Structure and Properties of Ln₂MoO₆ Oxymolybdates (Ln = La, Pr, Nd) Doped with Magnesium

Abstract

The literature data and the results obtained by the authors on the study of the structure and properties of a series of polycrystalline and single-crystal samples of pure and Mg-doped oxymolybdates Ln₂MoO₆ (Ln = La, Pr, Nd) are analyzed. Presumably, the high-temperature phase I4₁/acd of Nd₂MoO₆ single crystals is retained at room temperature. The reason for the loss of the center of symmetry in the structures of La₂MoO₆ and Pr₂MoO₆ and the transition to the space group I$\overline{4}$c2 is the displacement of oxygen atoms along the twofold diagonal axes. In all structures, Mg cations are localized near the positions of the Mo atoms, and the splitting of the positions of the atoms of rare-earth elements is found. Thermogravimetric studies, as well as infrared spectroscopy data for hydrated samples of Ln₂MoO₆ (Ln = La, Pr, Nd), pure and with an impurity of Mg, confirm their hygroscopic properties.

1. Introduction

During decades, tungstates, molybdates, and binary systems of the Ln₂O₃–Mo(W)O₃ type, especially in the concentration range 25–50 mol% of Ln₂O₃ oxide, have been intensively studied [1,2,3,4,5,6]. The different content of oxides determines the variety of compounds in these systems. Scheelite-like 1:3 phases (the 1:3 ratio of molar concentrations of oxides, i.e., the 1:3 composition) with excellent luminescent properties are distinguished among them. In the molybdates of these compounds, improper ferroelectrics were first discovered [7]. In 2000, the LAMOX family (1:2 composition) with a high oxygen conductivity of the order of 10^–2 S cm^–1 was discovered [8]. The cubic phase of the 5:6 composition (Ln₅Mo₃O₁₆) [9] has the same high conductivity. Finally, 1:1 compounds (Ln₂Mo(W)O₆), so-called oxymolybdates and oxytungstates, existing in all the binary systems mentioned above, exhibit polymorphism that depends on the ionic radius of the rare earth element and the synthesis temperature [1,2,3,4,5,6]. Oxymolybdates and oxytungstates are chemically stable and have high refractive indices. These compounds doped with Eu exhibit intense luminescence [10,11] and are also used as microwave ceramics [12,13]. Other properties of these compounds are poorly understood. Let us focus our attention on the study of a number of new properties of the rather well-studied phases of oxymolybdates and oxytungstates Ln₂Mo(W)O₆ existing in binary systems Ln₂O₃–Mo(W)O₃ [1,4,14,15,16,17].

Already in the first and subsequent structural studies, polymorphism in these compounds was revealed [1,2,4,14,15,16,17,18,19,20]. Oxytungstates were synthesized in two different monoclinic scheelite-like symmetry classes, depending on the ionic radius of the rare earth element [1]. As for oxymolybdates Ln₂MoО₆ with large rare earth cations (La, Pr, and Nd), a large number of works have been devoted to the study of their polymorphism, structure, and properties [14,15,16,17,19]. During the synthesis of these oxymolybdates at a temperature of about 1200 °C, a layered tetragonal phase is formed. With a subsequent decrease in the ionic radius, a monoclinic scheelite-like phase with a structure of the Nd₂WO₆ type was synthesized, which remained monoclinic when heated to 1400 °C [1,2,4,15]. Scheelite-like structures of the Nd₂WO₆-type for oxymolybdates Ln₂MoO₆ (Ln = Nd–Tb) have been studied many times [15,17,19]. Among Ln₂MoO₆ molybdates with large cations (Ln = La–Nd), only Nd₂MoO₆ can exist at low temperatures in the monoclinic phase, which irreversibly transforms into the tetragonal phase at 1010 °C [21]. The existence of this phase transition was confirmed by differential thermal analysis (DTA).

In [14], the structure of La₂MoO₆ single crystals obtained at 800 °C was studied for the first time. The authors revealed their tetragonal layered structure, space group I$\overline{4}$2m, and unit cell parameters a = 4.089, c = 15.99 Å. No monoclinic phase was found.

