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Undoped and Mg-doped Pr 2 MoO 6 oxymolybdate polycrystals and single crystals have been prepared by solid-state reactions and flux growth. The compounds have been characterized by powder X-ray diffraction, energydispersive spectroscopy, inductively coupled plasma mass spectrometry, scanning transmission electron microscopy, single crystal X-ray structure analysis, differential scanning calorimetry and thermogravimetry. The (MgO) x -(Pr 2 O 3 ) y (MoO 3 ) z (x + y + z = 1) solid solution series has been shown to extend to x = 0.03. The structure of the Mg-doped Pr 2 MoO 6 single crystals can be represented as superimposed lattices of the main matrix (Pr 2 MoO 6 ) and lattices in which Mo atoms are partially replaced by Mg. The incorporation of Mg atoms into the structure of Pr 2 MoO 6 results in the disordering of the praseodymium and oxygen lattices. Both the polycrystalline and single-crystal Mg-doped samples are hygroscopic.

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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]

LixLn5–xMo3O16.5–1.5xFx (0.8 ≤ x ≤ 1.4 (Ln = La), 0.4 ≤ x ≤ 1 (Ln = Pr), 0.6 ≤ x ≤ 1 (Ln = Nd)) solid solutions on the Ln5Mo3O16–LiLn4Mo3O15F joins of the Ln2O3–MoO3–LiF ternary systems, similar to the fluorite‐related cubic compound Nd5Mo3O16, have been prepared by solid‐state reactions. Substitutions of lithium on the rare‐earth site and fluorine on the oxygen site have been shown to lead to a reversible phase transition accompanied by an anomaly in permittivity and a jump in conductivity by one and half orders of magnitude. In a wet atmosphere, the LixLn5–xMo3O16.5–1.5xFx compounds may have proton conductivity.

Polycrystalline (PbO) x (La 2 MoO 6 ) (1-x)/2 (x = 0, 0.05, 0.1, 0.2, 0.34, 0.4) samples have been prepared by solid-state reactions near 900°C. Phase-pure compounds with the layered tetragonal La 2 MoO 6 structure have been obtained in the range 0 ≤ х ≤ 0.34. Pb doping of La 2 MoO 6 has been shown to cause [a] M.

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Fluorite-like oxyfluorides NaLn 4 Mo 3 O 15 F (Ln = La, Pr, Nd), which relate to the Nd 5 Mo 3 O 16 structural type, were obtained by solid-phase synthesis in the air. Codoping of the Nd 5 Mo 3 O 16+δ family with sodium and fluorine led to the appearance of a piezoelectric effect and a reversible phase transition in oxyfluorides, accompanied by a jump in the conductivity and an anomaly of the permittivity of the ferroelectric type. It [a]