ARTICLE IN PRESS
M. Isobe et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 888–889
889
All samples of K2V8ꢀxTixO16 ð0pxp8Þ are obtained as
a single phase with a simple tetragonal hollandite structure
at room temperature. This means Ti forms an entire solid
solution of K2V8ꢀxTixO16. Fig. 1 shows the lattice
parameters as a function of x in K V Ti O at room
1
1
0.10
0.00
3.00
2.98
2.96
2.94
2
8ꢀx
x
16
temperature. The lattice parameters of both a and c
increase with x, namely the volume increases with x. Fig. 2
shows temperature dependence of magnetic susceptibility
ðwÞ for K2V8ꢀxTixO16. The most remarkable point is a
significant rise of transition temperature by a slight
substitution of Ti. As mentioned above, the MI transition
of K V O is characterized by the reduction of magnetic
9
9
9
.90
.80
.70
2
8
16
susceptibility. The onset temperature of the reduction of
magnetic susceptibility goes upto 270 K at x ¼ 0:5 from
170 K at x ¼ 0. Simultaneously, the transition becomes
broad and smears out around x ¼ 1:0. On the other hand,
the magnetic susceptibility of low-temperature phase
increases with x and then above x ¼ 1, the magnetic
susceptibility continuously changes as a function of
temperature. There is no anomaly suggesting any transi-
tion.
a
2.92
a
0
1
2
3
4
5
6
7
8
x in K V M O
16
2
8-x
x
Fig. 1. Composition ðxÞ dependence of lattice parameters in
K2V8ꢀxTixO16. The inset shows a schematic structure of K
from the c-axis of the tetragonal structure.
2
V
8
O16 viewed
In general, phase transitions accompanied by some kinds
of order suffer a significant damage by the introduction of
disorder and the transition temperature could be lowered.
3
þ
4þ
1
2
1
1
0
5
0
5
0
K V O is a mixed valent compound with V =V ¼ .
2
8
16
3
Since a stable valence state of Ti is Ti in the coexistence
4þ
K V Ti O
8-x
2
x
16
x = 4.0
3þ
4þ
with V =V , the substitution of Ti for V must give a
significant disorder in the charge ordering at the transition.
Therefore, the observed enhancement of transition tem-
perature is not understood in terms of such general
disorder effect. The transition of K V O is accompanied
2
8
16
x = 1.0
x = 0.5
by a volume increase about 0.14% in the low-temperature
phase. On the other hand, the substitution of Ti for V also
increases the volume. This means the Ti-substitution gives
a negative pressure to the system as a chemical pressure,
and as a result the transition temperature might be raised
by a slight substitution of Ti. However, the further Ti-
substitution leads to a significant disorder, and finally the
transition smears out. The (chemical) pressure effect on the
transition is dominant at the initial stage of the Ti-
substitution, while the disorder effect becomes dominant
with the progress of the Ti-substitution.
x = 0
x = 0.3
x = 0.1
H = 0.1T
1
00
150
200
250
300
350
Temperature (K)
Fig. 2. Temperature dependence of magnetic susceptibility for various x
in K2V8ꢀxTixO16.
This study has been supported by a Grant-in-Aid for
Scientific Research (No. 18104008) from the Japan Society
for the Promotion of Science.
pressure of 4 GPa at 1473 K for 1 h. Starting reagents were
ground thoroughly and the mixture was wraped by
platinum foil and then put into a pressure cell. KVO3
was obtained by a reaction of K CO and V O . V O was
References
2
3
2
5
2
3
prepared by the reduction of V O under flowing H gas at
2
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5
2
[
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1
173 K. A cubic type apparatus was used for high pressure
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Isobe, Masahiko
