crystals
Article
Structure and Properties of Ln
2
MoO
6
Oxymolybdates
(Ln = La, Pr, Nd) Doped with Magnesium
Ekaterina Orlova
1
, Elena Kharitonova
1
, Timofei Sorokin
2
, Alexander Antipin
2
, Nataliya Novikova
2
,
Nataliya Sorokina
2
and Valentina Voronkova
1,
*
Citation: Orlova, E.; Kharitonova, E.;
Sorokin, T.; Antipin, A.; Novikova, N.;
Sorokina, N.; Voronkova, V. Structure
and Properties of Ln
2
MoO
6
Oxymolybdates (Ln = La, Pr, Nd)
Doped with Magnesium. Crystals
2021, 11, 611. https://doi.org/
10.3390/cryst11060611
Academic Editor: Daniel Errandonea
Received: 6 May 2021
Accepted: 26 May 2021
Published: 28 May 2021
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1
Faculty of Physics, Moscow State University, Moscow 119991, Russia; agapova@polly.phys.msu.ru (E.O.);
harit@polly.phys.msu.ru (E.K.)
2
Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics”,
Russian Academy of Sciences, Moscow 119333, Russia; tim29@inbox.ru (T.S.); antipin@physics.msu.ru (A.A.);
nata110565@mail.ru (N.N.); nsor@ns.crys.ras.ru (N.S.)
* Correspondence: voronk@polly.phys.msu.ru; Tel.: +7-495-939-28-83
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 Ln
2
MoO
6
(Ln = La, Pr, Nd) are analyzed. Presumably, the high-temperature phase
I4
1
/acd of Nd
2
MoO
6
single crystals is retained at room temperature. The reason for the loss of the
center of symmetry in the structures of La
2
MoO
6
and Pr
2
MoO
6
and the transition to the space group
I 4c2 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 Ln
2
MoO
6
(Ln = La, Pr, Nd), pure and with an impurity of Mg, confirm
their hygroscopic properties.
Keywords: oxymolybdates; rare-earth elements; crystal structure; X-ray diffraction; growth from
flux; solid-state reactions; thermogravimetry; infrared spectroscopy
1. 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 inten-
sively studied [1–6]. 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 discov-
ered [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 [1–6]. Oxymolybdates and oxytungstates are chemically stable
and have high refractive indices. These compounds doped with Eu exhibit intense lumines-
cence [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
[1,4,14–17].
Already in the first and subsequent structural studies, polymorphism in these com-
pounds was revealed [1,2,4,14–20]. Oxytungstates were synthesized in two different mon-
oclinic 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),
Crystals 2021, 11, 611. https://doi.org/10.3390/cryst11060611 https://www.mdpi.com/journal/crystals