Scheelite-Related Triple Molybdates KCaLn(MoO4)3 in K2MoO4–CaMoO4–Ln2(MoO4)3 (Ln = Nd, Sm, Eu, Gd) Systems: Synthesis and Characterization

Article abstract of DOI:10.1134/S0036023618020146

Subsolidus phase ratios in K₂MoO_4–CaMoO₄–Ln₂(MoO₄)₃ systems where Ln = Nd, Sm, Eu, and Gd, have been studied by vibrational spectroscopy, X-ray powder diffraction, and differential thermal analysis. Triple molybdates KCaLn(MoO₄)₃ (Ln = Nd, Sm, Eu, and Gd) have been synthesized; they have a scheelite-derivative structure, monoclinic crystal system (space group P21/n). Their unit cell parameters have been determined, and IR and Raman spectra characterized.

Full text of DOI:10.1134/S0036023618020146

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2018, Vol. 63, No. 2, pp. 157–161. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © N.M. Kozhevnikova, 2018, published in Zhurnal Neorganicheskoi Khimii, 2018, Vol. 63, No. 2, pp. 147–151.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Scheelite-Related Triple Molybdates KCaLn(MoO4)3
in К2MoO4–CaMoO4–Ln2(MoO4)3 (Ln = Nd, Sm, Eu, Gd)
Systems: Synthesis and Characterization
N. M. Kozhevnikova
Baikal Institute for Nature Management, Siberian Branch, Russian Academy of Sciences, Ulan-Ude, 670047 Russia
e-mail: nicas@binm.ru
Received May 10, 2016
Abstract—Subsolidus phase ratios in K2MoO4–CaMoO4–Ln2(MoO4)3 systems where Ln = Nd, Sm, Eu,
and Gd, have been studied by vibrational spectroscopy, X-ray powder diffraction, and differential thermal
analysis. Triple molybdates KCaLn(MoO4)3 (Ln = Nd, Sm, Eu, and Gd) have been synthesized; they have a
scheelite-derivative structure, monoclinic crystal system (space group P21/n). Their unit cell parameters have
been determined, and IR and Raman spectra characterized.
DOI: 10.1134/S0036023618020146
Scheelite molybdates and tungstates containing triple molybdates; and their characterization by X-ray
alkaline-earth and rare-earth elements have low ther- diffraction, differential thermal analysis, and vibra-
mal expansivities and high chemical and thermal sta- tional spectroscopy.
bilities. Owing to their tunable physicochemical and
optical characteristics, they find great application in
EXPERIMENTAL
optical devices, as laser materials, phosphors for light-
emitting diodes, catalysts, and solid electrolytes [1–12].
The initial components used to study phase forma-
tion in K2MoO4–CaMoO4–Ln2(MoO4)3 systems
were K2MoO4, CaMoO4, and Ln2(MoO4)3 samples
presynthesized by solid-phase reactions from K2CO3,
CaCO3, R2O3, and MoO3 at temperatures in the range
400–650°C with multiple intermittent grindings every
20–30 h. The calcination time at each temperature
was 100–120 h. After being calcined, samples were
slowly cooled with the furnace. Unequilibrated sam-
ples were annealed additionally; equilibrium was
regarded to be attained when the phase composition of
the sample remained unchanged upon two consecu-
tive anneals. Synthesis products were identified by
X-ray powder diffraction in an FR-552 monochroma-
tor chamber (CuKα radiation; Ge internal standard).
The X-ray diffraction patterns were calculated in the
Rentgen program.
The large extents of crystallization fields of
scheelite-related individual compounds and solid
solutions are due to the possibility of isovalent and
heterovalent substitutions of their alkaline-earth cat-
ions by cations of various characters and sizes at
framework and extra-framework sites. Extensive cat-
ionic isomorphism results in charge misbalance in the
scheelite structure due to the geometric features of
arrangement of nearest-neighbor polyhedra. It is the
appearing local and cooperative distortions that make
it possible to tune the properties of phases. These
structural specifics of scheelite-related molybdates are
responsible for their potential for use as functional
electronic materials, chemical sensors, and ion-
exchange materials, thereby stipulating extensive the-
oretical and experimental studies of this group of inor-
ganic compounds [3–12].
