Charge order and quasi-one-dimensional behavior in β(β′)-AxV2O5

Article abstract of DOI:10.1016/S0925-8388(00)01318-9

Quasi-one-dimensional conductor β(β′)-A_xV₂O₅ (A=Li, Na, Ag, Ca, Sr, Cu) was investigated for its electric and magnetic properties. The β-A_0.33V₂O₅ (A=Li, Na, Ag, Ca, Sr) phases show a phase transition accompanied by a charge separation and a charge order. The low temperature phase of β-A_0.33⁺V₂O₅ shows a long-range magnetic order at lower temperature whereas β-A_0.33²⁺V₂O₅ shows a formation of spin gap without any magnetic order. The magnetic properties of the low temperature phases strongly suggest a linear chain for β-A_0.33⁺V₂O₅ and a two-leg ladder chain for β-A_0.33²⁺V₂O₅ as a charge ordered pattern of magnetic V⁴⁺ ions. A quasi-one-dimensional conductor β′-Cu_0.65V₂O₅ shows a superconducting transition under high pressure around 4 GPa at 5 K.

Full text of DOI:10.1016/S0925-8388(00)01318-9

Journal of Alloys and Compounds 317–318 (2001) 109–114
L
www.elsevier.com/locate/jallcom
Charge order and quasi-one-dimensional behavior in b(b9)-A V O5
x 2
*
Yutaka Ueda , Hiroyuki Yamada, Masahiko Isobe, Toru Yamauchi
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
Abstract
Quasi-one-dimensional conductor b(b9)-A V O (A5Li, Na, Ag, Ca, Sr, Cu) was investigated for its electric and magnetic properties.
x
2
5
The b-A0.33V O (A5Li, Na, Ag, Ca, Sr) phases show a phase transition accompanied by a charge separation and a charge order. The low
2
5
1
21
0.33
temperature phase of b-A0.33V O shows a long-range magnetic order at lower temperature whereas b-A V O shows a formation of
2
5
2
5
spin gap without any magnetic order. The magnetic properties of the low temperature phases strongly suggest a linear chain for
1
21
0.33
41
b-A0.33V2O5 and a two-leg ladder chain for b-A V2O5 as a charge ordered pattern of magnetic V ions. A quasi-one-dimensional
conductor b9-Cu0.65V O shows a superconducting transition under high pressure around 4 GPa at 5 K.
 2001 Elsevier Science B .V .
2
5
All rights reserved.
Keywords: b(b9)-A V O ; Charge order transition; Quasi-one-dimensional behavior; Electromagnetic properties
x
2
5
1
. Introduction
respectively. The A9-sites are different from the A-sites by
/2 in the height along the b-axis. In the b-structure, the
1
Recently low dimensional vanadium oxides AV O (A5
A-cations cannot occupy simultaneously two nearest-
neighboring A-sites in the a–c plane but only either site,
because the two A-sites are too close. Therefore the
stoichiometric composition of the b-phase is A1 / 3V O or
2
5
Li, Na, Cs, Mg, Ca) have drawn much interest because of
their exotic behaviors resulting from an interplay among
charge, spin, orbital and lattice degrees of freedom,
particularly a phase transition accompanied with a charge-
order and a formation of spin-gap in NaV O [1]. The
2
5
AV O . On the other hand, there is not such limitation of
6
15
occupation of A-cations in the b9-phase. The two A9-sites
can be fully occupied and therefore the stoichiometric
composition is A2 / 3V2O5 or A V O . Both b- and b9-
2
5
structures of AV O take layer types consisting of V O
2
5
2
5
layers and interlayer A-atoms. The V O layer adopts a
2
5
2
6
15
special framework, so-called tr e´ llis lattice, where a ladder
or one-dimensional magnetic chain is realized according to
structures have a characteristic V O -framework formed by
2 5
the three crystallographically independent vanadium atoms
(V1, V2 and V3) and oxygen atoms. The V O -framework
4
1
1
51
a charge-ordered manner of V
(d , S51/2) and V
d ). These compounds are situated as end members of
vanadium bronze oxides A V O which appear in the
2
5
0
(
consists of three kinds of infinite chain along the b-axis
and each chain is a double chain, as shown in Fig. 1b. The
V1-sites have a sixfold octahedral coordination and form a
x
2
5
intercalation of A-cations between the layers of V O .
2
5
Some alkaline or alkaline earth atoms crystallize in a
zigzag chain of edge-sharing VO -octahedra. The V2-sites
6
monoclinic structure called b(b9)-structure with V O . The
with a similar octahedral coordination form a two-leg
2
5
b(b9) bronze is the most famous compound among the
vanadium bronze oxides. The b(b9) bronze has a one-
dimensional structure rather than a layered structure [2,3].
Fig. 1a shows a projection on the a–c plane in the
monoclinic b(b9)-structure. The A-cations are located in
the tunnel formed by a V O -framework and occupy the
ladder chain of corner-sharing VO -octahedra and the V3-
sites that have a fivefold square pyramidal coordination
6
form a zigzag chain of edge-sharing VO -square-pyramids.
5
From such structural characteristics the b(b9)-A V O has
x
2
5
long been known as a quasi-one-dimensional conductor
[4–6]. Actually two of the authors (HY and YU) reported
a clear one-dimensional conducting behavior and a metal-
insulator transition in b-A0.33V2O5 (A5Na, Ag) [7]. In
this paper we report a charge order transition and a
2
5
A-sites in the b-structure and A9-sites in the b9-structure,
*
Corresponding author.
0
925-8388/01/$ – see front matter
 2001 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(00)01318-9

