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

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,I = 153 K is centrosymmetric with superspace group Pnam(σ₁00)00s and σ₁ = 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 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.