Vibrational spectra of polycrystalline KCaLn(MoO4)3
samples were recorded on a Bruker FT-IR spectrome-
ter and a Specord М-80 spectrometer with 1.06-nm
Of the triple molybdates containing alkaline-earth
and rare-earth elements, the series of barium and
strontium compounds M(Ba)SrLn(MoO4)3 (M = K,
Rb, or Cs; Ln = Nd, Sm, Eu, and Gd) are known [3, 4]. laser excitation in the near-IR range (back-scattering
geometry, resolution: 3–5 cm–1). Test samples were
Nujol mulls on a polyethylene substrate or KBr pellets.
Molybdenum isotope-substituted samples were pre-
Whether other combinations of metal cations are pos-
sible has not been analyzed.
The focuses of this study are on subsolidus phase
equilibria in K2MoO4–CaMoO4–Ln2(MoO4)3 sys- pared using 92MoO3 and 100MoO3 that contained at
tems where Ln = Nd, Sm, Eu, and Gd; synthesis of least 95% of the major isotope.
157

158
KOZHEVNIKOVA
K2MoO4 with CaMoO4 yielded K2Ca(MoO4)2, which
K2MoO4
has the palmierite structure. CaLn2(MoO4)4 molyb-
dates having large homogeneity extents and a mono-
clinically distorted scheelite-related structure (space
group С2/с) [3], were synthesized in CaMoO4–
Ln2(MoO4)3 systems.
K5Nd(MoO4)4
Interactions along CaMoO4–KLn(MoO4)2 sec-
tions were studied in most detail (in 1–2 mol %); for
Ln = Nd, Sm, Eu, and Gd, KCaLn(MoO4)3 triple
molybdates were formed at 650–750°C. Reaction
mixtures were calcined for 140–160 h with intermit-
tent homogenization for equilibration.
KNd(MoO4)2
K2Ca(MoO4)2
T
The quasi-binary sections in K2MoO4–CaMoO4–
Ln2(MoO4)3
systems
are:
KLn(MoO4)2–
KCaLn(MoO4)3, K2Ca(MoO4)2–KCaLn(MoO4)3,
K2Ca(MoO4)2–K5LnR(MoO4)4, K2Ca(MoO4)2–
KLn(MoO4)2, CaMoO4–KCaLn(MoO4)3,
CaLn2(MoO4)4–KCaLn(MoO4)3, and
CaMoO4
CaNd2(MoO4)4
Nd2(MoO4)3
T = KCaNd(MoO4)3
CaLn2(MoO4)4–KLn(MoO4)2. X-ray powder diffrac-
tion showed that a KCaLn(MoO4)3 triple molybdate
dissolves 2–3 mol % CaMoO4; the solubility of a triple
molybdate in CaMoO4 is 10–12 mol %. In
CaLn2(MoO4)4, no more than 1.5–2 mol % of a triple
molybdate is dissolved. The solubility of KLn(MoO4)2
in KCaLn(MoO4)3 is 4–5 mol %; KLn(MoO4)2 dis-
solves no more than 2–3 mol % KCaLn(MoO4)3.
Palmierite continuous solid solutions between the ini-
tial components are identified along the
Fig. 1. 750°C isothermal section of the K MoO –
2
4
CaMoO –Nd (MoO ) system.
4
2
4 3
Differential thermal analysis experiments were per-
formed on an MOM OD-103 derivatograph (tempera-
ture elevation rate: 10 K/min; sample size: 0.3–0.4 g).
RESULTS AND DISCUSSION
Interactions in K2MoO4–CaMoO4–Ln2(MoO4)3
systems were studied by the crossing sections method
in the range 450–800°C. The studies of the K2MoO4–
CaMoO4–Ln2(MoO4)3 systems comprised two stages.
First, phase compositions were studied at the intersec-
tion points of the sections that radiated from the nor-
mal and double molybdates formed in the boundary
binaries. The second stage involved the study of the
sections that turned to be quasi-binary up to 750°C in
order to triangulate the systems (Fig. 1). In view of the
similarity of phase ratios in the studied systems, we
will not describe each of them; Fig. 1 shows phase
ratios in the K2MoO4–CaMoO4–Nd2(MoO4)3 system
by the way of example.
