ИСТИНА

Fluorite-like Nd5Mo3O16 and Pr5Mo3O16 compounds attracts attention as materials with mixed electron-oxygen conductivity [1, 2]. Depending on synthesis conditions and atmosphere oxygen content the formula of compounds can be written as Nd5Mo3O16+d or Pr5Mo3O16+d (0 < d < 0.5). Pr, Nd and Mo contents may also vary into compounds, whereby solid solutions with fluorite-like structure a formed in Nd2O3-MoO3 system at 42.9 - 46.7% Nd2O3 [3]. According to [4] in the case of Pr5Mo3O16 (d = 0) all oxygen positions in the structure are occupied, however extensive cavities were found in the structure. These cavities may be occupied by free interstitial oxygen. This oxygen may participate in the formation of the high conductivity of the compound. However, the vacancy mechanism of conductivity is also not excluded, as well as the possibility of increasing the oxygen conductivity, when heterovalent substitutions introduced into the Nd5Mo3O16+d structure. The aim of the present work is synthesis of Nd5Mo3O16-based solid solutions in Nd2O3-MoO3 system, pure and doped with Ca, Pb, Nb, Zr, and investigation of their dielectric and conductive properties. Polycrystalline samples of Nd6Mo4O21, Nd5Mo3O16.5, Nd14Mo8O45, PbNd4Mo3O16+d, Nd5-xZrxMo3O16+d (x = 0 - 0.1), Nd5-xCaxMo3O16+d (x = 0 - 1), Nd5Mo3-xNbxO16+d (x = 0 - 0.5) have been obtained by solid state synthesis in air. According to X-ray all of the samples belong to Nd5Mo3O16 fluorite-like structure. Electrophysical measurements shows that conductivity of xNd2O3+(1-x)MoO3 (x = 0.429 - 0.467) slightly increases with increasing of Mo content, whereas doping of heterovalent Ca, Pb, Zr and Nb substitutions does not change the conductivity compared with pure Nd5Mo3O16+d (Figure 1). These substitutions strongly affect on Nd5Mo3O16+d dielectric properties, resulting to a significant change in the intensity, width and temperature of dielectric permittivity peak, which was observed near 550 - 600 ºC in pure fluorite-like compounds (Figure 2). Heterovalent substitution Nd3+ with Ca2+, Pb2+, Zr4+ and Mo6+ with Nb5+ should influence on the amount of oxygen vacancies in the structure of the compound. However, as mentioned earlier, such substitution did not affect the electrical conductivity. Thus, the high oxygen conductivity of Nd5Mo3O16-based fluorite-like compounds presumably is not caused by the vacancy conduction mechanism. More likely, that oxygen conductivity may be connected with the presence of free movable oxygen in the interstices of the structure. [1] M. Tsai, M. Greenblatt, W.H. McCarroll, Oxide ion conductivity in Ln5Mo3O16+x (Ln = La, Pr, Nd, Sm, Gd, x ~ 0.5) with a fluorite-related structure, Chem. Mater., vol. 1, pp. 253-259, (1989). [2] V.I. Voronkova, E.P. Kharitonova, D.A. Belov, M.V. Patrakeev, O.N. Leonidova, I.A. Leonidov. Fluorite-like compound with the Nd5Mo3O16 type structure with mixed electronic-ionic conductivity. The 19th International Conference on Solid State Ionics, June 2-7, 2013. Kyoto, Japan. Program. pp. Mon-E-057. [3] V.I. Voronkova, E.P. Kharitonova, D.A. Belov, Synthesis and electrical properties of a new fluorite-like anionic conductor in the Nd2O3–MoO3 system (43–47 mol% Nd2O3), Solid State Ionics, vol. 225, pp. 654-657, (2012). [4] M.J. Martinez-Lope, J.A. Alonso, D. Sheptyakov, V. Pomyakushin, Preparation and structural study from neutron diffraction data of Pr5Mo3O16, J. Solid State Chem., vol. 183, pp. 2974-2978, (2010).