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Melilite‐like modulation and temperature‐dependent evolution in the framework structure of K 2 Sc[Si 2 O 6 ]F
Author(s) -
Hejny Clivia,
Kahlenberg Volker,
Eberhard Tim,
Krüger Hannes
Publication year - 2016
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520615024865
Subject(s) - melilite , modulation (music) , materials science , physics , acoustics , metallurgy , spinel
The crystal structure of synthetic K 2 Sc[Si 2 O 6 ]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 P 4 2 / mnm (α,α,0)000 s (−α,α,0)0000 [ a = 8.9878 (1), c = 8.2694 (2) Å, V = 668.01 (2) Å 3 ] with modulation wavevectors q 1 = 0.2982 (4)( a* + b* ) and q 2 = 0.2982 (4)(− a* + b* ). Structure refinement taking into account the modulation of positional and ADP parameters for all atoms from 3074 observed main hkl 00 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 [ScO 4 F 2 ] octahedra, [Si 4 O 12 ] 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 K 2 Sc[Si 2 O 6 ]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 P 4 2 / mnm , is identical to the room‐temperature average structure of K 2 Sc[Si 2 O 6 ]F.