ИСТИНА

The сubic La2Mo2O9 (1:2) and Nd5Mo3O16 (5:6) compounds in binary Ln2O3 - MoO3 systems have high oxygen or mixed conductivity of the order of 10-2 S/cm at 800oC. Oxymolybdate Ln2MoO6 (1:1) with large rare earth elements Ln = La, Pr, Nd synthesized at temperatures above 1000°C have the tetragonal layered structure constructed of a single layer [1] or two [2] layers of fluorite Ln2O2 and of a Mo- tetrahedra layer. An unusual behavior of Nd2MoO6 compounds doped with lead was detected in [3]. There were two said structures coupled by reversible first-order phase transition near 800oC. The conductivity of the high-temperature phase ran up 10-2 S/cm, In the present study we investigated the La2MoO6 - PbO cut in a ternary La2O3 - MoO3 - PbO system to determine the influence of Pb impurities on phase formation and electrical properties of a number of compositions. Previously, these properties have not been studied. Polycrystalline (PbO)x(La2MoO6)(1-x)/2 (x = 0, 0.05, 0.1, 0.2, 0.34, 0.4) samples were prepared by solid-state synthesis at the maximum heating temperature of 900 - 1000°C. Obtained samples were investigated by X-ray diffraction analysis (DRON-2.0), differential scanning calorimetry based on NETZSCH STA 449C system and by impedance spectroscopy (Novocontrol Beta-N instrument and Tesla BM 431 E bridge). All polycrystalline samples were synthesized in tetragonal phase except for the sample with x = 0.4, which was a phase mixture. The anomalies at about 870оС were detected by DSC in samples with x  0.05. Similar anomalies were observed earlier in [3] for Nd2MoO6 samples doped with Pb, and were associated with the transition from I41/acd phase [2] to I-42m phase [1] while cooling. The permittivity of undoped sample increased monotonically with temperature. The permittivity anomalies of -type appeared in the samples doped with Pb in the field of detected DSC thermal effects, and the conductivity increased abruptly by two orders of magnitude, reaching 10-2 S/cm at 800 ºC. The piezoelectric effect was found in the doped La2MoO6 compounds at room temperature, which might indicate the synthesis of compounds in the acentric phase. The electrical measurement of the sample with x = 0.33 revealed relaxation with an activation energy of 0.84 eV and the time t = 9.7 × 10-12 sec. associated with the charge motion in oxygen vacancies as a result of doping.