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Issue 20, 2012
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Reverse sequence of transitions in prototypic relaxor 1,4-diazabicyclo[2.2.2]octane

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Abstract

Two phase transitions consecutively reduce the crystal symmetry with rising temperature in 1,4-diazabicyclo[2.2.2]octane hydrobromide, [C6H13N2]+·Br, dabcoHBr. Low-temperature prototypic hexagonal phase III, space group P[6 with combining macron]m2, Z = 1 above 458 K transforms into orientational states of orthorhombic phase II, space group Cmc21, Z = 4 and above 471 K to orthorhombic phase I, space group Pca21, Z = 8. The reverse transitions have been attributed to entropy and enthalpy contributions of the proton disordering, disproportionation of dabcoH+ cations into neutral dabco molecules and dabcoH2+2 dications at nanodomain walls, opposite propeller versus planar dabco conformations and conversions between NH+⋯N and NH+⋯Br hydrogen bonds in dabcoHBr phase III, the NH+⋯N bonded relaxor material. All of the structural-disorder features in the NH+⋯N bonded aggregates are essential for the short-range polarization and giant dielectric response in dabcoHBr phase III. Despite the first-order type of the phase transitions, the lattice dimensions of phases I and II remain related to the prototypical relaxor phase III.

Graphical abstract: Reverse sequence of transitions in prototypic relaxor 1,4-diazabicyclo[2.2.2]octane

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Publication details

The article was received on 13 Mar 2012, accepted on 13 Jun 2012 and first published on 14 Jun 2012


Article type: Paper
DOI: 10.1039/C2CE25359C
Citation: CrystEngComm, 2012,14, 6428-6434
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    Reverse sequence of transitions in prototypic relaxor 1,4-diazabicyclo[2.2.2]octane

    W. Nowicki, A. Olejniczak, M. Andrzejewski and A. Katrusiak, CrystEngComm, 2012, 14, 6428
    DOI: 10.1039/C2CE25359C

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