According to [16,17], the space group of Ln₂MoO₆ (Ln = La, Nd) single crystals obtained by cooling from 1250 to 900 °C is centrosymmetric I4₁/acd with a doubled parameter c (a = 5.798, c = 32.036 Å). This conclusion was first made in [16] and was associated with additional reflections arising from the deformation of the crystal lattice during firing at high temperatures. Further investigation of the physical properties of the crystals revealed a weak piezoelectric effect [16], which made it necessary to reconsider the crystal symmetry. For crystals at room temperature, an asymmetric space group I$\overline{4}$c2 was chosen, which is a subgroup of the space group I4₁/acd. The authors of [16] assumed the existence of a second order phase transition between these groups, which was therefore not detected using DTA [22]. According to [16,17], the structure of La₂MoO₆ or Nd₂MoO₆ consists of two layers of LaO₈ or NdO₈ polyhedra and a layer of isolated MoO₄ tetrahedra. The conductivity of oxymolybdates does not exceed 10^–4 S cm^–1 [23]. Thermogravimetry shows that molybdates Ln₂MoO₆ (Ln = La, Pr, Nd) have hygroscopic properties up to 1000–1200 °C, which may indicate the possible proton conductivity in these compounds [24,25,26].

Currently, a number of works are known in which the rare earth element in oxymolybdates with large cations is partially replaced by divalent elements—lead or magnesium [21,24,25,26]. Solid solutions (PbO)_x(Nd₂MoO₆)_(1–x)/2 are formed in a wide range 0 ≤ x ≤ 0.6. In these solid solutions, in the region of 820 °C, a phase transition was observed between the low-temperature I$\overline{4}$2m and high-temperature I4₁/acd phases upon heating. In the phase transition region, the conductivity of neodymium oxymolybdate increased by two orders of magnitude and reached 10^–2 S cm^–1 and was comparable to the conductivity of molybdate with the LAMOX structure. When lanthanum oxymolybdate was doped with Pb, the field of solid solutions narrowed [27]. However, the physical properties changed upon doping in the same way as in the case of neodymium oxymolybdate: a phase transition occurred (775 °C), the conductivity increased.

The second divalent element, magnesium, during the synthesis of solid solutions according to the scheme (MgO)_x(Ln₂MoO₆)_(1–x)/2, where Ln = La, Pr, Nd [24,25,26], led to the formation of a very narrow field of solid solutions with x < 0.10. Mg-doped single crystals were prepared by flux growth. According to the X-ray diffraction study of the compounds obtained, the Mg atom replaces Mo and leads to the splitting of the positions of rare earth elements, while the phase transition does not occur. In [24,25,26], the features of the structure and properties of Mg-doped oxymolybdates are considered. The presented work is a brief review in which we summarize, analyze and compare the structural features and properties of Mg-doped Ln₂MoO₆ oxymolybdates with large rare earth cations (Ln = La, Pr, Nd), previously published and supplemented by studies of a number of new properties using powder X-ray diffraction, thermogravimetry, and infrared spectroscopy.

2. Materials and Methods

Polycrystalline samples were prepared by solid-state reactions in air in ternary systems MgO–Ln₂O₃–MoO₃ (Ln = La, Pr, Nd) in accordance with the scheme (MgO)_x(Ln₂O₃)_y(MoO₃)_z (x, y, z are the molar fractions of oxides, x + y + z = 1, y = z = (1 – x)/2). These compositions correspond to the Ln₂MoO₆–MgO binary join of the above ternary system, (MgO)_x(Ln₂MoO₆)_(1–x)/2, x = 0, 0.03, 0.05, 0.10. High purity (99.9%) reagents of magnesium oxide MgO and rare earth oxides La₂O₃, Pr₂O₃, and Nd₂O₃, due to their hygroscopicity, were preliminarily dried before weighing. The tablets of the required composition were compressed using a hydraulic press (10 MPa). Two-stage firing of specimens at temperatures of 800 and 1250 °C for 24 h with intermediate grinding in an agate mortar was applied. The heating and cooling rate was 300 °C h^–1.

The surface of the samples of pure and Mg-doped Pr₂MoO₆ was studied by scanning electron microscopy (SEM) using a Scios DualBeam system (scanning electron microscope (SEM)/focus ion beam) (Thermo Fisher Scientific, Waltham, MA, USA). The samples were mounted on an aluminum holder with carbon adhesive tape. SEM image of pure Pr₂MoO₆ ceramics shows weakly faceted crystallites up to 5 μm in size (Figure 1a). The incorporation of Mg into ceramics (Figure 1b) led to an increase in the crystallite size to 10–50 µm and a decrease in the number of pores.

