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ABSTRACT:The model approaches and quantum chemical calculations are employed to explain the peculiarities of the ferroelectric behavior of new "zerodimensional" H-bonded materials, i.e., 5-bromo and 5-iodo derivatives of 9-hydroxyphenalenone (9HPO) and its deuteroxy analogue (9DPO). The tunneling parameters ⍀ (H/D) and the Ising model coupling parameters J ij are evaluated and discussed. Analysis of these parameters for the hydroxy and deuteroxy species demonstrates the quantum paraelectric behavior of both Br and I 9HPO derivatives due to the large values of the ⍀(H)/J 0 relation, where J 0 is the molecular field parameter describing the coupling of any given H-bond proton with all rest ones. In contrast, small values ⍀(D)/J 0 for their 9DPO analogues favor the low-temperature structural phase transition into an ordered phase, which has a rather antiferroelectric than ferroelectric character. The estimates obtained and the resulting conclusions are in line with the overall observed trends in behavior of the substances under examination that emerge from the available experimental data.

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