Electrical and Structural Properties of Bi-Substituted La2Mo2O9 Annealed at Intermediate Temperature

Abstract

Introduction Oxide-ion conductors find important applications in solid oxide fuel cells and other solid ionic devices. High-temperature polymorph of lanthanum molybdate (La2Mo2O9) has been attracting extensive attention in recent years[1]. The high temperature phase (β phase) of La2Mo2O9, a cubic structure with space group P213, exhibits good ionic conductivity as high as 0.06 S/cm at 1073 K. However, a phase transition from the β phase to low temperature monoclinic α phase takes place around 560-580 °C and leads to a drop in conductivity by nearly two orders of magnitude. Partial substitution of bismuth for lanthanum as La2 – xBixMo2O9 can effectively suppress the α–β phase transition stabilizing β-type structure down to room temperature and restrict the drop in conductivity. Differential scanning calorimetry (DSC) curves of La2-xBixMo2O9 proved the restriction of α–β transition. However, for relatively lower bismuth concentration as x = 0.06, a small endothermic peak sometimes appears around 450–500 °C in DSC traces, which seems to differ from the α-β phase transition [2]. In this work, We report the low temperature stable phase β' from the thermal analyses after various heat treatments. While the newly found β' phase might be related to the dynamic oxygen disorder [3] for the slight difference in XRD patterns between the β and β' phase. Nevertheless, β' phase has been only detected by thermal analysis so far. Then, in the present study, we measured the conductivity and electron diffraction on the annealed samples of La2 – xBixMo2O9 (x = 0.06) to reveal the electric and structural properties of β' phase. Experiment La2-xBixMo2O9 (x = 0.06) sample is prepared by means of conventional sintering method from the mixture of La2O3 (Kishida Chemical, 99.99%), Bi2O3 (Kishida Chemical, 99.9%) and MoO3 (Wako Pure Chemical, 99%) reagents. The calcining and sintering temperatures were selected as500°C and 1000°C, respectively. The sintered sample of β phase was then annealed in a Pt crucible at 460°C to achieve β' phase. Electrical conductivities and structures of the annealed La2 – xBixMo2O9 (x = 0.06) have been investigated by means of a.c. impedance method and transmission electron microscope. Results and Discussion Fig.1 shows the Arrhenius plots of the conductivity measured on annealed La2-xBixMo2O9 (x = 0.06). The conductivity data obtained in the heating and cooling directions are plotted by red and black symbols, respectively. In the lower temperature, both datum disagree each other while they coincide at higher temperature. This indicates that the sample of initially taking β' phase transforms into β phase on heating and the β' phase would supercooled to room temperature. On the other hand, the as-sintered sample shows a concurrence of conductivity curves in both directions. We also found that the as-sintered and annealed have different electron diffraction pattern, while there is only slight difference in XRD patterns. Fig.2 shows the electron diffraction pattern of annealed sample (β' phase) . This pattern indicates that body-centered cubic subcell with a lattice constant of 12.06A accompanied by superlattice reflections. We are now investigating the phase relations between β and β' phase.