M.Zdanowska-Fršczek(*), P.Czarnecki(**), M.Krupski(*), J.Jakubas(***)
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(*) Institute of Molecular Physics, Polish Academy of Sciences,
60-194 Poznań, Smoluchowskiego 17
(**) Phaculty of Physics, A.Mickiewicz University,
61-614 Poznań, Umultowska 85,
(***) Institute of Chemistry, Wrosław University,
50-385 Wrocław, Joliot-Curie 14
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Ferroelectric chloroacetates belong to a large class of strongly hydrogen bonded compounds containing anions of general formula [RCOOH...OOCR]- in which two chloroacetate radicals are kept together by very short hydrogen bonds. It was found that only those containing NH4+ and N(CH3)4+ cations and asymmetrical dimeric hydrogen bonds exhibit ferroelectric properties.The differences between ferroelectric phase and paraelectric one are reflected in some ammonium and tetramethylammonium bands as well as in changes in symmetry of the dimmeric unit.
The results of numerous experimental technique studies and the analysis of our previous high pressure NQR and NMR results indicate the importance of hydrogen bonds associated with the dimer anion in ferroelectric phase transition but molecular mechanism of the phase transition in chloroacetates is still unclear.
For better anderstanding of the mechanism of phase transition in chloroacetates we were studied dielectric behaviour of NH4H(ClH2CCOO)2 crystal under high hydrostatic pressure. The temperature anomalies in dielectric permittivity were studying by us under isobaric conditions up to 7.5*102 Mpa. The transition temperature decreases linearly with increasing pressure with pressure coefficient dTc/dp = -1.3*102 KMPa-1.
The pressure effect on Tc indicate that the mechanism of phase transition seems to be well described by means of a phenomenological explanation with a pseudo-spin -lattice coupled mode model.In interpretation of the results one can suggest that the phase transition in AHCA is a combined effect of structural changes and of proton disordering in hydrogen bonds.

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