The phase relations along the Nd 2 MoO 6 -PbO join of the ternary Nd 2 O 3 -MoO 3 -PbO system have been studied by means of solid-state synthesis in air. The samples with high Pb content underwent a reversible first-order phase transition near 820°C. XRD analysis revealed two tetragonal phases, the hightemperature centric phase (I4 1 /acd) and the low-temperature acentric phase (I42m). In the region of the phase transition, the permittivity of the Pb-containing samples show a strong lambda-type anomaly and electrical conductivity increasesIn the 1960s and 1970s, rare-earth molybdates of various compositions (1:3, 1:2, 5:6, and 1:1) were synthesized as Ln 2 O 3 -MoO 3 binary systems containing 25-50 mol-% Ln 2 O 3 . Many of these molybdates possess functional properties of practical interest. Borchardt and Bierstedt found that the 1:3 compounds, Ln 2 (MoO 4 ) 3 , show ferroelectric properties. [1] In 2000, Lacorre et al. reported La 2 Mo 2 O 9 (1:2 compound) to have high oxygen-ion conductivity (0.06 S/cm at 800°C). [2] In air, 1:2 compounds can only be synthesized for Ln = La and Pr. [2][3][4][5] A number of Ln 2 O 3 -MoO 3 systems have been found to contain Ln 5 Mo 3 O 16+δ compounds of variable composition with a fluorite-like cubic structure. [6][7][8][9][10][11] Compounds with such a structure can be synthesized for Ln = La, Pr, Nd, Sm, and Gd under reducing conditions, [9] and only for Pr and Nd in air. [6,7,10,11] The Ln 5 Mo 3 O 16+δ compounds were found to have mixed oxygen-ion/electronic conductivity, which opens up the possibility of using them in electrochemical devices. [9,10] At a slightly higher rare-earth content, 1:1 Ln 2 MoO 6 compounds, so-called oxymolybdates, [12][13][14] exist for all of the rare earths and have different structures depending on the ionic radius of the rare-earth element and the synthesis temperature.The structures and polymorphism of the oxymolybdates have been the subject of several studies. [12][13][14][15][16][17] The 1:1 molybdates have a tetragonal (La 2 MoO 6 -type) or monoclinic (Nd 2 WO 6type [18] ) structure depending on the ionic radius of the rareearth element and the synthesis temperature. The tetragonal layered structure of Ln 2 MoO 6 (Ln = La, Pr, Nd) compounds is [a]
