Investigation of electronic parameters, carrier transport mechanisms via the correlated barrier hopping model, electrothermal NTCR effects, and polarization contributions to the dielectric response of Ni3(PO4)2 orthophosphates synthesized by the sintering process

Abstract

Nickel orthophosphate, Ni₃(PO₄)₂, was prepared through a conventional solid-state synthesis route and its crystal structure was verified by X-ray diffraction, confirming a monoclinic phase with space group P2₁/c. Optical studies using UV-Vis spectroscopy revealed strong absorption in the ultraviolet region, consistent with its semiconducting nature. The dielectric and electrical behaviors were systematically examined as functions of both temperature and frequency using impedance and electric modulus approaches, which allowed a clear distinction between grain and grain-boundary contributions, relaxation processes, and charge transport mechanisms. The temperature-dependent variation of the power-law exponent s(T) showed a decreasing trend, pointing to the correlated barrier hopping (CBH) mechanism as the dominant conduction process within the investigated temperature range (393–633 K). Based on this model, the maximum barrier energy was estimated as W_M = 0.66 eV. Analysis of the impedance and modulus spectra confirmed a non-Debye type relaxation with well-resolved grain and grain-boundary contributions. The impedance data were successfully fitted using an equivalent R//C//CPE circuit, and the variation in resistance further confirmed the NTCR behavior. The material displays a notably high dielectric constant at low frequencies (ε′ ≈ 1.58 × 10⁴ at 633 K), reflecting effective polarization processes and thermally activated charge carrier motion. At higher frequencies, it preserves stable dielectric characteristics accompanied by moderate energy losses. Overall, these results establish a strong correlation between relaxation phenomena and hopping-based charge transport, highlighting Ni₃(PO₄)₂ as a potential candidate for high-frequency dielectric and microelectronic device applications.