A Comparative Study of Electrical Conductivity and Dielectric Relaxation in Lithium- and Vanadium-Based P2O5–ZnO–CaO Phosphate Glasses

Phosphate glasses with the compositions 60P2O5–15ZnO–15CaO–10Li2O (PZCL) and 60P2O5–15ZnO–15CaO–10V2O5 (PZCV) (mol%, expressed as oxides) were synthesized by the melt-quenching method. The prepared glasses were characterized by using X-ray powder diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). X-ray diffraction shows only broad diffuse halos without Bragg reflections, indicating that both glasses are X-ray amorphous. Raman analysis confirms that PZCL behaves as a typical alkali-modified ultraphosphate glass containing Q3 and Q2 units, whereas PZCV contains characteristic V–O vibrational modes indicating that vanadate groups are integrated into the phosphate backbone. SEM observations reveal smooth surfaces at low magnification for both glasses; at higher magnification, PZCV shows a compact and homogeneous surface with fine, uniformly dispersed granules, whereas PZCL displays a more textured surface with agglomerates and plate-like features. EIS analysis using the Havriliak–Negami model reveals that the free-charge carrier contribution appears as a step in M′(ω,T) and a peak in M″(ω,T), allowing a clear distinction between conductivity relaxation and interfacial polarization or dipolar effects, with conductivity derived from the M″(ω,T) peak frequency. PZCV shows dc conductivity approximately one order of magnitude higher than PZCL (1.5 × 10−9 vs. 1.8 × 10−10 S·cm−1 at 120 °C). The temperature dependence follows Arrhenius behavior, with activation energies of about 0.89 eV for PZCV and 1.04 eV for PZCL. The electrode polarization observed at low frequencies with ion-blocking gold electrodes indicates a significant ionic contribution in both glasses, although a polaronic (V4+/V5+) contribution to the conductivity of PZCV cannot be excluded. Capacitance analysis reveals a slight increase with temperature for both samples, with PZCV consistently showing higher values (~7 pF) than PZCL (~5 pF). Although the absolute conductivities remain far below those required for practical solid electrolytes, these results clarify how the replacement of Li2O by V2O5 modifies charge transport and dielectric relaxation in P2O5-rich Zn–Ca phosphate glasses.