Citation: Popov, V.V.; Zubavichus,
Y.V.; Menushenkov, A.P.; Yastrebtsev,
A.A.; Gaynanov, B.R.; Rudakov, S.G.;
Ivanov, A.A.; Dubyago, F.E.;
Svetogorov, R.D.; Khramov, E.V.; et al.
Features of the Phase Preferences,
Long- and Short-Range Order in
Ln
2
(WO
4
)
3
(Ln = Gd, Dy, Ho, Yb)
with Their Relation to Hydration
Behavior. Crystals 2022, 12, 892.
https://doi.org/10.3390/
cryst12070892
Academic Editors: Maria Milanova
and Martin Tsvetkov
Received: 4 May 2022
Accepted: 17 June 2022
Published: 23 June 2022
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crystals
Article
Features of the Phase Preferences, Long- and Short-Range Order
in Ln
2
(WO
4
)
3
(Ln = Gd, Dy, Ho, Yb) with Their Relation to
Hydration Behavior
VictorV. Popov
1,2,
* , YanV. Zubavichus
3,
* , Alexey P. Menushenkov
1
, Alexey A. Yastrebtsev
1
,
Bulat R. Gaynanov
1
, Sergey G. Rudakov
1
, Andrey A. Ivanov
1
, Fyodor E. Dubyago
1
,
Roman D. Svetogorov
2
, EvgenyV. Khramov
2
, Nadezhda A. Tsarenko
4
, NataliyaV. Ognevskaya
4
and IgorV. Shchetinin
5
1
Department of Solid State Physics and Nanosystems, National Research Nuclear University MEPhI (Moscow
Engineering Physics Institute), 115409 Moscow, Russia; apmenushenkov@mephi.ru (A.P.M.);
alexyastrebtsev@gmail.com (A.A.Y.); brgaynanov@gmail.com (B.R.G.); sgrudakov@mephi.ru (S.G.R.);
andrej.ivanov@gmail.com (A.A.I.); walrus9f@gmail.com (F.E.D.)
2
Kurchatov Synchrotron Radiation Source, National Research Center Kurchatov Institute,
123182 Moscow, Russia; rdsvetov@gmail.com (R.D.S.); evxramov@gmail.com (E.V.K.)
3
Synchrotron Radiation Facility SKIF, Boreskov Institute of Catalysis SB RAS, 630559 Koltsovo, Russia
4
JSC Design & Survey and Research & Development Institute of Industrial Technology,
115409 Moscow, Russia; nadatsar@gmail.com (N.A.T.); ognevskayanv@mail.ru (N.V.O.)
5
Material Science Department, National University of Science and Technology MISiS, 119049 Moscow, Russia;
ingvvar@gmail.com
* Correspondence: vvpopov@mephi.ru (V.V.P.); yvz@catalysis.ru (Y.V.Z.)
Abstract: The effect of synthesis conditions on the features of the long- and short-range order of
Ln
2
(WO
4
)
3
(Ln = Gd, Dy, Ho, Yb) powders synthesized via coprecipitation of salts has been studied
by a complex of physico-chemical techniques including synchrotron X-ray powder diffraction, X-ray
absorption spectroscopy, Raman and infrared spectroscopy, and simultaneous thermal analysis. It was
found that crystallization of amorphous precursors begins at 600
◦
C/3 h and leads to the formation
of the monoclinic structure with sp. gr. C12/c1(15) for Ln
2
(WO
4
)
3
(Ln = Gd, Dy) and with sp. gr.
P12
1
/a1(14) for Ln = Yb, whereas crystallization of Ho precursor requires even higher temperature.
After annealing at 1000
◦
C, the P12
1
/ a1(14) phase becomes the dominant phase component for all
heavy lanthanoid types except for Ln = Gd. It was shown that the Ln (Ln = Dy, Ho, and Yb) tungstates
with the P12
1
/ a1(14) monoclinic structure correspond to trihydrates Ln
2
(WO
4
)
3
·3H
2
O formed due to
a rapid spontaneous hydration under ambient conditions. It was concluded that the proneness to
hydration is due to a specific structure of the P12
1
/ a1(14) phase with large voids available to water
molecules. Modifications in the local structure of Ln-O coordination shell accompanying the structure
type change and hydration are monitored using EXAFS spectroscopy.
Keywords: lanthanoid tungstates; phase transitions; hydration; Synchrotron XRD; X-ray Absorption
Fine Structure (XAFS); simultaneous thermal analysis; raman spectroscopy; FT-IR spectroscopy
1. Introduction
The lanthanoid (Ln) tungstates present a wide group of inorganic complex oxides
of transition metal and rare earth elements, which attract great interest both for basic
science (great chemical flexibility, rich polymorphism and phase transformations, high
ionic conductivity, etc.), and for practical uses (from electronics to biology) due to their
multifunctional properties (optical, electronic, luminescence and so on) [1–4]. The most
typical Ln tritungstates Ln
2
(WO
4
)
3
with the 2:3 lanthanoid:tungsten atomic ratio have been
known for many decades [5–7]. In particular, a considerable interest in the Ln
2
(WO
4
)
3
Crystals 2022, 12, 892. https://doi.org/10.3390/cryst12070892 https://www.mdpi.com/journal/crystals