Single crystals of rare earth oxymolybdates Ln₂MoO₆ (Ln = La, Pr, Nd) were prepared by flux growth in the Li₂O–MoO₃–Ln₂O₃, Ln = La, Pr, Nd, ternary systems from high-temperature melts with a composition of 12.5 mol% Ln₂O₃, 30 mol% Li₂O, and 57.5 mol% MoO₃. To obtain Mg-doped single-crystal samples, magnesium oxide was added to the melt. The maximum melt temperature was 1350 °C. All the obtained single crystals had a plate-like shape, sizes up to 5 mm. Depending on the rare earth cation, the crystals were colorless (Ln = La), yellow (Ln = Pr), or violet (Ln = Nd) in the light of an incandescent lamp (Figure 2a). The good quality of the obtained single crystals was confirmed by SEM. In the SEM image of a Mg-doped Pr₂MoO₆ single crystal, no domains and block boundaries were found (Figure 2b).

The chemical composition of Mg-doped Nd₂MoO₆ and Pr₂MoO₆ was determine in [24,25] using energy dispersive X-ray spectrometry. More precisely, the low concentration of magnesium in Pr₂MoO₆ single crystals was determined by inductively coupled plasma-mass-spectrometry (ICP-MS technique) [25].

X-ray phase analysis was carried out at room temperature using a Rigaku Miniflex 600 diffractometer (Cu Kα radiation, 2θ = 20°–60°).

To estimate the density of the obtained polycrystalline samples, we used the ratio of the density obtained by hydrostatic weighing in toluene and the X-ray density of the samples, calculated by the formula:

ρ_calc = MZ 1.66/V,

where M is the molecular weight (g), Z is the number of formula units, V is the volume of the unit cell (ų). The density of pure and Mg-containing polycrystalline samples averaged 95% of the calculated one.

Thermogravimetric (TG) studies of all obtained phases were carried out on NETZSCH STA 449C equipment in the temperature range 20–1250 °С in air at a heating and cooling rate of 10 °С min^–1. Infrared (IR) spectroscopy of the compounds under study was carried out on a Bruker Vertex 70 IR Fourier spectrometer in the frequency range 500–5000 cm^–1. A sample of 1.0 mg ground into powder was pressed with potassium bromide into a tablet weighing 200 mg and 0.5 mm thick. Polycrystalline and single-crystal samples for IR and TG studies were preliminary hydrated by immersion in distilled water at room temperature for one–three weeks.

X-ray diffraction studies of Ln₂MoO₆ (Ln = La, Pr, Nd) single crystals, pure and doped with Mg, were carried out on an Xcalibur Eos S2 diffractometer (Rigaku Oxford Diffraction) at room temperature. The CrysAlisPro program [28] was used for data reduction. Absorption correction was made analytically, with allowance for the crystal shape and sizes [29], and the JANA2006 program [30] was used to refine the structural model, which was determined with the Superflip program [31] using the charge-flipping algorithm. The experimental data, including the main characteristics of the crystals under study, the results of the refinement of structures, the coordinates, occupancies of positions, and equivalent atomic displacement parameters are given in [24,25,26].

3. Results and Discussion

3.1. Powder X-ray Diffraction

In powder diffraction patterns of ground polycrystalline samples (MgO)_x(Ln₂MoO₆)_(1–x)/2 (х = 0, 0.03, 0.05, 0.10), Ln = La, Pr (Figure 3 and Figure 4) at х ≥ 0.05, a peak of a second phase, MgO, emerges (θ = 43°). Samples with x = 0.03 are isostructural with pure oxymolybdates La₂MoO₆ and Pr₂MoO₆. For Mg-containing neodymium oxymolybdates (Figure 5), the solid solution range with a tetragonal structure of Nd₂MoO₆ exists at x < 0.10. It should be noted that a wider range of solid solutions is indicated in [24,25] for (MgO)_x(Ln₂MoO₆)_(1–x)/2 (Ln = Pr, Nd) samples, which is associated with a lower diffractometer resolution. The unit cell parameters of pure and Mg-doped samples of oxymolybdates Ln₂MoO₆, Ln = La, Pr, Nd, change insignificantly with the introduction of Mg atoms into the structure of the compounds under study (

Table 1. Composition, unit cell parameters and volume of Mg-doped oxymolybdates (MgO)_x(Ln₂MoO₆)_(1–x)/2, Ln = La, Pr, Nd.

