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| Home > Publications database > Ferroelectricity of Phenazine--Chloranilic Acid at T = 100 K |
| Journal Article | PUBDB-2014-04193 |
; ; ; ; ;
2014
Springer Science + Business Media B.V
Dordrecht [u.a.]
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Please use a persistent id in citations: doi:10.1007/s10870-014-0527-1
Abstract: The co-crystal of phenazine (Phz) and chloroanilic acid (H2ca) is ferroelectric below the temperature TIc=253 K (FE-I phase). Upon cooling, two more phase transitions involve a further reduction of symmetry, until Phz-H 2 ca is triclinic in the second ferroelectric phase (FE-II phase) stable below TIIc= 137 K. Ferroelectricity in all low-temperature phases is believed to be related to partial proton transfer within the hydrogen bonds between the molecules Phz and H2ca . Here we present the crystal structure of the FE-II phase at T=100 K. Experimental positions of hydrogen atoms indicate that ferroelectricity is mainly governed by half of the hydrogen-bonded chains, whereby proton transfer is observed within one of the two hydrogen bonds in which each molecule participates. A simple point charge model quantitatively reproduces the polarisation of this material. However, a possible contribution to the polarisation is proposed of the O–H ⋯ N hydrogen bonds of the second half of the mixed chains, which show elongated O–H bonds similar to those in the FE-I phase. The twofold superstructure with P1 symmetry was successfully solved as commensurately modulated structure employing the monoclinic superspace group P21(1/2σ21/2)0 . The latter shows that the distortions at low temperatures follow a single normal mode of the space group P21 of the FE-I phase, and it thus explains that the direction of the polarisation remains close to the monoclinic axis, despite the lowering towards triclinic symmetry
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