K2Ca(MoO4)2–K5Ln(MoO4)4
sections.
The
K2Ca(MoO4)2–KLn(MoO4)2 sections are simple
eutectics with component solubility of up to 2 mol %.
Rare-earth molybdates KCaLn(MoO4)3 have a
monoclinically distorted scheelite structure, space
group P21/n, Z = 9. The compounds are isostructural
to each other and to the double molybdate
α-KSm(MoO4)2 and KBaLn(MoO4)3 triple molyb-
dates where Ln = La–Lu or Y [2, 3]. The
KCaLn(MoO4)3 structure may be regarded as the
product of heterovalent substitution of the alkaline-
earth element in the scheelite structure by the scheme
2Ca = K+ + Ln3+. Table 1 lists the crystallographic
The boundary binary K2MoO4–Ln2(MoO4)3 was and thermal characteristics of KCaLn(MoO4)3, which
found to form 1 : 1 and 5 : 1 molybdates, which crys- melt incongruently at temperatures in the range 1020–
tallize in different structural types [1]. Reacting 985°С without experiencing polymorphic transitions
Table 1. Crystallographic and thermal characteristics of KCaLn(MoO4)3
Unit cell parameters
Melting
point, °С
V/z, Å3
Compound
a, Å
b, Å
c, Å
β, deg
KCaNd(MoO4)3
KCaSm(MoO4)3
KCaEu(MoO4)3
KCaGd(MoO4)3
12.178(2)
12.174(1)
12.166(3)
12.161(2)
11.689(3)
11.467(2)
11.253(1)
11.132(3)
16.798(2)
16.783(2)
16.761(3)
16.751(4)
95.66(3)
95.72(1)
95.48(2)
94.91(4)
264.1
259.2
254.1
252.2
1020
1005
998
985
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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SCHEELITE-RELATED TRIPLE MOLYBDATES
Table 2. Indexing of the X-ray diffraction pattern for KCaGd(MoO4)3
159
2
2
× 10–4
× 10–4
I
d, Å
h
k
l
dobs
dcalc
20
10
5
10
5
3.32501
5.06147
4.91822
4.83731
3.88094
4.44155
3.28750
3.21702
3.05533
3.4805
144.04
0
0
2
3
3
144.02
389.03
418.10
427.94
666.0
844.0
924.0
975.1
1073.5
1078.1
1089.1
1088.5
1297.0
1413.0
1460.1
390.30
413.45
427.41
664.5
844.3
925.5
0
10
0
2
2
3
0
3
3
4
4
4
2
1
0
2
2
1
1
3
0
1
0
0
0
3
1
2
–2
–4
–3
3
–4
3
–1
0
2
2
100
100
5
80
5
966.0
1071.6
1076.9
1090.0
3.02868
40
50
2
3
5
2.77749
2.66123
2.61687
2.59354
2.57586
1297.0
1412.1
1460.8
–4
–6
1
1507.1
1
4
1
4
4
0
4
1
–3
3
3
1507.5
1508.0
1559.4
1587.1
1585.2
1651.5
1652.0
1729.4
1736.0
1757.1
1764.5
1772.5
184.6
2035.1
2106.2
2196.3
2217.1
2572.6
2487.2
2587.0
2659.6
2788.8
3000.2
3080.0
3121.0
3146.6
3243.6
3341.0
3380.1
3476.8
3594.0
3651.6
3709.2
10
1
2.53235
2.51098
1560.1
1586.0
–4
1
2.46090
1651.0
3
3
3
1
5
5
5
5
3
3
2
2
0
6
2
3
4
1
1
2
0
5
4
7
6
7
2
1
3
2
3
0
0
1
1
1
4
1
1
4
3
0
4
0
3
0
5
6
6
4
2
1
1
2
6
7
3
–5
–4
–7
–2
–1
0
0
–2
6
7
–5
7
–2
–6
–8
5
5
2
10
10
15
3
5
5
5
10
90
60
3
3
60
1
50
50
10
1
1
2
2.41070
2.40031
2.38644
2.38109
2.37393
2.32763
2.21680
2.18068
2.13440
2.12430
2.05343
2.00434
1.96637
1.93903
1.89350
1.82593
1.80190
1.79083
1.78263
1.75564
1.73008
1.72017
1.69595
1.66765
1.65458
1.64212
1720.0
1736.0
1757.1
1764.1
1774.0
1845.0
2035.1
2105.1
2196.2
2217.1
2373.1
2489.2
2586.2
2660.1
2789.1
2999.4
3080.0
3118.0
3146.9
3244.4
3340.9
3379.5
3477.0
3594.0
3652.8
3709.1
9
–6
–8
4
–4
7
–1
–6
–1
–5
–2
70
60
1
50
