Structural implications of anomalous thermal expansion and glass-like dielectric response in pyridinium halogenoaurates.

The crystals of [C(5)NH(6)](+)[AuCl(4)](-), [C(5)NH(6)](+)[AuBr(4)](-), and [C(5)NH(6)](+)[AuI(4)](-) have been studied by single-crystal X-ray diffraction and dielectric spectroscopy. The structures of the chloride and bromide are isosymmetric, with the monoclinic space group C2/m, and both are built of sheets of pyridinium cations and tetrahalogenoaurate anions alternately arranged in the direction [001]. The anomalous thermal expansion and dielectric response characteristic of dipolar glass formation observed in these compounds have been interpreted in terms of the development of the short-range dipolar order and the competition between the ferroelectric and antiferroelectric interactions. The frustration of dipolar interactions, leading to glassy behavior, results from the strongly anisotropic properties of the layered crystal structure. These features have not been observed for [C(5)NH(6)](+)[AuI(4)](-), which crystallizes in the space group P2(1)/c and exhibits a nonlayered structural packing because of the larger size of the anion.