668.01 (2) Å3] with modulation wavevectorsq1= 0.2982 (4)(a* + b*) andq2= 0.2982 (4)(−a* + b*). Structure refinement taking into account the modulation of positional and ADP parameters for all atoms from 3074 observed mainhkl00 and satellite reflectionshklmnof first order with single,m·n= 0, and mixed,m·n= ±1, indices converged to a finalRvalue of 0.0514. The structure is a mixed octahedral–tetrahedral framework composed of [ScO4F2] octahedra, [Si4O12] rings and K in variable coordination. Due to the modulation the O atoms move into and out of the first coordination sphere of K leading to a minimum of five and a maximum of 10 interatomic K—O distances up to 3.1 Å. Although this feature is comparable to observations in modulated fresnoite and melilite group compounds, these structures differ from K2Sc[Si2O6]F with respect to their topology. On temperature increase the intensity of the satellite reflections decreases until they disappear just above 443 K. The high-temperature normal structure, in space groupP42/mnm, is identical to the room-temperature average structure of K2Sc[Si2O6]F.