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

0479715 - FZÚ 2018 RIV GB eng J - Journal Article
Noohinejad, L. - van Smaalen, S. - Petříček, Václav - Schönleber, A.
Incommensurately modulated structure of morpholinium tetrafluoroborate and configurational versus chemical entropies at the incommensurate and lock-in phase transitions.
Acta Crystallographica Section B-Structural Science Crystal Engineering and Materials. Roč. 73, Jun (2017), s. 836-843. E-ISSN 2052-5206
R&D Projects: GA ČR(CZ) GA15-12653S
Institutional support: RVO:68378271
Keywords : incommensurately modulated structure * morpholinium tetrafluoroborate * configurational entropy
OECD category: Condensed matter physics (including formerly solid state physics, supercond.)
Impact factor: 6.467, year: 2017

Morpholinium tetrafluoroborate belongs to a class of ferroelectric compounds ABX4. However, it does not develop ferroelectric properties because the incommensurate phase below Tc,I = 153 K is centrosymmetric, the threefold superstructure below Tc,II = 117–118 K possesses the acentric but non-ferroelectric space group P212121. A structure model for the incommensurately modulated phase 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] anions into this orientation.
Permanent Link: http://hdl.handle.net/11104/0275716