Humidity-dependent dielectric and electrical behavior of magnesium iron vanadate (Mg₂FeVO₆) ceramics

Mg₂FeVO₆ ceramic with a double perovskite-related structure was synthesized by the sol-gel method and characterized using XRD, SEM, DLS, and zeta potential. XRD identified three crystalline phases with a dominant Ia-3̄d structure and a crystallite size of 26.1 nm. SEM showed solid particles with an average size of ∼75.6 nm. The zeta potential of -19 mV indicated negative surface charge and hydroxyl groups. A humidity sensor was fabricated by drop casting on an FTO substrate and tested across 11% to 97% RH at frequencies from 1 kHz to 5 MHz. Impedance decreased and capacitance increased with increasing RH, with the best response at 1 kHz. A capacitance sensitivity of 56.04% RH^-1, based on a capacitance range of 0.86 pF to 42.31 pF across 11% to 97% RH, exceeded impedance sensitivity by a factor of 48. The sensor showed good repeatability over four cycles and excellent short-term stability. Response and recovery times were 760 s and 25 s, respectively. Phase angle and Nyquist analysis confirmed a transition from capacitive to resistive behavior with increasing RH. Permittivity, dielectric loss, and AC conductivity increased with RH at all frequencies. The sensing mechanism involves water film formation at grain boundaries with protonic conduction via the Grotthuss mechanism at high RH. This work provides the first humidity-dependent dielectric and electrical dataset for Mg₂FeVO₆.