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首页> 外文期刊>Acta Crystallographica, Section B. Structural science, crystal engineering and materials >Incommensurately modulated structure of morpholinium tetrafluoroborate and configurational versus chemical entropies at the incommensurate and lock-in phase transitions
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Incommensurately modulated structure of morpholinium tetrafluoroborate and configurational versus chemical entropies at the incommensurate and lock-in phase transitions

机译：在不加密和锁定相转变的情况下，在不加氟硼酸盐和型化化学熵的情况下的调节结构

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摘要

Morpholinium tetrafluoroborate, [C_4H_(10)NO]~+[BF_4]~-, belongs to a class of ferroelectric compounds ABX_4. However, [C_4H_(10)NO]~+[BF_4]~- does not develop ferroelectric properties because the incommensurate phase below T_(c,I) = 153 K is centrosymmetric with superspace group Pnam(σ_100)00s and σ_1 = 0.42193 (12) at T = 130 K; the threefold superstructure below T_(c,II) = 117-118 K possesses the acentric but non-ferroelectric space group P2_12_12_1. At ambient conditions, [C_4H_(10)NO]~+[BF_4]~- comprises orientationally disordered [BF_4]~- anions accommodated in cavities between four morpholinium cations. A structure model for the incommensurately modulated phase, which involves modulated orientational ordering of [BF_4]~- together with modulated distortions and displacements of the morpholinium ions is reported. A mechanism is proposed for the phase transitions, whereby at low temperatures morpholinium cations are shaped around the tetrafluoroborate anion in order to optimize the interactions with one orientation of this anion and, thus, forcing [BF_4]~- into this orientation. This mechanism is essentially different from a pure order-disorder phase transition. It is supported by consideration of the transition entropy. The difference in configurational entropy between the disordered and incommensurate phases has been computed from the structure models. It is shown to be much smaller than the experimental transition entropy reported by Owczarek et al. [Chem. Phys. (2011), 381, 11-20]. These features show that the order-disorder contribution is only a minor contribution to the transition entropy and that other factors, such as conformational changes, play a larger role in the phase transitions.
机译：吗啉氨基丙酸，[C_4H_（10）否]〜+ [BF_4]〜 - ，属于一类铁电化合物ABX_4。然而，[C_4H_（10）否]〜+ [BF_4]〜 - 不产生铁电特性，因为下面是T_（C，I）= 153k的中非晶相是带有超对对称的超对称PSM（σ_100）00s和σ_1= 0.42193（ 12）在t = 130 k处;以下T_（C，II）= 117-118k的三倍上层建筑具有锐焦但非铁电空间组P2_12_12_1。在环境条件下，[C_4H_（10）否]〜+ [BF_4]〜 - 包括定义无序的[BF_4]〜 - 容纳在四个吗啉个阳离子之间的空腔中的阴离子。报道了涉及[BF_4]〜 - 与调节扭曲和大胆碱离子的调节失真一起调节定向阶段的结构模型。提出了一种用于相转变的机制，由此在低温下，在低氟硼酸盐阴离子周围成形，以优化与这种阴离子的一种取向的相互作用，从而迫使[BF_4]〜 - 进入该方向。这种机制与纯秩序紊乱相转变基本上不同。通过考虑过渡熵，支持它。已经从结构模型计算了无序和联交相之间的配置熵之间的差异。显示比OWCZAREK等人报告的实验过渡熵远小得多。 [化学。物理。 （2011），381,11-20]。这些特征表明，订单障碍贡献只是对转型熵的微小贡献，以及其他因素，如构象变化，在相变中发挥更大的作用。

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