ИСТИНА

A large LAMOX family of compounds based on La2Mo2O9, doped with various dopants in the lanthanum position and in the position of molybdenum, was discovered in 2000 by Lacorre et al. [1]. These compounds attract attention because of their high oxygen conductivity (0.06 S/cm at 800ºC) and possibility to use it in various electrochemical systems. Pure La2Mo2O9 undergoes a reversible first order phase transition from monoclinic to high-temperature cubic phase with high oxygen conductivity. Addition of dopants promotes suppression of this phase transition. In this case a metastable cubic phase is stabilized at room temperature. The metastable cubic phase transforms into high temperature cubic phase upon heating. Stability of LAMOX compounds increases at partial substitution of Mo with W. For this purpose in present work we study phase formation, polymorphism and conductive properties of La2Mo2O9-based compounds in La2Mo2O9 - Ln2W2O9 - Ln2Mo2O9 (Ln = Pr, Nd, Sm, Gd) ternary systems. Polycrystalline samples in the above systems were prepared by solid state reaction in air and characterized by X-ray, DSC and conductivity measurements. X-ray investigation shows that wide field of La2Mo2O9-based compounds restricted by La2Mo2O9 - La0.6Pr1.4Mo0.6W1.4O9 - Pr2Mo0.9W1.1O9 - Pr2Mo2O9 exist in Pr-contained ternary system. For Ln = Nd the above field becomes narrower and restricted by compositions La2Mo2O9 - La0.8Nd1.2Mo0.8W1.2O9 - Nd2MoWO9 - Nd2Mo1.86W0.14O9 - La0.4Nd1.6Mo2O9. "Nd2Mo2O9" does not exist in pure form, but Nd2Mo2-xWxO9 solid solutions (0.14  x  1) have the same structure as La2Mo2O9. In the system with Sm and Gd field of the existence of compound with the La2Mo2O9 structure is significantly narrower and limited by La2Mo2O9 - La0.8Sm1.2Mo0.8W1.2O9 - LaSmMo2O9 and La2Mo2O9 - La1.2Gd0.8Mo1.2W0.8O9 - La1.4Gd0.6Mo2O9 triangles. So, decrease of the ion radius of rare-earth element leads to decreasing of crystallization field of La2Mo2O9-based compounds. According to DSC data, substitution of Mo with W and La with Nd, Pr, Sm, Gd leads to disappearance of monoclinic phase and stabilization of metastable cubic phase at room temperature. Phase transition between low temperature and high temperature cubic phases was observed at 450 - 600ºC depending on dopant content. Pure La2Mo2O9, Pr2Mo2O9 and samples in La2Mo2O9 - Nd2W2O9 - Nd2Mo2O9 and La2Mo2O9 - Gd2W2O9 - Gd2Mo2O9 systems demonstrate the same conductivity behavior. The conductivity of low temperature phase obeys Arrhenius law, for high temperature cubic phase Vogel-Tammann-Fulcher law can be used to describe the conductivity. The conductivity of these samples varies weakly depending on the content of tungsten and the rare earth element. In the case of Pr and Sm contained systems the conductivity behaves different. Independently of W content, for the samples with high amount of Pr and Sm, their conductivity curves can be divided into two parts, each of which is well approximated by the Arrhenius law. The border between these two parts corresponds to a phase transition between metastable low temperature cubic phase and high temperature cubic phase. In this case conductivity of the compounds in high temperature region is significantly lower than that of pure La2Mo2O9 and Pr2Mo2O9. Thus, our studies show that the behavior of the conductivity does not depend on the ionic radius of the rare earth dopant. It can be assumed that the change in behavior of the conductivity for samples with high Pr and Sm content may be related to the ability to change the valence of these elements. [1] Lacorre P. Goutenoire F. Bohnke O. Retoux R. Laligant Y. (2000) Nature. 404. 856-859.