Crystal structure, thermal expansion and high-temperature electrical conductivity of A-site deficient La2-zCo1+y(MgxNb1-x)1-yO6 double perovskites

Journal article, 2015

New La-deficient double perovskites with P21/n symmetry, La∼1.90(Co2+1-xMg2+x)(Co3+1/3Nb5+2/3)O6 with x=0, 0.13 and 0.33, and La2(Co2+1/2Mg2+1/2) (Co3+1/2Nb5+1/2)O6 were prepared by solid state reaction at 1450 °C. Their crystal structures were refined using time-of-flight neutron powder diffraction data. Our results show that certain cations such as Nb5+, with very strong B-O bonds in the perovskite structure, can induce A-site vacancies in double perovskites. Upon heating in N2 gas atmosphere at 1200 °C ∼1% O atom vacancies are formed together with a partial reduction of the Co3+ content. The average thermal expansion coefficient between 25 and 900 °C of La1.90(Co2+2/3Mg2+1/3)(Co3+1/3Nb5+2/3)O6 was determined to be 17.4 ppm K-1. Four-point electronic conductivity measurements showed that the compounds are semiconductors, with conductivities varying between 3.7·10-2 and 7.7·10-2 S cm-1 at 600 °C and activation energies between 0.77 and 0.81 eV. Partial replacement of La3+ with Sr2+ does not lead to any increase of conductivity, while replacement of Mg2+ with Cu2+ in La1.9CoCu1/3Nb2/3O6 and La1.8CoCu1/2Nb1/2O6 leads to ∼100 times larger conductivities at 600 °C, 0.35 and 1.0 S cm-1, respectively, and lower activation energies, 0.57 and 0.73 eV, respectively.