Investigations of heavy p-block elements (Sn, Sb, Bi) in 2-amino-5-methylpyridinium halogenometallate complexes: optical absorption and electrical properties

Abstract

The interaction between 2-amino-5-methylpyridinium and heavy p-block metal chlorides (Sn, Sb, and Bi) results in the formation of three hybrid compounds: (C₆H₉N₂)₂SnCl₆, (C₆H₉N₂)₃[BiCl₆], and (C₆H₉N₂)₂[Sb₂Cl₈]. Their structural, optical, and electrical properties were systematically examined by powder X-ray diffraction, UV–visible spectroscopy, and complex impedance spectroscopy in order to elucidate the effect of metal-center substitution on their physical behavior. Optical analysis reveals a gradual narrowing of the band gap from Sb- to Bi-containing compounds. The estimated band gap energies are 3.49 eV for (C₆H₉N₂)₂[Sb₂Cl₈], 3.36 eV for (C₆H₉N₂)₂SnCl₆, and 3.10 eV for (C₆H₉N₂)₃[BiCl₆], indicating enhanced electronic delocalization with increasing atomic number. Electrical measurements demonstrate a negative temperature coefficient of resistance (NTCR) for all samples in the temperature range 343–383 K, confirming their semiconducting character. The DC conductivity exhibits thermally activated behavior consistent with the Arrhenius model. The calculated activation energies are 0.71 eV (Sb-based), 0.54 eV (Sn-based), and 0.40 eV (Bi-based). Although the Bi-containing compound shows the lowest activation energy, the overall conductivity decreases in the order (C₆H₉N₂)₂[Sb₂Cl₈] > (C₆H₉N₂)₂SnCl₆ > (C₆H₉N₂)₃[BiCl₆]. These findings highlight that metal-ion substitution represents an effective approach to modulate the electronic structure and charge transport properties of 2-amino-5-methylpyridinium-based hybrid materials, underscoring their potential for semiconducting and electronic device applications.