Structural modulation in K2V3O8

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

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 group P42/mnm(α,α,0)000s(-α,α,0)0000 [a = 8.9878 (1), c = 8.2694 (2) Å, V = 668.01 (2) Å(3)] with modulation wavevectors q1 = 0.2982 (4)(a* + b*) and q2 = 0.2982 (4)(-a* + b*). Structure refinement taking into account the modulation of positional and ADP parameters for all atoms from 3074 observed main hkl00 and satellite reflections hklmn of first order with single, m·n = 0, and mixed, m·n = ±1, indices converged to a final R value 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 group P42/mnm, is identical to the room-temperature average structure of K2Sc[Si2O6]F.