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首页> 外文期刊>Acta Crystallographica, Section B. Structural science, crystal engineering and materials >Low-temperature behaviour of K_2Sc[Si_2O_6]F: determination of the lock-in phase and its relationships with fresnoite- and melilite-type compounds
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Low-temperature behaviour of K_2Sc[Si_2O_6]F: determination of the lock-in phase and its relationships with fresnoite- and melilite-type compounds

机译：K_2SC [Si_2O_6] F的低温行为：锁定相位的测定及其与弗雷特伯特和微金属型化合物的关系

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摘要

K_2Sc[Si_2O_6]F exhibits, at room temperature, a (3 + 2)-dimensional incommensurately modulated structure [a = 8.9878 (1), c = 8.2694 (2) ? , V = 668.01 (2) ?~3; superspace group P4_2/mnm(α,α,0)000s(-α,α,0)0000] with modulation wavevectors q_1 = 0.2982 (4)(a* + b*) and q_2 = 0.2982 (4)(-a* + b*). Its lowtemperature behaviour has been studied by single-crystal X-ray diffraction. Down to 45 K, the irrational component α of the modulation wavevectors is quite constant varying from 0.2982 (4) (RT), through 0.2955 (8) (120 K), 0.297 (1) (90 K), 0.298 (1) (75 K), to 0.299 (1) (45 K). At 25 K it approaches the commensurate value of one-third [i.e. 0.332 (3)]: thus indicating that the incommensurate-commensurate phase transition takes place between 45 K and 25 K. The commensurate lock-in phase of K_2Sc[Si_2O_6]F has been solved and refined with a 3 × 3 × 1 supercell compared with the tetragonal incommensurately modulated structure stable at room temperature. This corresponds to a 3 × 1 × 3 supercell in the pseudo-orthorhombic monoclinic setting of the lowtemperature structure, space group P2/m, with lattice parameters a = 26.786 (3), b = 8.245 (2) c = 26.824 (3) ? , β = 90.00 (1)°. The structure is a mixed tetrahedral-octahedral framework composed of chains of [ScO_4F_2] octahedra that are interconnected by [Si_4O_(12)] rings with K atoms in fourfold to ninefold coordination. Distorted [ScO_4F_2] octahedra are connected to distorted Si tetrahedra to form octagonal arrangements closely resembling those observed in the incommensurate structure of fresnoite- and melilite-type compounds.
机译：k_2sc [si_2O_6] f在室温下展示（3 + 2） - 二维不计均否的结构[a = 8.9878（1），c = 8.2694（2）？ ，v = 668.01（2）？〜3; Superspace组P4_2 / MNM（α，α，0）0000s（-α，α，0）0000]，调制波浪Q_1 = 0.2982（4）（A * + B *）和Q_2 = 0.2982（4）（ - a * + B *）。通过单晶X射线衍射研究了其低温行为。降至45 k，调制波浪的非理性分量α从0.2982（4）（4）（4）（4）（120k），0.297（1）（90 k），0.298（1）（ 75 k），至0.299（1）（45 k）。在25 k处，它接近三分之一的相称值[即因此，表明，在45 k和25k之间发生了联合致密的相变相转变。[Si_2O_6] F的相称锁定相位已经解决并用3×3×1超级细胞进行了解决和精制通过四边形的不膨胀地调节结构在室温下稳定。这对应于低温结构的伪正式晶晶型的3×1×3超级胶片，空间组P2 / m，晶格参数A = 26.786（3），B = 8.245（2）C = 26.824（3）还，β= 90.00（1）°。该结构是由[SCO_4F_2]八面体的链组成的混合四面体 - 八面体框架，其由k原子与四倍与九倍配位相互联系的[Si_4O_（12）]环与K原子相互连接。扭曲的[sco_4f_2]八面体连接到扭曲的Si tetrahedra以形成八角形布置，与在弗雷特诺钛矿和微金属型化合物的中非含量结构中观察到的八边形布置。

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