938
KOZHEVNIKOVA, KOPYLOVA
–1
of atoms of three cationic sublattices and are described 90 cm was assigned to the vibrations of the barium subꢀ
by the following irreducible representations: lattice. The translational vibrations of molybdenum are
for lithium cations at the axes of the second order mixed with the vibrations of the atoms of gadolinium
–
–
1
(
in octahedra),
ГLi = 1Ag + 2Bg + 1Au + 2Bu
for barium and rareꢀearth cations at the axes of
the second order (position M1),
ГМ1 = 1Ag + 2Bg + 1Au + 2Bu
for lithium, barium, and rareꢀearth cations in
sublattice and have frequencies of 265 and 186 cm .
In the longꢀwave range of the spectra of the comꢀ
pounds studied, two more bands are observed at 307–
97 and 263–258 cm ; the bands demonstrate the
isotope effect. Assignment of the vibration bands of
atoms of the alkali metal sublattices was performed
based on the band shifts after lithium isotope substituꢀ
tion or the substitution of sodium for lithium. The
greatest shifts were found for two groups of absorption
bands at 325–319, 307–297, 290–286, and 263–258,
,
–
–1
2
,
–
general position (position M2 in eightꢀapix polyheꢀ
dra),
–1
ГМ2 = 3Ag + 3Bg + 3Au + 3Bu
.
232–227 cm .
The first group of bands was assigned to the vibraꢀ
tions of lithium cations in octahedral positions, and the
second one to those in the eightꢀvertex polyhedra. It is
precisely this range where two additional (compared
These vibrations are active in IR or Raman absorption
spectra in compliance with the alternative forbiddenness
–1
rule and should have frequencies below 500 cm . In
addition, 44 bands assigned to deformation, libraꢀ
tional, and translational vibrations of MoO4 groups
should be observed in the same spectrum range.
The IR spectra of the compounds studied have no disꢀ
withBaLn (MoO ) absorption bands are observed, and
2
4 4
this is in good agreement with the known literature data
on the IR spectra of LiLn (MoO ) [4].
2
4 2
–1
tinctions in their highꢀfrequency range (950–650 cm ).
On isomorphic substitution of a sodium atom for a
The absorption bands of the stretching vibrations of lithium one in a unit cell, two new absorption bands at
–1
MoO4 groups are poorly resolved in this spectrum 216 and 204 cm appear in the IR spectra. These
range. This is caused by the aboveꢀdescribed cation wavenumbers correspond to sodium cation vibrations
distribution over the positions and the corresponding in eightꢀvertex polyhedra. However, the bands at 263–
–1
statistics on distortions of tetrahedral anions.
258 and 232–227 cm still remain in the spectra at the
In the range of the deformation vibrations of МоО4 same positions. At the same time, the intensity of the
–1
groups and the vibrations of the cationic sublattices band at 352–350–344 cm decreases, and the inteꢀ
–1
(
below 500 cm ), the spectrum of Li Ba Tm (MoO )
gral intensity of the appearing group of bands at 350–
3
2
3
4 8
–1
contains up to 14 absorption bands, much less than 320 cm increases, which confirms that lithium and
was expected. These distinctions between the results of sodium cations are distributed over different crystalloꢀ
the groupꢀtheory analysis and the experimental numꢀ graphic positions.
ber of vibrations are likely due to a factorꢀgroup splitꢀ
ting of inner (deformation) and outer vibrations of
МоО4 groups. At the same time, the spectra of the
REFERENCES
compounds under consideration in this range are very
1. V. K. Trunov, V. A. Efremov, and Yu. A. Velikodnyi,
Crystallochemistry and Properties of Double Molybdates
and Tungstates (Nauka, Moscow, 1986) [in Russian].
close to those of BaLn (MoO ) double molybdates.
2
4 4
This indicates that the vibrations of the cationic subꢀ
lattices are close to each other and to the deformation
vibrations of MoO4 tetrahedra.
The frequencies of the outer vibrations of MoO4
groups and the vibrations of the sublattices of heavy
cations are about the same values as in case of
2
. N. M. Kozhevnikova and M. V. Mokhosoev, Triple
Molybdates (Izd–vo Buryatsk. Gos. Univ., UlanꢀUde,
2000) [in Russian].
3
. I. I. Kiseleva, M. I. Sirota, R. P. Ozerov, et al., Kristalꢀ
lografiya 24 (6), 1277 (1979).
BaLn (MoO )
The bands at 164 and 156 cm were assigned to the
librational vibrations of MoO4 groups, and the band at
.
2
4 4
4. K. I. Petrov, M. E. Poloznikova, Kh. T. Sharipov, and
V. V. Fomichev, Vibration Spectra of Molybdates and
Tungstates (FAN, Tashkent, 1990) [in Russian].
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 6 2011