3
86
KOZHEVNIKOVA, KOPYLOVA
group (R = Dy–Lu, Y), the phase equilibria at 700°C
are characterized by the following quasibinary
sections: Li MoO –Li Ba R (MoO ) , Li R (MoO ) –
2
4
3
2
3
4 8
7
3
4 8
Li Ba R (MoO ) , BaMoO –Li Ba R (MoO ) , Li Ba R ·
3
2
3
4 8
4
3
2
3
4 8
3
2 3
(
MoO ) –LiR(MoO ) , Li Ba R (MoO ) –BaR (MoO ) ,
4 8 4 2 3 2 3 4 8 2 4 4
and BaR (MoO ) –LiR(MoO ) (Fig. 1b). Li Ba R ·
2
4 4
4 2
3
2
3
(
MoO ) incongruently melt in the temperature range
4 8
9
20–1015°C without undergoing any polymorphic
transformations.
According to X-ray diffraction data, the lithium–
barium–rare-earth molybdates Li Ba R (MoO ) belong
3
2
3
4 8
to the structural type of a monoclinically distorted
sheelite, sp.gr. C2/c, Z = 2. The compounds are iso-
structural with each other and with double molybdates
BaR (MoO ) (R = Ce–Dy [1, 4]). The structure of
2
4 4
these latter is constituted by honeycomb-like layers of
R-octagons, with Mo-tetrahedra connected to both
sides of a layer via common oxygen vertices. Lithium
atoms occupy different crystallographic positions in
the structure of the prototype BaR (MoO ) , one third
of lithium atoms are statistically situated in positions
of the rare-earth element with a coordination number
CN = 8. The remaining two thirds of lithium atoms are
localized in a partial position on a second-order axis,
with octahedral coordination with respect to oxygen. In
this case, these positions are occupied in an ordered or
partially ordered way, which does not lead to a new
superstructure, although a new structure does appear.
Fig. 2. Dependence of the electrical conductivity σ on the
temperature T, K. (1) Li Ba (MoO , (2) Li Ba Eu
MoO , (3) Li Ba Sm (MoO , and (4) Li Ba La (MoO
2
4 4
3
2
Y
8
3
4
)
8
3
2
3
·
) .
4 8
(
)
4 8
3
2
3
4
)
3
2
3
1
–1.5 mm thick, with platinum electrodes. The
electrodes were deposited by brazing a platinum paste.
The electrical properties were measured with an
E8-4 ac bridge at a frequency of 103 Hz. A P-5025
capacitance box was used, which extended the range in
which the dielectric loss tangent could be measured
A
variant of the cation distribution among
crystallographic positions (leading to Do = 2.10%) was
taken into account when we derived the crystallo-
graphic formula of the compounds Li (Ba0.85R ) ·
(
determination accuracy ±5%).
2
0.15 2
The dc current was measured with an E6-13A
teraohmmeter at a voltage of 30–50 mV.
(
R0.675Ba0.075Li ) (MoO4)8.
0.25 4
The crystallographic parameters of Li Ba R (MoO )
4 8
3
2
3
The reproducibility of measurement results was
verified under heating and cooling in the temperature
range 20–600°C. The sample temperatures were measured
in the course of our electrical studies with a chromel–
alumel thermocouple connected to a V7-21A volt-
meter, with an accuracy of ±2°.
are listed in the table. The unit cell volumes of the
phases steadily decrease in the order La–Lu, which is
due to manifestation of the “lanthanide” contraction in
the series of rare-earth elements.
The conductivity of the ternary molybdates
–
8
Li Ba R (MoO ) varies within the range from 10 to
3
2
3
4 8
As regards the nature of phase transformations, the
systems under study can be divided into two groups:
–2 –1
–1
1
0 Ω cm as temperature increases from 20 to 500°C,
with the conductivity mostly having ionic nature (ti =
.82–0.87). The temperature dependences of σ are
(
1) R = La–Tb and (2) R = Dy–Lu, Y, which is due to
0
the different phase compositions of the boundary
systems Li MoO –R (MoO ) . For the systems with
characterized by a single break, which probably
corresponds to transition from impurity conductivity to
that of the intrinsic type (Fig. 2).
2
4
2
4 3
R = La–Tb, the quasibinary sections are Li MoO –
2
4
Li Ba R (MoO ) , BaMoO –Li Ba R (MoO ) , BaR ·
3
2
3
4 8
4
3
2
3
4 8
2
–
2
–1
–1
(
MoO ) –Li Ba R (MoO ) , BaR (MoO ) –LiR (MoO ) ,
The conductivity of up to 10
Ω
cm is due to
4
4
3
2
3
4 8
2
4 4
5
4 8
Li Ba R (MoO ) –LiR (MoO ) , and Li Ba R (MoO ) –
the rather large size of rare-earth ions and voids in the
sheelite-like skeleton, which substantially reduces the
3
2
3
4 8
5
4 8
3
2
3
4 8
LiR(MoO ) (Fig. 1a). In the systems of the second
4
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 3 2011