Composition	a, c, Å	V, Å3
(MgO)x(La2MoO6)(1–x)/2, x = 0	5.795(7), 32.054(4)	1076.44(5)
(MgO)x(La2MoO6)(1–x)/2, x = 0.03	5.791(2), 32.03(1)	1074.15(2)
(MgO)x(Pr2MoO6)(1–x)/2, x = 0	5.693(9), 31.656(4)	1039.7(9)
(MgO)x(Pr2MoO6)(1–x)/2, x = 0.03	5.691(1), 31.646(5)	1040.68(0)
(MgO)x(Nd2MoO6)(1–x)/2, x = 0	5.665(1), 31.638(5)	1015.33(4)
(MgO)x(Nd2MoO6)(1–x)/2, x = 0.03	5.659(1), 31.622(9)	1012.8(5)
(MgO)x(Nd2MoO6)(1–x)/2, x = 0.05	5.662(1), 31.616(8)	1013.5(4)

).

Figure 6 shows powder diffraction patterns of ground single crystals of pure and Mg-doped Ln₂MoO₆, Ln = La, Pr, Nd [24,25,26]. There is a significant increase in the intensity of the 0012 reflection in comparison with diffractograms obtained from polycrystalline samples, which is explained by the layered structure of oxymolybdate crystals. The largest face of the crystal is (001) plane (Figure 2a), normal to the c axis.

3.2. Thermogravimetry

In [24,25,26], the hygroscopic properties of pure and Mg-containing compounds Ln₂MoO₆ (Ln = La, Pr, Nd) were found. In Figure 7 and Figure 8, TG curves of preliminary hydrated polycrystalline and single-crystal Ln₂MoO₆ samples doped with Mg are shown. For all the samples under study, after the first heating, weight losses of the order of 0.1–0.7% were recorded associated with the evaporation of water. No noticeable weight loss was observed upon reheating, indicating that the water had evaporated. The hygroscopicity of single-crystal samples of La and Nd oxymolybdates was first detected in this work. The weight loss of ceramics and single crystals at high temperatures confirms the presence of water in the crystal structure of the samples [32], not only in the ceramics pores or on the surface.

The water loss was approximately the same for the ceramics, because ceramics with the same magnesium content were examined ((MgO)_0.03(Ln₂MoO₆)_0.485, Ln = La, Pr, Nd) (Figure 7). For single crystals, the amount of absorbed water turned out to be different due to the different concentration of magnesium in the crystals (Figure 8). The largest amount of magnesium was found in neodymium crystals, and the amount of absorbed water for them was the maximum. Apparently, the incorporation of magnesium into the structure increases the hygroscopicity of the samples.

3.3. IR Spectroscopy

IR spectroscopy of Mg-doped polycrystalline oxymolybdate samples is performed for the first time. In Figure 9, the IR spectra of hydrated oxymolybdates (MgO)_x(Ln₂MoO₆)_(1–x)/2, Ln = La, Pr, Nd, x = 0.03, are shown. The vibration spectrum of the crystal lattice of the samples under study is recorded below 900 cm^–1 [33]. Consequently, the region 4500–900 cm^–1 can be attributed to the vibration of OH bonds. For all studied hydrated phases of Mg-containing oxymolybdates, a strong broad band at about 3460 cm^–1 is observed, which characterizes the stretching vibrations of the O–H bond and confirms the presence of various oxygen-hydrogen groups in the samples. The band corresponding to deformation vibrations of an isolated water molecule shifts to short-wave region and is recorded at 1644 cm^–1, which occurs during association of water molecules, for example, due to hydrogen bonds [34,35]. The absorption band at 1730 cm^–1 unambiguously indicates the existence of the hydroxonium ion Н₃О⁺. Thus, it can be concluded that in the studied phases of oxymolybdates, oxygen-hydrogen groups are Н₃О⁺.

3.4. Crystal Structure

X-ray diffraction analysis of Mg-doped Ln₂MoO₆ (Ln = La, Pr, Nd) tetragonal single crystals showed that no radical structural changes occurred at an Mg concentration of 2.6–11.4% [24,25,26]. The structure of these crystals was solved in space group I4₁/acd for Mg-containing samples of neodymium oxymolybdate [24] and in space group I$\overline{4}$c2 for lanthanum and praseodymium oxymolybdates with magnesium [25,26]. The reason for the loss of the center of symmetry and the transition to the space group I$\overline{4}$c2 can be the displacement of O atoms, which form the nearest environment of the atoms of the rare earth elements, along the twofold diagonal axes [16].

