Polar and antiferroelectric behaviour of a hybrid crystal – piperazinium perchlorate

Abstract

Monoprotonated piperazinium perchlorate, ijNH 2 IJCH 2 ) 4 NH]ijClO 4 ], appeared to be a novel room temperature polar material (P1). Its acentric symmetry was confirmed by single-crystal X-ray diffraction, second harmonic generation (SHG) and pyroelectric measurements. Differential scanning calorimetry (DSC) measurements revealed a complex sequence of phase transitions above room temperature: I ↔ II at 433/422 K (heating-cooling), II ↔ III at 417/411 K, III ↔ IV at 403/395 K and IV ↔ V at 397 K (the lowest temperature phase transition recorded only upon heating). The characteristic feature of the structure of ijNH 2 IJCH 2 ) 4 NH]ijClO 4 ] is the presence of two parallel cationic chains which are connected with each other by strong N-H⋯N hydrogen bonds. In phase V, these strongly polar non-equivalent chains contribute to spontaneous polarization. 1 H NMR measurements disclosed the reorientational motions of the piperazinium IJ[NH 2 IJCH 2 )NH] + ) cations as well as the proton motion in the N-H⋯N hydrogen bonds along the piperazine chain. Over phase I, the overall motions of the ClO 4 − anions and reorientational motion of cations are postulated. The dielectric response, ε′IJT), accompanying the PT I ↔ II indicates possible antiferroelectricity in phase II.

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“…In turn, a surprising dielectric response (between phases III and IV ) is observed along [001] direction (Figure c). The dielectric response accompanying the last transformation seems to be unexpected, especially if one takes into account the structural requirements and/or optical (SHG) results that exclude the appearance of ferro- or antiferroelectric properties. − …”

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