Crystal structure, phase transition, electrical and optical properties of p-hydroxypyridinium iodate [C5H6NO (IO3)

Abstract

The structural investigation of a new hybrid iodate compound [C5H6NO (IO3)]2 denoted as HPI was performed using single crystal and powder X-ray diffraction. The synthesized compound was also characterized using DSC, FT-IR, 13C Mass/NMR, diffuse reflectance (DRS), fluorescence and complex impedance spectroscopy (CIS). The new hybrid iodate salt crystallizes in the monoclinic system and in the acentric space group “Pn”. DSC and VT-XRPD measurements revealed that HPI undergoes a first order phase transition at 383 K showing a discontinuous increase of the molar volume and entropy. The temperature variation of AC and bulk conductivity plots of HPI showed two straight-line regions with a change in slope around 380 K corresponding to the structural phase transition. The bulk resistance, deduced from the thermal variation of Z′ and Z″ in both phases is found to decrease with rise in temperature, suggesting a negative temperature coefficient of resistance (NTCR) behavior of this compound. The electrical σtot (ω, T) conductivity obeyed to the Jonscher's power law and the temperature dependence of the S (T) parameter showed that the electrical conductivity of the HPI compound may be treated using correlated barrier hopping (CBH) model. The hopping of the proton H+ as a charge carrier may take place through the NH⋯O and OH⋯O hydrogen bonds.