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 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.

Low-temperature behaviour of K2Sc[Si2O6]F: determination of the lock-in phase and its relationships with fresnoite- and melilite-type compounds

“…At room temperature, the component of the modulation wavevectors was determined by the least-squares method from 206 satellites. The value, = 0.2983 (6), is in excellent agreement with that observed by Hejny et al (2016). All the reflections observed on collected frames ( indexed by five integers hklmn and with respect to the five-dimensional base h = ha* + kb* + lc* +mq 1 + nq 2 , where a*, b* and c* are the reciprocal axes of the basic structure (de Wolff, 1974).…”

“…A gem-quality crystal (60 Â 70 Â 90 mm) was chosen from the same batch of high-temperature synthesis used by Hejny et al (2016) for the structure solution of the room-temperature modulated and high-temperature normal structure. The crystal was mounted on an Oxford Diffraction Xcalibur 3 diffractometer, fitted with a Sapphire 2 CCD detector and equipped with a Helijet cooling device.…”

“…5a). The observed Sc-O and Sc-F distances range from 1.9450 (4) to 2.2252 (4) Å and 2.008 (1) to 2.123 (1) Å , respectively, and have a wider spread than the respective values in the IC structure at room temperature [Sc-O: 2.069 (2)-2.093 (2) Å ; Sc-F: 2.065 (1)-2.074 (1) Å ; Hejny et al, 2016], hence the octahedra in the low-temperature commensurate structure are more distorted. According to the octahedral angle variance (calculated according to Robinson et al, 1971) the Sc1, Sc4 and Sc7 octahedra show the greatest distortion.…”

“…A new (3 + 2)-dimensional incommensurately modulated (IC) structure has been recently described for K 2 Sc[Si 2 O 6 ]F in superspace group P4 2 /mnm(,,0)000s(À,,0)0000 with modulation wavevectors q 1 = 0.2982 (4)(a* + b*) and q 2 = 0.2982 (4)(Àa* + b*) (Hejny et al, 2016). In the structure, [ScO 4 F 2 ] octahedral chains and [Si 4 O 12 ] rings are interconnected to form a mixed octahedral-tetrahedral framework, where large open voids are filled with potassium in variable coordination ranging from five-to tenfold (Fig.…”

“…The lock-in phase was not observed in K 2 Sc[Si 2 O 6 ]F at temperatures as low as 173 K (Hejny et al, 2016). However, due to the resemblance of most of the other structural features between K 2 Sc[Si 2 O 6 ]F and the compounds with a melilite-type structure, it was suspected that the title compound might also transform to a commensurate superstructure at sufficiently low temperatures.…”