Effects of Zr doping on the phase composition, crystal structure and ionic conductivity of (Zr, Al)-doped lanthanum germinates

Abstract

The crystal structure and ionic conductivities of (Zr, Al)-doped lanthanum germinate have been investigated in this paper. Apatite-type La10−xZr x Ge5.5Al0.5O26.75+x/2 (x = 0, 0.05, 0.1, 0.2, 0.5, 1.0) were prepared by solid state reaction. The XRD analysis exhibited that Zr and Al were substituting La and Ge site, respectively. And the unit volume of the samples increased with the increasing Zr content. In addition, there was no second phase formed when x ≤ 0.2. The relative density and thermal expansion coefficient (TEC) of La10−xZr x Ge5.5Al0.5O26.75+x/2 was improved due to Zr doping. Much interesting, the results of AC impedance spectroscopy exhibited that the conductivity increased with the increasing Zr content (x = 0.05, 0.1, 0.2). The improvement of conductivity was because the number of interstitial oxide ions increased, which assisted the migration of oxide ions. However, the conductivity decreased when the Zr content increased (x = 0.5, 1.0). There was an optimum doping amount of Zr. La9.8Zr0.2Ge5.5Al0.5O26.85 exhibited the highest conductivity of 1.25 × 10−2 S cm−1 at 800 °C and increasing TEC of 9.71 × 10−6 K−1 from room temperature to 800 °C, which suggests that La9.8Zr0.2Ge5.5Al0.5O26.85 is a potential electrolyte material.