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

Leila Noohinejad, Sander van Smaalen, Václav Petříček, Andreas Schönleber
  • Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, September 2017, International Union of Crystallography
  • DOI: 10.1107/s2052520617009398

Modulated structure of morpholinium tetrafluoroborate and its phase transitions

What is it about?

Morpholinium tetrafluoroborate, C_(4)H_(10)NO^(+) BF_(4)^(-) belongs to a class of ferroelectric compounds ABX_(4). However, it does not develop ferroelectric properties because the incommensurate phase below T_(c,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,II) = 117-118 K possesses the acentric but non-ferroelectric space group P2_81)2_81)2_(1). At ambient conditions, C_(4)H_(10)NO^(+) BF_(4)^(-) comprises orientationally disordered BF_(4)^(-) anions accommodated in cavities between four morpholinium cations.

Why is it important?

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 earlier. 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.

Perspectives

Priv.-Doz. Dr. Andreas Schönleber
Universitat Bayreuth

The incommensurately modulated crystal structure of C_(4)H_(10)NO^(+) BF_(4)^(-) involves partial ordering of BF_(4)^(−) 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.

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http://dx.doi.org/10.1107/s2052520617009398

The following have contributed to this page: Sander van Smaalen and Priv.-Doz. Dr. Andreas Schönleber