Impedance analysis and protonic conduction mechanism in RbH ₂PO₄/SiO₂ composite systems

The protonic conductivity of (1 - x)RbH₂PO₄-xSiO ₂ composites prepared by mechanical milling has been investigated in the humid atmosphere. In comparison with pure RbH₂PO₄, the conductivity enhancement was observed and it was strongly dependent on the amount of SiO₂ additives. At low-temperature (LT) phase, the conductivity of composites increased with the addition of SiO₂ till x = 0.6 at which the maximum conductivity reached. At high-temperature (HT) phase, it was approximately equal to that of pure RbH₂PO₄ and almost kept unchanged till x = 0.5. Therefore, the jump in conductivity at ∼280 °C (superprotonic phase transition temperature of RbH ₂PO₄) tended to decrease as x increased and finally disappeared. Furthermore, the conductivity of both LT and HT phases went down at large x values. The conductivity enhancement has been confirmed not to be influenced by absorbed water on the surface of SiO₂ particles. In fact, it is due to the formation of structurally disordered or amorphous interface phase. Also, we summarized the different theoretical models of the conductivity enhancement in composites.