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The crystal structure and dielectric characteristics were evaluated for Ba4Sm2Ti4Ta6O30 and Ba5SmTi3Ta7O30 ceramics. The X-ray diffraction patterns confirmed the tetragonal tungsten bronze structure with space group P4 bm. A normal ferroelectric phase transition was observed at 332 K for the former, while the typical relaxor ferroelectric behavior with a spin glass nature was determined for the latter, where a broad permittivity peak with a strong frequency dispersion was observed around 150 K, which followed the Vogel–Fulcher relationship well. These different dielectric behaviors originated from the different crystal structures governed by the occupancy configuration of cations at A1 and A2 sites. [ABSTRACT FROM AUTHOR]

Two new isostructural complexes [M(H2biim)3][M(btc)(Hbiim)]•2H2O (M = Co, (1); M = Ni, (2)) (btc = 1,3,5-benzenetricarboxylate; H2biim = 2,2′-biimidazole) have been synthesized and characterized by single-crystal X-ray diffraction. They present a unique structure consisting of two distinct units: the monomeric cations [M(H2biim)3]2+ and the two-dimensional (2D) anionic polymer [M(Hbiim)(btc)]2-. In the anionic moiety, the Hbiim monoanion is simultaneously coordinated to one metal atom in a bidentate mode and further to another metal atom in a monodentate mode. The imidazolate groups bridge the two adjacent metal ions into a helical chain which is further arranged in left- and right-handed manners. These chains are bridged by btc ligands into a 2D brick wall structure. The most interesting aspect is that the [M(H2biim)3]2+ cations act as pillars and link the anionic layers via robust heteromeric hydrogen-bonded synthons R²2(9) and R¹2(7) formed by the uncoordinated oxygen atoms of carboxylate groups and the H2biim ligands, resulting in a microporous metal-organic framework with one-dimensional (1D) channels (ca. 11.85 Å ⊗ 11.85 Å for 1 and 11.43 Å ⊗ 11.43 Å for 2). Magnetic properties of these two complexes have also been studied in the temperature range of 2–300 K, and their magnetic susceptibilities obey the Curie-Weiss law in the temperature range of 20–300 K (for 1) and 2–300 K (for 2), respectively, showing anti-ferromagnetic coupling through imidazolate bridging. Taking into consideration the Heisenberg infinite chain model as well as the possibility of chain-to-chain and chain-to-cation interactions, the anti-ferromagnetic exchange of 2 is analyzed via a correction for the molecular field, giving the values of gcat = 2.296, gNi = 2.564, J = -13.30 cm-1, and zJ′ = -0.017 cm-1. The microporous frameworks are stable at ca. 350 °C. They do not collapse after removal of the guest water molecules in the channels, and they adsorb methanol molecules selectively. [ABSTRACT FROM AUTHOR]