1
10
Y. Ueda et al. / Journal of Alloys and Compounds 317–318 (2001) 109–114
Table 1
b(b9)-A V O investigated in this work
x
2
5
4
1
51
Compounds
b-Li V O (2.4 , x , 0.38)
V
/V
T (K)
TN (K)
t
x
2
5
b-LiV O (x 5 1/3)
1/5
1/5
1/5
2/4
2/4
180
136
90
7
6
15
b-Na V O (0.23 , x , 0.35)
x
2
5
b-NaV O (x 5 1/3)
24
24
–
6
15
b-Ag V O (0.23 , x , 0.41)
x
2
5
b-AgV O (x 5 1/3)
6
15
b-Ca V O (0.30 # x , 0.34)
x
2
5
b-CaV O (x 5 1/3)
150
170
–
6
15
b-Sr V O (0.30 # x , 0.34)
x
2
5
b-SrV O (x 5 1/3)
–
6
15
b9-Cu V O (0.26 , x , 0.66)
x
2
5
b9-Cu0.65V O
|2/4
T 55 K
2
5
C
3
. Results and discussion
The b(b9)-A V O compounds investigated in this work
x
2
5
are summarized in Table 1. All are mixed valent oxides of
4
1
51
V
and V whose ratio depends on the kind of A-cation
and its composition. The stoichiometric b-compounds
show some kind of phase transition at T as a common
t
property, as shown in Table 1. At T the lattice parameters
t
do not exhibit a significant change as shown in Fig. 2 but
the phase transition is accompanied by a change of electric
resistivity or specific heat [7]. The b-phase is a quasi-one-
dimensional conductor with a metallic conductivity along
the b-axis [7]. Fig. 3 shows the temperature dependence of
electric resistivity of b-A0.33V O (A5Li, Na, Ag) mea-
Fig. 1. (a) A schematic crystal structure of b(b9)-A V O projected on
x
2
5
a–c plane. V1, V2 and V3 represent crystallographically independent
vanadium atoms. A and A9 are A-cation sites for b- and b9-phase,
respectively. (b) Three kinds of chain formed by (V1)O , (V2)O and
2
5
sured along the b-axis. The resistivity changes from a
6
6
metallic to semiconductive one at each T . Fig. 4 shows the
(
V3)O5.
t
temperature dependence of magnetic susceptibility for b-
A0.33V O (A5Na, Ag, Ca and Sr). Na- or Ag-compound
2
5
shows a slight change around T and a successive increase
t
quasi-one-dimensional behavior as common properties in
below T in the magnetic susceptibility, and then orders
t
b(b9)-A0.33V O .
magnetically at lower temperature (T ). On the other hand,
2
5
N
the magnetic susceptibility for Ca- or Sr-compound has
also a slight jump at T but below T it shows a low-
t
t
dimensional behavior and does not show any evidence for
a long-range magnetic order down to the lowest tempera-
ture. Especially the magnetic susceptibility of Sr-com-
pound decreases to a very small value at low temperature,
2
. Experimental
Powder samples were synthesized by a solid state
5
1
reaction of V O ,V O , and AVO or A(VO ) for A V O
suggesting a spin-gap state as a ground state. The
V
2
5
2
3
3
3
2
x
2
5
(
A5Li, Na, Ag, Ca, Sr) and also CuO or Cu O for
NMR studies confirmed a character of charge order
2
5
1
Cu V O in an evacuated silica tube. AVO or A(VO )
transition at T . Fig. 5 shows the results of V NMR for
x
2
5
3
3
2
t
was prepared by a solid state reaction of V O and A CO
b-Sr0.33V O obtained by spin echo method with powder
2
5
2
3
2
5
or ACO in air. Single crystals of A V O (A5Li, Na, Ag
samples. Above T only one signal is observable, sug-
3
x
2
5
t
and Cu) were grown by a self-flux method. The details of
sample preparation were described in Refs. [7,8]. Single
crystals of A V O (A5Ca, Sr) have been unsuccessfully
grown. Electrical resistivity was measured by an ordinary
four-probe method with single crystals. Magnetic suscep-
tibility and magnetization were measured using a SQUID
magnetometer. NMR experiments were performed by a
spin-echo method using a coherent pulsed spectrometer.
gesting a uniform electronic state, while below T two
t
signals can be observed. One is assigned to nonmagnetic
5
1
V
sites with a very small and constant value of Knight
x
2
5
41
shift and the other is due to the magnetic V sites whose
Knight shift shows a similar temperature dependence as
5
1
the magnetic susceptibility. In V NMR experiments
using single crystals of b-Na0.33V2O5 two signals with
slightly different negative Knight shifts can be observed