Table 2. Atomic coordinates in the structure of Ln₂MoO₆ single crystals in space groups I4₁/acd and I$\overline{4}$c2 and their difference Δ.

Atom	Ln	I41/acd			$\mathit{I}\overline{4}\mathit{c}2$			Δ
		x/a	y/b	z/c	x/a	y/b	z/c	x/a	y/b	z/c
La1	La	0	0	0.16351(1)	0	0	0.16343(1)	0	0	0.00008
La1i/La2		0.5	0	0.08649(1)	0.5	0	0.08639(1)	0	0	0.0001
Mo1		0.5	0	0.25	0.5	0	0.25	0	0	0
Mo1		0	0	0	0	0	0	0	0	0
Pr1	Pr	0	0	0.16311(1)	0	0	0.16318(1)	0	0	–0.00007
Pr1i/Pr2		0.5	0	0.08689(1)	0.5	0	0.08698(1)	0	0	–0.00009
Mo1		0.5	0	0.25	0.5	0	0.25	0	0	0
Mo1i/Mo2		0	0	0	0	0	0	0	0	0
Nd1	Nd	0	0	0.16294(1)	0	0	0.16323(1)	0	0	–0.00029
Nd1i/Nd2		0.5	0	0.08706(1)	0.5	0	0.08734(1)	0	0	–0.00028
Mo1		0.5	0	0.25	0.5	0	0.25	0	0	0
Mo1i/Mo2		0	0	0	0	0	0	0	0	0
O1	La	0.75	0.2409(2)	0.125	0.7443(1)	0.2411(1)	0.1249(1)	0.0057	–0.0002	0.0001
O2_1		0.3329(1)	0.1683(1)	0.2147(1)	0.3341(1)	0.1683(1)	0.2148(1)	–0.0012	0	–0.0001
O2_1ii/O2_2		0.6683(1)	0.3329(1)	0.4647(1)	0.6682(1)	0.3312(1)	0.5353(1)	0.0001	0.017	–0.0706
O1	Pr	0.75	0.2669(3)	0.125	0.7322(1)	0.2459(6)	0.1255(1)	0.0178	0.021	–0.0005
O2_1		0.3355(1)	0.1758(1)	0.2143(1)	0.3322(1)	0.1691(3)	0.2871(1)	0.0033	0.0067	–0.0728
O2_1ii/O2_2		0.6758(1)	0.3354(1)	0.4643(1)	0.6733(1)	0.3295(4)	0.4662(1)	0.0025	0.0059	–0.0019
O1	Nd	0.75	0.2567(1)	0.125	0.7524(1)	0.2567(1)	0.1249(1)	–0.0024	0	0.0001
O2_1		0.3348(1)	0.1757(1)	0.2859(1)	0.3314(1)	0.1679(1)	0.2859(1)	0.0034	0.0078	0
O2_1ii/O2_2		0.6757(1)	0.1757(1)	0.5359(1)	0.6793(1)	0.3337(1)	0.5359(1)	–0.0036	0.0011	0

Symmetry operations of I4₁/acd space group: (i) –x + ½, y, –z + ¼; (ii) y + ½, x, z + ¼. La2, Pr2, Nd2, Mo2, and O2_2 atoms correspond to I$\overline{4}$c2 space group. La1, Pr1, Nd1, Mo1, O1, and O2_1 atoms correspond to both space groups. The maximum displacements of oxygen atoms are highlighted in bold.

lists the coordinates of the Ln (La, Pr, Nd), Mo, and O atoms in three structures Ln₂MoO₆ in space groups I4₁/acd and I$\overline{4}$c2, as well as the coordinate difference calculated from the data of [24,25,26]. The arrangement of Mo atoms in both models of structures is the same, the coordinates of heavy Ln atoms are close, while oxygen atoms deviate significantly from their centrosymmetric positions. The minimum deviations of the coordinates of oxygen atoms in space group I$\overline{4}$c2 from their positions in non-polar space group I4₁/acd are observed for the Nd₂MoO₆ structure in comparison with similar values for the La₂MoO₆ and Pr₂MoO₆ structures (Table 2), which indicates that the high-temperature phase I4₁/acd of the Nd₂MoO₆ single crystal is presumably fixed at room temperature.

