Ti-doping effect on the MI transition of hollandite vanadate, K2V8O16

Article abstract of DOI:10.1016/j.jmmm.2006.10.1087

Ti-doping effect on the metal-insulator transition of hollandite vanadate, K₂V₈O₁₆ has been investigated. The transition is accompanied by the charge ordering and the structural change from tetragonal to monoclinic structures with the increase of volume in the low-temperature monoclinic insulator phase. A slight amount of Ti-substitution for V atoms raises the transition temperature up to 270 K at x = 0.5 in K₂ V_{8 - x} Ti_x O₁₆ from 170 K at x = 0. This significant rise of transition temperature can be understood as a negative chemical pressure effect caused by the Ti-substitution. On the other hand, the disorder effect from the Ti-substitution makes the transition broad and finally the transition smears out around x = 1.0.

Full text of DOI:10.1016/j.jmmm.2006.10.1087

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 888–889
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Ti-doping effect on the MI transition of hollandite vanadate, K V O
2
8 16
Ã
Masahiko Isobe, Shigenori Koishi, Yutaka Ueda
Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Available online 27 November 2006
Abstract
Ti-doping effect on the metal-insulator transition of hollandite vanadate, K
2 8
V O16 has been investigated. The transition is
accompanied by the charge ordering and the structural change from tetragonal to monoclinic structures with the increase of volume in
the low-temperature monoclinic insulator phase. A slight amount of Ti-substitution for V atoms raises the transition temperature up to
2
70 K at x ¼ 0:5 in K2V8ꢀxTixO16 from 170 K at x ¼ 0. This significant rise of transition temperature can be understood as a negative
chemical pressure effect caused by the Ti-substitution. On the other hand, the disorder effect from the Ti-substitution makes the
transition broad and finally the transition smears out around x ¼ 1:0.
r 2006 Elsevier B.V. All rights reserved.
PACS: 72.80.Ga; 72.60.+g; 75.30.Kz
Keywords: Hollandite vanadium oxide; Metal-insulator transition; Substitution effect; Magnetic susceptibility
4
þ 4þ
Vanadium oxides exhibit a rich variety of electromagnetic
properties. The most characteristic property among them is
metal-insulator (MI) transition as a function of temperature,
as typically observed in binary vanadium oxides [1]. The MI
transitions in mixed valent vanadium oxides such as V O are
(see Fig. 2), suggesting the formation of V –V singlet
3þ
3þ
pairs and V –V pairs in the low-temperature insulator
phase [3]. The transition is of first order accompanied by the
structural change from a tetragonal to a monoclinic structure,
and the low-temperature phase has a superlattice of
4
7
p
p
accompanied by the charge separation and charge order, and
2a ꢁ 2a ꢁ 2c, where a and c denote the primitive cell
4
þ
4þ
spin singlet V –V pairs are formed in the low-temperature
insulator phases, resulting in the large reduction of magnetic
susceptibility at the MI transitions [2]. In our current study
on vanadium oxides, an MI transition at 170 K was recently
found in a hollandite vanadium oxide K V O [3]. ‘‘Hollan-
parameters for the simple tetragonal hollandite structure,
4
þ
3þ
suggesting a charge ordering of V and V [3]. From these
results we proposed a possible charge order model, where two
3
þ
4þ
kinds of double-chain formed by V and V chains and by
single V chains, respectively, order in a manner that gives a
4þ
2
8 16
p
p
4
þ
4þ
dite’’ is one of manganese minerals. Hollandite-type com-
pounds have the general chemical formula A M O ðxp2Þ.
superlattice of 2 ꢁ 2 in the a–b plane, and the V –V
3 3þ
þ
and V –V pairs are formed in each chain along the c-axis,
resulting in a duplication of the c-axis [3]. In order to
elucidate the origin of the transition, we have investigated the
substitution effect of Ti for V sites. We found the significant
enhancement of transition temperature by the substitution.
This means the transition is not of a simple charge order
transition in which a significant damage on the transition is
expected by the introduction of disorder. In this short paper,
we report the structural and magnetic properties of
K V Ti O and discuss the Ti-substitution effect on the
x
8
16
The hollandite structure consists of the tubular M O -
16
8
network and A-cations at the tunnel sites of the M8O16-
framework, as shown in the inset of Fig. 1. The M8O16-
framework is constructed from the double-chains running
along the c-axis in the tetragonal structure and each double-
chain is formed by sharing the edges of MO6 octahedra. The
MI transition of K V O is accompanied by two step jumps
2
8 16
about three orders in the resistivity along the c-axis and the
reduction of magnetic susceptibility down to a small value
2
8ꢀx
x
16
transition.
Powdered samples were prepared by a solid-state
reaction of KVO , V O , V O and TiO2 under high
Ã
Corresponding author. Tel./fax: +81 4 7136 3435.
E-mail address: yueda@issp.u-tokyo.ac.jp (Y. Ueda).
3
2
3
2
5
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.10.1087

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
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2
3
2
5
2
3
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2
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