INFLUENCE OF CATIONS ON THE VIBRATIONAL SPECTRA AND STRUCTURE
427
z+
tially covalent bond with the M cation. The ∆k (γ) quency increases steadily with the electronegativity of
M
z+
the M element within a given group of the Periodic
value depends on the electronegativity γ of the M cat-
Table. In addition, the vibrational frequency of the
ion. The atomic mass m* in (2) can be written as m* =
2
–
z+
m + ∆m (γ), where ∆m (γ) takes into account the cor- [WO
]
4
anions depends on the atomic mass of M ,
O
M
M
z+
which can be understood in terms of the cation environ-
ment of the [WO ] complexes in tungstate melts. If
the melt contains several types of M cations, the cat-
ion environment distorts the cubic symmetry character-
rection for the dynamic M –O bonding. The value of
2
–
z+
∆
m (γ) depends on both the atomic mass of the M
cation and the M –O bond energy, which can be esti-
mated from the electronegativity of M . Thus, partially
covalent bonding of the M cation will influence both
the numerator and the denominator in Eq. (2). There-
4
M
z+
z+
z+
2
–
z+
istic of a free [WO ] complex.
4
z+
fore, an increase in the electronegativity of M may be
ACKNOWLEDGMENTS
accompanied by a rise in ν because of the increase in
1
This work was supported by the Russian Foundation
for Basic Research, project no. 01-02-16098.
numerator in (2). At the same time, this increase may be
slowed down because of the changes in m* in the
denominator in (2).
These observations shed some light on the origin of
REFERENCES
the observed variation of ν (A ) with γ (Fig. 4b). For
1
1
1. Janz, G. and James, D.W., Raman Spectra and Ionic
Interactions in Molten Nitrates, J. Chem. Phys., 1961,
vol. 35, no. 2, pp. 739–744.
2. Child, W.C., Begun, G.M., and Smith, D.H., Raman
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Soc., Faraday Trans., 1981, vol. 2, pp. 2237–2247.
3. Brooker, M.H. and Breding, M.A., Significance of Both
Polarizability and Polarizing Power of Cations in Nitrate
Vibrational Spectra, J. Chem. Phys., 1973, vol. 58,
no. 12, pp. 5319–5321.
. Walrafen, G.E., Raman Spectra of Molten Sulfates, J.
Chem. Phys., 1965, vol. 43, no. 2, pp. 479–482.
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trum and Vibrational Assignment for Molten Thallous
Nitrates, J. Chem. Phys., 1963, vol. 39, no. 7,
pp. 1809−1812.
6. Voron’ko, Yu.K., Kudryavtsev, A.B., Osiko, V.V., and
Sobol’, A.A., High-Temperature Raman Scattering
Studies of Melt Structure and Crystallization Processes,
Rost Krist., 1988, vol. 16, pp. 178–195.
. Lazarev, A.N., Mirgorodskii, A.P., and Ignat’ev, I.S.,
Kolebatel’nye spektry slozhnykh okislov (Vibrational
Spectra of Mixed Oxides), Moscow: Nauka, 1975,
p. 210.
8. Basiev, T.T., Sobol, A.A., Zverev, P.G., et al., Compara-
tive Spontaneous Raman Spectroscopy of Crystals for
Raman Lasers, Appl. Opt., 1999, vol. 38, no. 3,
pp. 594−598.
molten tungstates of alkali and alkaline-earth metals
with low γ values, the correction to m* is insignificant,
and the linear rise in ν (A ) with γ is mainly due to the
1
1
z+
effect of the partially covalent bonding of the M cat-
ion on the force constants of the fully symmetric mode.
+
2+
In going from Cs to Mg , ν (A ) increases from 888
1
1
–
1
to 962 cm . Further rise in ν (A ) with γ (rare-earth and
1
1
zinc tungstates) is impossible because of the correction
of m* in the denominator in Eq. (2). Finally, for heavy
4
z+
M cations with relatively high values of γ (Cd, Pb, and
Bi), ν (A ) decreases with increasing γ because of the
1
1
5
z+
increase in m* (Fig. 4b). The M cation may have a
2
–
similar effect on the ν (A ) frequency of the [WO ]
1
1
4
complex in crystalline tungstates containing isolated
2
–
[
WO ] groups, in which the crystal-field and Davydov
4
splitting effects are weak. For example, we observed a
–
1
monotonic increase in ν (A ) (in the range 916–941 cm )
1
1
for crystalline tungstates (Cs–Li), just as in the melts.
7
Among the tungstates studied, the highest ν (A ) fre-
1
1
–
1
quency (975 cm ) was observed in molten and crystal-
line (α form) MgWO (Fig. 2). The spectrum of crys-
4
–
1
talline PbWO showed a low ν (A ) frequency, 905 cm ,
4
1
1
characteristic of melts containing heavy cations with
relatively high electronegativity values.
Note that the effect of the electronegativity of Mz+
9
. Basiev, T.T., Sobol, A.A., Zverev, P.G., et al., Raman
Spectroscopy of Crystals for Stimulated Raman Scatter-
ing, Opt. Mater., 1999, vol. 11, no. 3, pp. 307–314.
cations on the ν (A ) frequency may be obscured
1
1
severely by Davydov splitting, which takes place in a
number of crystalline tungstates. In particular, in the
case of BaWO , SrWO , and CaWO , ν (A ) was
1
1
0. Basiev, T.T., Sobol, A.A., Voronko,Yu.K., and Zverev, P.G.,
Spontaneous Raman Spectroscopy of Tungstates and
Molybdates Crystals for Raman Lasers, Opt. Mater.,
4
4
4
1
1
observed to increase in going from Ba to Ca (with
increasing γ) for melts and to decrease for crystalline
materials [11].
2
000, vol. 15, no. 8, pp. 205–216.
1. Zverev, P.G., Basiev, T.T., Sobol’, A.A., et al., Stimu-
lated Raman Scattering Study of Alkali Tungstate Crys-
tals, Kvantovaya Elektron. (Moscow), 2000, vol. 30,
no. 1, pp. 55–59.
CONCLUSIONS
z+
The nature of the M cation has a significant effect 12. Porto, S.P.S. and Scott, J.F., Raman Spectra of CaWO4,
on the frequency of the fully symmetric mode ν (A ) of
1
1
SrWO , CaMoO , and SrMoO , Phys. Rev., 1967,
4 4 4
2
–
isolated [WO4] anions in tungstate melts. This fre-
vol. 157, no. 3, pp. 716–719.
INORGANIC MATERIALS Vol. 41 No. 4 2005