Crystal structure and characterization of the novel image hydrogen bonded polar crystal image

Dielectric properties and phase transitions of the piperazinium tetrafluoroborate ([NH2(CH2)4NH][BF4][NH2(CH2)4NH][BF4], abbreviated as PFB) crystal are related to the one-dimensional arrangement of the cations linked by the bistable NH+⋯NNH+⋯N hydrogen bonds and molecular motions of the [BF4]−[BF4]− units. The crystal structure of [NH2(CH2)4NH][BF4][NH2(CH2)4NH][BF4] is monoclinic at room temperature with the polar space group Pn . The polar/acentric properties of the room temperature phase IV have been confirmed by the piezoelectric and pyroelectric measurements. DSC measurements show that the compound undergoes three first-order structural phase transitions: at 421/411 K (heating/cooling), at 386/372 K and at 364/349 K. 1H and 19F NMR measurements indicate the reorientational motions of [BF4]−[BF4]− anions and piperazinium(+) cations as well as the proton motion in the hydrogen-bonded chains of piperazine along the [001] direction. Over the phase I the isotropic reorientational motions or even self-diffusion of the cations and anions are expected. The conductivity measurements in the vicinity of the II–I PT indicate a superionic phase over the phase I.