ИСТИНА

INTRODUCTION In the 60s-70s the compounds of different compositions (1:3, 1:2, 5:6, 1:1) were synthesized in binary Ln2O3 - MoO3 systems in the range of 25 - 50 mol.% Ln2O3. Borchard et al [1] discovered ferroelectric behavior of the compound with 1:3 composition, Ln2(MoO3)4. In 2000, a group of Lacorre [2] revealed high oxygen conductivity (~ 0.06 S/cm at 800 ºC) in the 1:2 compound, La2Mo2O9. The 1:2 compounds are synthesized only for La and Pr [1,3,4]. Class of compounds with the La2Mo2O9 structure can be expanded. For example, at partial substitution of molybdenum with tungsten, one can observe the stabilization of this structure for Nd [5]. The 5:6 compounds, Ln5Mo3O16.5, with fluorite-like structure exist in the air only with Pr and Nd. With other rare-earth elements they are synthesized only under reducing conditions with composition Ln5Mo3O16. These compounds are of variable composition with mixed oxygen-electron conductivity [6]. The 1:1 compounds, Ln2MoO6, or oxymolybdates are formed in the air with all the rare-earth elements [7], but their conductive and other properties have not been studied. Ln2MoO6 compounds with large cations (Ln = La-Nd) synthesized at temperatures above 1000°C, have a tetragonal structure at room temperature. Starting with Ln = Sm, compounds at room temperature are monoclinic [7]. Oxymolybdate structures are fluorite-type structures derivatives [7]. The present work is intended to study of phase formation and conductivity properties of compounds in the ternary Nd2O3 - MoO3 - MeO (Ме =Pb, Ca) systems near the Nd5Mo3O16.5 and Nd2MoO6 compounds. EXPERIMENTAL Polycrystalline samples were obtained by solid state synthesis in air. XRD characterization was performed for powdered samples with a DRON-2.0 diffractometer (Cu Kα-radiation, scanning interval 2θ = 20°-60° with a step of 0.05°). A differential scanning calorimetry (DSC) has been made using NETZSCH STA 449C equipment (30-1200oC, 10 K/min, Al2O3-crucibles, air atmosphere). Temperature dependences of conductivity were measured using a Tesla BM 431 E bridge (20-1000oC, 1 MHz, Pt-electrodes). The phase formation study showed that in the binary Nd2MoO6 – PbO cut there are compounds with tetragonal Nd2MoO6 structure up to 60 mol.% PbO. In the Nd2O3 - MoO3 - CaO system near Nd2MoO6 and Nd5Mo3O16.5 compounds one can observe an extensive field of compounds with the cubic structure of fluorite-related Nd5Mo3O16.5. Electrophysical studies showed that the identified compounds have reasonably high conductivity close to 10-2 S/cm at 800оС (Figure). In Pb-contained compounds noticeable anomalies were observed in the temperature dependences of permittivity and conductivity near 800 ºC. CONCLUSIONS In the ternary Nd2O3 - MoO3 - MeO systems, where Me = Pb, Ca, near Nd5Mo3O16.5 and Nd2MoO6, the extensive areas of compounds with tetragonal (Me = Pb) or cubic (Me = Ca) structures were obtained. The compounds had relatively high conductivity close to 10-2 S/cm. REFERENCES 1. H.J. Borchardt, P.E. Bierstedt. J. Appl. Phys. (1967) 2057. 2. P. Lacorre, F. Goutenoire, O. Bonkem R. Retoux, Y.Laligant. Nature 404 (2000) 856. 3. T. Yamazaki, T. Shimazaki, T. Terayama. J. Mater. Sci. Lett. 21 (2002) 29. 4. V.I. Voronkova, E.P. Kharitonova, E.I. Orlova, D.S. Kolesnikova. Crystallogr. Reports. 56 (2011) 1135. 5. V.I. Voronkova, E.P. Kharitonova, E.I. Orlova, D.A. Belov. J. Solid State Chem. 196 (2012) 45. 6. M. Tsai, M. Greenblatt, W.H. McCarroll. Chem. Mater. 1 (1989) 253. 7. L.H. Brixner, A.W. Sleight, M.S.Licis. J. Solid State Chem. 5 (1972) 186.