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首页> 外文期刊>Acta crystallographica. Section F, Structural biology communications >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, [C4H10NO](+)[BF4](-), belongs to a class of ferroelectric compounds ABX(4). However, [C4H10NO](+)[BF4](-) does not develop ferroelectric properties because the incommensurate phase below T-c,T-I = 153 K is centrosymmetric with superspace group Pnam(sigma(1)00)00s and sigma(1) = 0.42193 (12) at T = 130 K; the threefold superstructure below T-c,T-II = 117-118 K possesses the acentric but non-ferroelectric space group P2(1)2(1)2(1). At ambient conditions, [C4H10NO](+)[BF4](-) comprises orientationally disordered [BF4](-) anions accommodated in cavities between four morpholinium cations. A structure model for the incommensurately modulated phase, which involves modulated orientational ordering of [BF4](-) 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 [BF4](-) 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：[C4H10NO]（+）[BF4]（ - ），属于一类铁电化合物ABX（4）。然而，[C4H10NO]（+）[BF4]（ - ）不产生铁电性能，因为在TC下链= 153k，Ti = 153k是用超空白组Pnam的邻摩托（Sigma（1）00）00s和Sigma（1）= 0.42193（12）在t = 130 k处;低于T-C，T-II = 117-118k的三倍上层建筑具有锐焦，但非铁电空间组P2（1）2（1）2（1）。在环境条件下，[C4H10NO]（+）[BF4]（ - ）包括定义无序的[BF4]（ - ）阴离子，其容纳在四个吗啉阳离子之间的空腔中。据报道，涉及涉及[BF4]（ - ）的调节定义排序与调节扭曲和大胆碱离子的转移的结构模型。提出了一种用于相转变的机制，由此在低温下，大卟啉阳离子在四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，并因此迫使[BF4]（ - ）进入该取向。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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