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 2 2018

160
Table 3. Vibration frequencies (cm–1) in Raman and IR spectra of KCaLn(MoO4)3 and CaMoO4
KCaNd(MoO4)3 KCaSm(MoO4)3 KCaGd(MoO4)3 CaMoO4
KOZHEVNIKOVA
raman
IR
Assignment
raman
IR
raman
IR
raman
IR
92Mo
100Mo
92Mo
927
100Mo
927
947
928
886
869
945
928
886
869
932
919
896
868
934
915
895
936
879
918
ν1
855
841
823
806
783
758
846
704
840
698
837
816
804
774
749
736
843
842
813
803
771
748
737
705
848
813
836
817
796
768
748
801
760
794.5
ν3
738
686
706
705
682
456
418
386
354
474
430
427
375
426
375
403.5
392
431
329
ν2
384
385
366
383
365
372
384
ν4
ν2
ν4
331
322
327
300
323.5
329
309
325
305
322
321
320
309
331
320
286
229
286
285
228
285
224
284
237
284
262
284
262
269
205
210
189
205
R
T
200
187
187
191.5
144.5
177
155
202
170
144
170
168
157
138
132
125
157
138
–
146
140
145
139
129
126
103
149
137
131
153
R
R
R
177
155
128
105
125
127
78
102
112
Т
T
101
77
(Fig. 2). Table 2 shows the results of indexing the vibrations (the librations (R) in MoO4 group and the
KCaGd(MoO4)3 X-ray diffraction pattern. The mea- translational vibrations (T) in various subslattices of
the crystal).
sured IR and Raman vibrational frequencies in
KCaLn(MoO4)3 spectra are shown in Table 3 and in
Fig. 3. Here are also shown the vibrational frequencies
for CaMoO4 and their assignment to the internal
vibrations in MoO4 groups and the crystal-lattice
The data shown indicate the similarity of CaMoO4
and KCaLn(MoO4)3 vibrational spectra, thereby mak-
ing possible satisfactory interpretation. The differ-
ences are explainable by the changes in interatomic
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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SCHEELITE-RELATED TRIPLE MOLYBDATES
161
the complete removal of degeneration from the triply
degenerate vibrations and the manifestation of reso-
nance interactions of vibrations in translationally non-
equivalent MoO4 groups [13].
T, °C
T
1000
DTG
The measured spectra have an increased number of
librations of MoO4 groups, which are IR-active. Their
frequencies are insensitive to molybdenum isotope
substitution, are almost unchanged along the series of
the compounds studied, and are close to the respective
frequencies in CaMoO4. The observed number of
translational vibrations corresponds to the number
expected from theory. The relevant frequencies in
some case experience the isotopic effect and undergo
systematic modification upon cationic substitutions.
The data we gained can serve as evidence that all cat-
ions are equally involved in the translational vibrations
of KCaLn(MoO4)3. The observed lowering of crystal
symmetry upon heterovalent cationic substitutions in
the scheelite structure is due to distortions of molyb-
denum–oxygen tetrahedra.
800
600
400
200
DTA
1020
Time
0
5
TG
Δm, mg
Fig. 2. Derivative thermal curves for KCaGd(MoO ) .
4 3
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Translated by O. Fedorova
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 63 No. 2 2018