Phase Relations and Physical Properties of Layered Pb‐Containing Nd₂MoO₆ Compounds

Abstract

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]

Effect of Sodium and Fluorine Co‐Doping on the Properties of Fluorite‐Like Rare‐Earth Molybdates of Nd5Mo3O16 Type

“…The appearance of the above features in the properties of NaLn 4 Mo 3 O 15 F compounds may be due to very insignificant structural changes since their diffractograms do not differ from that of the pure Nd 5 Mo 3 O 16 . It should be noted that the appearance of ferroelectric‐superionic phase transition was also observed in lead doped oxymolybdates Ln 2 MoO 6 (Ln = La, Nd) with a layered tetragonal structure having a fluorite‐like Ln2O2 layer and close in composition to Ln 5 Mo 3 O 16 molybdates …”

Synthesis, structure and properties of layered Pr2MoO6-based oxymolybdates doped with Mg

“…According to their results, the oxymolybdates prepared at 1473 K or higher temperatures have a tetragonal structure. A low-temperature monoclinic phase has been identified only for Nd 2 MoO 6 (Pokrovskii et al, 1969;Voronkova et al, 2016). At 1283 K, this phase irreversibly transforms into the high-temperature tetragonal phase (Voronkova et al, 2016).…”

“…A low-temperature monoclinic phase has been identified only for Nd 2 MoO 6 (Pokrovskii et al, 1969;Voronkova et al, 2016). At 1283 K, this phase irreversibly transforms into the high-temperature tetragonal phase (Voronkova et al, 2016). The unit-cell parameters of La 2 MoO 6 were reported to be a = 4.089 and b = 15.99 Å (space group I " 4 42m) (Sillé n & Lundborg, 1943).…”

“…They were found to be made up of fluorite layers, each containing two sheets of NdO 8 or LaO 8 polyhedra, with a sheet of isolated MoO 4 tetrahedra sandwiched between them. Analysis of X-ray diffraction data for (PbO) x (Nd 2 MoO 6 ) (1-x)/2 and (PbO) x (La 2 MoO 6 ) (1-x)/2 samples, doped with divalent Pb (Voronkova et al, 2016(Voronkova et al, , 2017, indicated that the layered tetragonal phase had an extended stability region. Near 1073 K, an antiferroelectric phase transition was identified, due to the transformation of the I4 1 /acd phase into an I " 4 42m phase on cooling.…”

“…However, doping of the oxymolybdates, e.g. with Pb, was found to cause drastic changes in their conductivity and other properties, as mentioned above (Voronkova et al, 2016(Voronkova et al, , 2017. Among other bivalent impurities doping Nd 2 MoO 6 , the structure with magnesium is known (Voronkova et al, 2019).…”