Incommensurately modulated structure of morpholinium tetrafluoroborate and configurational versus chemical entropies at the incommensurate and lock‐in phase transitions

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Structural science, crystal engineering and materials, 2017-10, Vol.73 (5), p.836-843
• Main Authors: Noohinejad, Leila, van Smaalen, Sander, Petříček, Václav, Schönleber, Andreas
• Format: Article
• Language: English
• Subjects: Anions; Cations; configurational entropy; Entropy; Ferroelectric materials; incommensurately modulated structure; Low temperature; morpholinium tetrafluoroborate; Order disorder; Phase transitions
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520617009398

Morpholinium tetrafluoroborate, [C4H10NO]+[BF4]−, belongs to a class of ferroelectric compounds ABX4. However, [C4H10NO]+[BF4]− does not develop ferroelectric properties because the incommensurate phase below Tc,I = 153 K is centrosymmetric with superspace group Pnam(σ100)00s and σ1 = 0.42193 (12) at T = 130 K; the threefold superstructure below Tc,II = 117–118 K possesses the acentric but non‐ferroelectric space group P212121. 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. The incommensurately modulated crystal structure of [C4H10NO]+[BF4]− involves partial ordering of [BF4]− along with conformational changes of the morpholinium cation. Both features are essential for the phase transition. A pure order–disorder transition is excluded on the basis of computations of the configurational entropy of the incommensurate structure.