Abstract
The halogenoantimonates(III) and halogenobismuthates(III) of the general formula RaMbX3b+a (where R denotes organic cations, X = Cl, Br, I and M = Sb, Bi) form a large group of ferroelectric crystals [1]. The title crystal [4-NH2C5H4NH][BiCl4] (abbreviation – 4APCB) is closely related to the recently found antimonate(III) analog [4-NH2C5H4NH][SbCl4] [2] , which reveals interesting ferroelectric properties with intermediate incommensurate phases. 4APCB undergoes at 252 K structural phase transition (PT) of first order. The room temperature structure (phase I) is monoclinic, space group Cc with a = 13.229, b = 13.505 c = 7.350 A and β = 120.31. The structure is composed of BiCl4 ions which form infinite chains through the crystals via chloride linkages. Each bismuth lies in a distorted octahedral environment. The 4-aminopyridinium cation reveals substantial disorder. The low temperature phase (II) is monoclinic, space group P21\c with a = 11.1660(7), b = 13.5857(8), c = 7.2750(4) A and β = 93.777(5). Over the phase II the organic cations are fully ordered. The structural analysis, calorimetric (DSC) and spectroscopic studies (IR) show that the PT is governed by the dynamics of the 4-aminopyridinium cations.
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