Optical behavior, ionic conductivity, and dielectric performance of lithium bismuth diphosphate: relevance for...

In the present study, single-phase lithium bismuth diphosphate (LiBiP₂O₇) was prepared by conventional solid-state synthesis. Structural characterization verifies the formation of a monoclinic phase, with an average crystallite size close to 60.6 nm. The average grain size of the sample was estimated at 2.056 µm. Fourier-transform infrared (FTIR) analysis confirms the presence of typical vibrational bands related to the (P₂O₇)^4- pyrophosphate units. Optical characterization derived from absorbance data reveals semiconducting features, with a direct band gap estimated at 3.94 ± 0.02 eV. Impedance measurements carried out over a wide frequency range (0.1 Hz to 1 MHz) and temperature span (453-673 K) indicate that the electrical response is mainly dominated by grain effects. The material displays a strong NTCR response, evidenced by a high thermistor constant (β ≈ 9633 K) and a stability factor (SF ≈ 3.091), pointing to its potential use in high-temperature sensing devices. The Nyquist diagrams were well described using an equivalent circuit composed of parallel R//C//Q elements. A detailed investigation of the AC and DC conductivity, together with electric modulus analysis, reveals that charge transport is predominantly governed by the long-range migration of charge carriers, while localized dipolar relaxation processes make only a minor contribution to the overall electrical response. The conduction mechanism is consistent with the Correlated Barrier Hopping (CBH) model, yielding an activation energy of about 0.82 ± 0.04 eV and a potential barrier height W _M ≈ 0.14 eV. Dielectric analysis indicates a deviation from ideal Debye relaxation behavior. The dielectric constant reaches values on the order of 10⁴ at 673 K, while the dielectric loss (tan δ _max ≈ 14.68) increases significantly at low frequencies and elevated temperatures, suggesting the dominance of interfacial and space-charge polarization phenomena. In summary, these results deepen the understanding of the electrical transport and dielectric response in pyrophosphate systems and identify LiBiP₂O₇ as a promising material for high-frequency dielectric applications and energy storage technologies.