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

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 I4¯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.


Introduction
During decades, tungstates, molybdates, and binary systems of the Ln 2 O 3 -Mo(W)O 3 type, especially in the concentration range 25-50 mol% of Ln 2 O 3 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 (Ln 5 Mo 3 O 16 ) [9] has the same high conductivity. Finally, 1:1 compounds (Ln 2 Mo(W)O 6 ), 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 Ln 2 Mo(W)O 6 existing in binary systems Ln 2 O 3 -Mo(W)O 3 [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 Ln 2 MoО 6 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 Nd 2 WO 6 type was synthesized, which remained monoclinic when heated to 1400 • C [1,2,4,15]. Scheelite-like structures of the Nd 2 WO 6 -type for oxymolybdates Ln 2 MoO 6 (Ln = Nd-Tb) have been studied many times [15,17,19]. Among Ln 2 MoO 6 molybdates with large cations (Ln = La-Nd), only Nd 2 MoO 6 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 La 2 MoO 6 single crystals obtained at 800 • C was studied for the first time. The authors revealed their tetragonal layered structure, space group I42m, and unit cell parameters a = 4.089, c = 15.99 Å. No monoclinic phase was found.
According to [16,17], the space group of Ln 2 MoO 6 (Ln = La, Nd) single crystals obtained by cooling from 1250 to 900 • C is centrosymmetric I4 1 /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 I4c2 was chosen, which is a subgroup of the space group I4 1 /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 La 2 MoO 6 or Nd 2 MoO 6 consists of two layers of LaO 8 or NdO 8 polyhedra and a layer of isolated MoO 4 tetrahedra. The conductivity of oxymolybdates does not exceed 10 -4 S cm -1 [23]. Thermogravimetry shows that molybdates Ln 2 MoO 6 (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 (Nd 2 MoO 6 ) (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 I42m and high-temperature I4 1 /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 (Ln 2 MoO 6 ) (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 Ln 2 MoO 6 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.

Materials and Methods
Polycrystalline samples were prepared by solid-state reactions in air in ternary systems MgO-Ln 2 O 3 -MoO 3 (Ln = La, Pr, Nd) in accordance with the scheme (MgO) x (Ln 2 O 3 ) y (MoO 3 ) z (x, y, z are the molar fractions of oxides, x + y + z = 1, y = z = (1 -x)/2). These compositions correspond to the Ln 2 MoO 6 -MgO binary join of the above ternary system, (MgO) x (Ln 2 MoO 6 ) (1-x)/2 , x = 0, 0.03, 0.05, 0.10. High purity (99.9%) reagents of magnesium oxide MgO and rare earth oxides La 2 O 3 , Pr 2 O 3 , and Nd 2 O 3 , 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 Pr 2 MoO 6 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 Pr 2 MoO 6 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 Ln 2 MoO 6 (Ln = La, Pr, Nd) were prepared by flux growth in the Li 2 O-MoO 3 -Ln 2 O 3 , Ln = La, Pr, Nd, ternary systems from hightemperature melts with a composition of 12.5 mol% Ln 2 O 3 , 30 mol% Li 2 O, and 57.5 mol% MoO 3 . 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 Pr 2 MoO 6 single crystal, no domains and block boundaries were found ( Figure 2b). The chemical composition of Mg-doped Nd 2 MoO 6 and Pr 2 MoO 6 was determine in [24,25] using energy dispersive X-ray spectrometry. More precisely, the low concentration of magnesium in Pr 2 MoO 6 single crystals was determined by inductively coupled plasmamass-spectrometry (ICP-MS technique) [25].
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: where M is the molecular weight (g), Z is the number of formula units, V is the volume of the unit cell (Å 3 ). 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 • C in air at a heating and cooling rate of 10 • C 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 Ln 2 MoO 6 (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].

Thermogravimetry
In [24][25][26], the hygroscopic properties of pure and Mg-containing compounds Ln 2 MoO 6 (Ln = La, Pr, Nd) were found. In Figures 7 and 8, TG curves of preliminary hydrated polycrystalline and single-crystal Ln 2 MoO 6 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 singlecrystal 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 (Ln 2 MoO 6 ) 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.

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 (Ln 2 MoO 6 ) (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 Mgcontaining 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 Н 3 О + . Thus, it can be concluded that in the studied phases of oxymolybdates, oxygen-hydrogen groups are Н 3 О + .

Crystal Structure
X-ray diffraction analysis of Mg-doped Ln 2 MoO 6 (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 I4 1 /acd for Mg-containing samples of neodymium oxymolybdate [24] and in space group I4c2 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 I4c2 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 lists the coordinates of the Ln (La, Pr, Nd), Mo, and O atoms in three structures Ln 2 MoO 6 in space groups I4 1 /acd and I4c2, 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 I4c2 from their positions in non-polar space group I4 1 /acd are observed for the Nd 2 MoO 6 structure in comparison with similar values for the La 2 MoO 6 and Pr 2 MoO 6 structures (Table 2), which indicates that the high-temperature phase I4 1 /acd of the Nd 2 MoO 6 single crystal is presumably fixed at room temperature. Table 2. Atomic coordinates in the structure of Ln 2 MoO 6 single crystals in space groups I4 1 /acd and I4c2 and their difference ∆.

Atom
Ln 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 Ln 2 MoO 6 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  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.

Conclusions
Based on the analysis of the structures of single-crystal samples of Ln 2 MoO 6 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 I4c2 from their positions in non-polar space group I4 1 /acd are observed for the Nd 2 MoO 6 structure compared to La 2 MoO 6 and Pr 2 MoO 6 , which indicates that the high-temperature phase I4 1 /acd of the Nd 2 MoO 6 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 Nd 2 MoO 6 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 Ln 2 MoO 6 (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 Mg 2+ atoms for Мo 6+ , the proton component of the conductivity of oxymolybdates of large rare earth metals is probable.
Author Contributions: Synthesis of polycrystalline samples and IR spectroscopy, writing-original draft preparation, E.O.; TG and DSC analysis; analysis, validation, E.K.; X-ray phase analysis, T.S.; X-ray diffraction analysis, visualization, A.A.; X-ray diffraction analysis, visualization, translation, data curation, N.N.; X-ray diffraction analysis, writing-original draft preparation, supervision, N.S.; crystal growth by the flux method, writing-original draft preparation, as well as review and editing, project administration, V.V. All authors have read and agreed to the published version of the manuscript.

Funding:
The following funding is acknowledged: Russian Foundation for Basic Research (award No. 18-29-12005). This work was performed using the equipment of the Shared Research Center FSRC "Crystallography and Photonics" RAS and was supported by the Ministry of Science and Higher Education of the Russian Federation within the State assignment FSRC "Crystallography and Photonics" RAS with respect to the X-ray phase and structure analyses and by the Russian Foundation for Basic Research with respect to the synthesis of the crystals and the study of their thermogravimetric and spectroscopic properties.