Y. Ueda et al. / Journal of Alloys and Compounds 317–318 (2001) 109–114
111
fields of 0, 16, 27 and 72 kOe in frequency-swept NMR
under zero external fields [9]. The internal field of 72 kOe
4
1
is a typical value for magnetic V ions and the other three
are the transferred hyperfine fields from the magnetic sites.
These results indicate a rather uniform charge state above
T and below T a clear charge separation and charge order.
t
t
Particularly the NMR experiments with single crystals
clearly show a charge transfer from the V3-sites to the V1-
or V2-sites at T . The definite internal fields observed
t
below T indicate that the magnetically ordered state is not
N
a spin-glass like state but a long-range magnetic order.
Now we confirmed a charge separation and charge order
transition at T in b-A V O . The magnetic properties
t
0.33
2
5
below T give some information on charge-ordered man-
t
1
ner. The compound with monovalent A -cation shows a
maximum of magnetic susceptibility around 30 K and it
orders magnetically slightly below the temperature at the
maximum. On the other hand the magnetic susceptibility
2
1
of the compound with divalent A -cation has a maximum
around 50 K and then decreases smoothly to a small value
with decreasing temperature, suggesting a spin-singlet as a
ground state. These results suggest a condensation of
4
1
magnetic V ions into one of the three V-sites, that is one
4
1
of the three chains. The ratio of V to total V ions is 1/6
1
21
in A -compound and 2/6 in A -compound. In the b(b9)-
structure there are three V-sites and each site forms a
double chain (six chains in total), as shown in Fig. 1b.
Therefore a low dimensional behavior of magnetic suscep-
tibility cannot be expected in random distribution of
4
1
magnetic V
ions into the three V-sites. When the
4
1
magnetic V ions preferably precipitate into one of th1e
three V-sites at T they occupy half of the chain in A -
t
compound and on the other hand fully occupy the chain in
2
1
A -compound, and as a result one-dimensional behaviors
can appear in both cases. The magnetic susceptibility for
1
A -compound shows a weak ferromagnetic behavior
below T . Fig. 6 shows the magnetization curves of
N
b-Na0.33V O measured along the a9-, b- and c-axis at 5
2
5
K. On the whole the magnetization increases almost
linearly against applied field, as shown in Fig. 6a, sug-
gesting an antiferromagnetic order. Looking at the curves
in detail, however, the magnetization curve along the
b-axis has a weak ferromagnetic component and further-
more a clear spin-flop is observed along the a9-axis (the
direction perpendicular to the b–c plane) around 0.5 T, as
clearly seen in Fig. 6b. A slight spin-flop behavior is also
observed along the c-axis. These results indicate that the
antiferromagnetic spin axis is perpendicular to the b-axis
and parallel to the a-axis and it easily flops to the direction
of applied field. The weak ferromagnetism originates in a
spin-canting caused by Dzyaloshinsky–Moriya (DM)-in-
teraction along the b-axis. A spin-canting angle can be
estimated to be 0.28 from the b-axis. Since the DM-
Fig. 2. Temperature dependences of lattice parameters of b-Na0.33V O .
2
5
above T . From the analyses of the data, one with less
t
Knight shift is assigned to the V3-sites and the other is to
the V1- and V2-sites. Below T the V3-sites lose the Knight
t
shift and the signal from the V1- and V2-sites splits into
three, one of which has a large negative Knight shift and
the other two have very small values of the Knight shift.
Since the Knight shift corresponds to a local spin suscep-
tibility at each site the sites with almost zero and large
negative Knight shift can be regarded to be the nonmag-
interaction is effective in a noncentrosymmetric relation
5
1
41
41
netic V -sites and magnetic V -sites, respectively.
between neighboring spins the magnetic V
ions are
Below TN we can observe four signals with the internal
regarded to form a linear chain along the b-axis as shown

