Hydrogen-bonded organo-amino phosphonium halides: dielectric, piezoelectric and possible ferroelectric properties.

Molecular ferroelectric materials are an exciting class of materials for potential applications in energy and electronics. Herein, we report examples of hydrogen-bonded binary salts of diphenyl diisopropylamino phosphonium halides [Ph2(iPrNH)2P]·X [DPDP·X, X = Cl, Br, I] which show dielectric, piezoelectric and NLO properties and some potentially ferroelectric attributes at room temperature. The phosphonium bromide salt was prepared by bromination of the phosphine precursor Ph2PCl and its subsequent treatment with isopropyl amine. The chloride and iodide salts were synthesized by the halogen exchange reaction of the bromide salt. The variable temperature single crystal X-ray analysis indicates the retention of the polar non-centrosymmetric phase of these materials for a wide range of temperatures from 100 to 400 K and above. All these assemblies were shown to exhibit 1D H-bonded chain structures along the crystallographic b-axis. The P-E loop measurements of these salts gave curves similar to those of non-linear leaky dielectric materials. However, the vertical piezoresponse force microscopy (V-PFM) analyses showed the existence of polarizable domain inversions indicating the possibility of ferroelectric behaviour in these materials. The temperature dependent dielectric measurements on these salts support the absence of phase transition temperatures in these assemblies. Also, bias-dependent PFM studies reveal their piezoelectric nature as the obtained converse piezoelectric coefficients are consistent with the d33 values obtained by the direct quasi-static methods.

Hierarchical Frameworks of Metal-Organic Cages with Axial Ferroelectric Anisotropy.

Designing molecular crystals with switchable dipoles for ferroelectric applications is challenging and often serendipitous. Herein, we show a systematic approach toward hierarchical 1D, 2D and 3D frameworks that are assembled through successive linkage of metal-organic cages [Cu6 (H2 O)12 (TPTA)8 ]12+ with chloride ions. Their ferroelectric properties are due to the displacement of channel-bound nitrate counterions and solvated water molecules relative to the framework of cages. Ferroelectric measurements of crystals of discrete and 1D-framework assemblies showed axial ferroelectric anisotropy with high remnant polarisation. Both, the reversible formation of cage-connected networks and the observation of ferroelectric anisotropic behaviour are rare among metal-ligand cage assemblies.

A Flexible Composite Mechanical Energy Harvester from a Ferroelectric Organoamino Phosphonium Salt.

A new binary organic salt diphenyl diisopropylamino phosphonium hexaflurophosphate (DPDP⋅PF6 ) was shown to exhibit a good ferroelectric response and employed for mechanical energy harvesting application. The phosphonium salt crystallizes in the monoclinic noncentrosymmetric space group Cc and exhibits an H-bonded 1D chain structure due to N-H⋅⋅⋅F interactions. Ferroelectric measurements on the single crystals of DPDP⋅PF6 gave a well-saturated rectangular hysteresis loop with a remnant (Pr ) polarization value of 6 µC cm-2 . Further, composite devices based on polydimethylsiloxane (PDMS) films for various weight percentages (3, 5, 7, 10 and 20 wt %) of DPDP⋅PF6 were prepared and examined for power generation by using an impact test setup. A maximum output peak-to-peak voltage (VPP ) of 8.5 V and an output peak-to-peak current (IPP ) of 0.5 µA was obtained for the non-poled composite film with 10 wt % of DPDP⋅PF6 . These results show the efficacy of organic ferroelectric substances as potential micropower generators.