On the different Fourier maps of the electron density, peaks were found near the cation positions in the structure, which were not observed on the different Fourier maps of undoped Ln₂MoO₆ crystals. This allowed the authors of [24,25,26] to assume that such distribution of the residual electron density was associated with the incorporation of magnesium into the structure of the compounds. The presence of magnesium in the samples was additionally confirmed by scanning transmission electron microscopy and ICP-MS. Based on the data of electron microscopy, Mg cations were concluded to replace Mo cations in the structure. In the crystal structure of Ln₂MoO₆ oxymolybdates (Ln = La, Pr, Nd), impurity Mg atoms were localized near the positions of the Mo atoms Mo1 and Mo2 at a distance of ~0.3 Å. The peaks located on the difference Fourier maps near the Ln positions at distances of up to ~0.6 Å were interpreted as split positions of rare earth cations.

Structural models of the Ln₂MoO₆ (Ln = La, Pr, Nd) single crystals doped with Mg can be represented as superimposed lattices of the main matrix, Ln₂MoO₆, (Figure 10) and lattices in which rare earth cation positions are split, and Mo cations are replaced by Mg. Since the Mg concentration in the crystal is low, there are few such lattices. The environment of the main cationic positions corresponds to the environment in the undoped Ln₂MoO₆. The La, Pr, and Nd cations are located in the center of distorted cubes composed of O anions occupying the main positions; Mo atoms have a tetrahedral oxygen environment. The rare earth cations in the split positions are partially surrounded by oxygen atoms in the additional positions found. Surrounding the cations with oxygen anions located in low-occupancy positions leads to an even greater distortion of the shape of polyhedra in comparison with the undoped Ln₂MoO₆. The doping Mg atoms in the structures are surrounded by eight O anions at distances of 2.1–2.9 Å, including two O atoms in additional positions. Thus, the partial replacement of Мo⁶⁺ with Mg²⁺ leads to disordering of O atoms, decreasing in the occupancy of their positions and, as a consequence, to the appearance of oxygen vacancies in the structure of Mg-containing Ln₂MoO₆ oxymolybdates.

As shown above, significant weight loss on heating, as well as the corresponding absorption bands in the IR spectra, indicate the hygroscopic properties of Mg-doped oxymolybdates. It is known [35] that acceptor doping and the formation of oxygen vacancies in the structure leads to the dissolution of a significant amount of water from the gaseous atmosphere and the appearance of high-temperature proton conductivity as a functional property of the material. Consequently, there is a possibility of a proton component of the electrical conductivity of oxymolybdates of large rare earth metals with magnesium impurities in the structure.

4. Conclusions

Based on the analysis of the structures of single-crystal samples of Ln₂MoO₆ oxymolybdates (Ln = La, Pr, Nd), pure and doped with Mg, the motif of heavy atoms was found to be centrosymmetric in all compounds. The violation of the center of symmetry occurs due to displacements of the oxygen atoms surrounding the atoms of the rare earth element. Minimum deviations of the coordinates of oxygen atoms in space group I$\overline{4}$c2 from their positions in non-polar space group I4₁/acd are observed for the Nd₂MoO₆ structure compared to La₂MoO₆ and Pr₂MoO₆, which indicates that the high-temperature phase I4₁/acd of the Nd₂MoO₆ single crystal is presumably fixed at room temperature. It should also be noted that among the three molybdates with the rare-earth cations La, Pr, and Nd, only the Nd₂MoO₆ compound at room temperature has a monoclinic scheelite-type structure, which at a temperature of about 1010 °C transforms into the tetragonal phase. Since the transition from the monoclinic phase to the high-temperature tetragonal phase is irreversible, tetragonal oxymolybdates synthesized at 1000 °C and higher temperatures are in a metastable state at room temperature. In all structures of Mg-doped single-crystal samples of oxymolybdates Ln₂MoO₆ (Ln = La, Pr, Nd), Mg cations were localized near the positions of Mo atoms, and the splitting of the positions of rare earth atoms was revealed.

The hygroscopic properties of the studied oxymolybdates were confirmed by thermogravimetry and IR spectroscopy. The incorporation of magnesium into the structure of oxymolybdates enhances the hygroscopic properties of the samples. Taking into account the formation of oxygen vacancies in the structure upon partial substitution of Mg²⁺ atoms for Мo⁶⁺, the proton component of the conductivity of oxymolybdates of large rare earth metals is probable.