Electrical properties and microstructure of nanocrystalline La 2−xA xMo 2O 9−δ (A = Ca, Sr, Ba, K) films

Abstract

Dense, crack-free and uniform nanocrystalline La 2−xA xMo 2O 9−δ (A = Ca, Sr, Ba, and K; x = 0, 0.05) (LAMOX) films were successfully synthesized on polycrystalline Al 2O 3 substrates by a modified sol–gel method. Pure LAMOX phases with an average grain size of 200–300 nm were obtained when the annealing temperature was about 650 °C. The cell parameter of nanocrystalline LAMOX films is larger than that of the corresponding microcrystalline samples. The conductivity of the nanocrystalline pure La 2Mo 2O 9 film reaches 0.07 S/cm at 600 °C, much higher than that of the corresponding bulk materials. The 2.5%Ca, Sr, Ba, and K-doping can suppress the phase transition occurred at 580 °C in pure La 2Mo 2O 9 and enhance the low-temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La 2Mo 2O 9 films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones. The increase of unit cell free volume resulted from the intrinsically larger cell parameter of nanocrystalline grains may be mainly responsible for the enhancement of conductivity and decrease of activation energy.