1
12
Y. Ueda et al. / Journal of Alloys and Compounds 317–318 (2001) 109–114
Fig. 3. Temperature dependences of electric resistivities of b-A0.33V O (A5Li, Na, Ag) measured along b-axis.
2
5
2
1
in Fig. 7a. The spin-gap behavior observed in A -com-
Each b-compound has a wide nonstoichiometric region
of A-cation, as shown in Table 1. The charge order
transition is rapidly suppressed by the nonstoichiometry of
A-cations and as a result the magnetic order disappears in
the nonstoichiometric compounds [7]. The metallic be-
havior also disappears in the nonstoichiometric com-
pounds, as reported in Ref. [7]. Therefore the b-phases
show metal-insulator transitions as functions of both
temperature and composition. The nonstoichiometry of
A-cation results in a disorder of arrangement of A-cations,
which could make metallic property unstable, particularly
in a quasi-one-dimensional conductor. It also could
weaken a charge-phonon coupling and suppress a charge
4
1
pound strongly suggests that the magnetic V ions occupy
the V2-sites to form a two-leg ladder chain as shown in
Fig. 7b, because a two-leg ladder chain with S51/2 is
responsible for a spin-gap behavior without any help of
lattice distortion.
5
1
Fig. 4. Temperature dependences of magnetic susceptibilities of b-
A0.33V O (A5Na, Ag, Ca, Sr).
Fig. 5. Temperature dependences of V Knight shift of b-Sr0.33V O
measured using powder samples.
2
5
2
5

Y. Ueda et al. / Journal of Alloys and Compounds 317–318 (2001) 109–114
113
Fig. 7. Plausible charge ordered patterns for low temperature phases of
1
21
.33 2 5 0.33 2 5
(
a) b-A0 V O and (b) b-A V O .
and the mixed valent state of vanadium ions in b9-
2
1
Cu0.65V2O5 is close to that in b-A V2O5 because the
0.33
1
valence state of copper ions is considered to be Cu . The
absence of a charge order transition in b9-Cu0.65V O may
2
5
be due to absence of charge density in the V3-sites, that is
5
1
the occupation of the V3-sites by V
ions. Actually a
5
1
preliminary V NMR experiment with single crystals of
b9-Cu0.65V O have revealed that a signal assigned to the
2
5
4
1
V3-sites has almost no shift [11]. If V
ions mainly
occupy the V2-sites in b9-Cu0.65V O it may be regarded
2
5
as a doped ladder system and therefore the observed
pressure-induced superconductivity may be understood in a
similar scenario in Sr(Ca) Cu O .
Fig. 6. Field dependences of magnetization of b-Na0.33V O measured
along b-, c- and a9-axis at 5 K (a) up to 5 T and (b) up to 2.5 T.
2
5
1
4
24 41
In summary, a quasi-one-dimensional conductor b-
A0.33V O (A5Li, Na, Ag, Ca, Sr) shows a charge order
transfer from the V3-sites to the V2- or V1-sites, and as a
result the charge order transition could be suppressed.
The b9-Cu0.65V2O5 is also a quasi-one-dimensional
conductor but it does not show any electric or magnetic
transition [8]. The stoichiometric b9-Cu2 / 3V O has been
2
5
transition at T . Above T the charge state is rather
t
t
uniform. At T a charge transfer and a charge localization
t
4
1
occur, and magnetic V ions condense to one of the three
1
V-sites. The low temperature phase of b-A V O shows
2
5
0.33
2
5
hardly synthesized in the present study. Very recently we
observed a superconducting behavior in b9-Cu0.65V2O5
under high-pressure around 4 GPa [10]. Beyond 5 GPa the
superconducting behavior disappears. The b9-phase has a
V O -framework similar to the b-phase in the structure
a long-range magnetic order at lower temperature and on
21
the other hand that of b-A V2O5 does a spin gap
0.33
behavior without any magnetic order. The magnetic prop-
erties of the low temperature phases strongly suggest a
1
linear chain for b-A V O and a two-leg ladder chain
2
5
0.33
2
5

1
14
Y. Ueda et al. / Journal of Alloys and Compounds 317–318 (2001) 109–114
2
0
1
.33
[
4] R.H. Wallis, N. Sol, A. Zylbersztejn, Solid State Commun. 23
for b-A V O as a charge ordered pattern of magnetic
2
5
4
1
(1977) 539.
V
ions.
A
quasi-one-dimensional conductor b9-
[5] D. Kaplan, A. Zylbersztejn, J. de Physique 37 (1976) L123.
Cu0.65V2O shows a superconducting transition under
high-pressure around 4 GPa at 5 K.
5
[6] H. Kobayashi, Bull. Chem. Soc. Jpn. 52 (1979) 1315.
[7] H. Yamada, Y. Ueda, J. Phys. Soc. Jpn. 68 (1999) 2735.
[8] H. Yamada, Y. Ueda, J. Phys. Soc. Jpn. 69 (2000) 1437.
[9] M. Itoh, N. Akimoto, H. Yamada, M. Isobe, Y. Ueda, J. Phys. Soc.
Jpn., Vol. 69, Suppl. B, 2000, 155 pp.
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