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.