Dielectric properties induced by the framework in the hybrid organic–inorganic compounds M(dca)2pyz M = Fe, Co and Zn

Abstract

In this work we prepare and study the M(dca)2pyz compounds, where dca=polar dicyanamide N(CN)2− ligand, pyz=pyrazine (C4H4N2) ligand and M=Fe2+, Co2+ and Zn2+ that, according to DSC display a first order phase transition, at Tt∼270K (Fe), ∼250K (Co) and ∼240K (Zn). Very interestingly these compounds, which consist of a rather robust framework with incorporated polar dca ligands, and which does not host guest species in the cavities of the structure, display a dielectric anomaly associated to such phase transition. This latter in fact corresponds to the structural transition from the disordered HT-phase (space group Pnma) to the ordered LT-phase (space group P21/n). From a detailed analysis of the dipole distributions in each of these crystal structures we conclude that the polar dca ligands, which bridge two transition metal cations, display a cooperative electrical arrangement, in which the dipoles of each of the two interpenetrated networks show a ferroelectric (FE) arrangement; nevertheless, the relative antiparallel orientation of both networks finally results in net antiferroelectric (AFE) arrangements both in the LT- and HT-polymorphs. Very interestingly, the dynamic disorder of the dca ligands at Tt gives rise to the observed dielectric anomaly. These materials constitute, therefore, an example of hybrid organic–inorganic compounds where the dielectric response directly arises from the framework –and not from polar guest species– which is a very interesting and novel approach in the search for materials in which the dielectric and magnetic properties can couple more strongly.