High pressure behavior of α-NaVO3: A Raman scattering study

Article abstract of DOI:10.1016/j.jssc.2007.07.023

The reported pressure-induced amorphization in α-NaVO₃ has been re-investigated using Raman spectroscopy. Discontinuous changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in frequency of the V-O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a mixture of crystalline α-phase, traces of crystalline β-phase and highly disordered phase consisting of V-O units in five- and six-fold coordination.

Full text of DOI:10.1016/j.jssc.2007.07.023

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Journal of Solid State Chemistry 180 (2007) 2824–2829
www.elsevier.com/locate/jssc
High pressure behavior of a-NaVO : A Raman scattering study
3
a,
Rekha Rao , T. Sakuntala , A.M. Shaikh , S.K. Deb
Ã
a
b
a
a
High Pressure Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India
b
Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400085, India
Received 23 March 2007; received in revised form 10 July 2007; accepted 23 July 2007
Available online 10 August 2007
Abstract
The reported pressure-induced amorphization in a-NaVO3 has been re-investigated using Raman spectroscopy. Discontinuous
changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in
frequency of the V–O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the
high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase
gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-
existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a
mixture of crystalline a-phase, traces of crystalline b-phase and highly disordered phase consisting of V–O units in five- and six-fold
coordination.
r 2007 Elsevier Inc. All rights reserved.
Keywords: Raman spectroscopy; High pressure; Phase transition; NaVO
3
1
. Introduction
Vanadium exists in different oxidation states in different
transition behavior of these compounds has led to high
temperature–high pressure synthesis of novel phases [8,9].
Studies on the high pressure behavior of alkali metavana-
dates have shown that KVO , RbVO [10] and CsVO [11]
exhibit structural transitions at moderate pressure of
5–10 GPa. The observed trend in transition pressure was
understood on the basis of cation sizes. All these
compounds. The interesting physical properties of the
various vanadium oxides depend on the electronic struc-
ture, and in turn, on the valance state of the vanadium in
the compound [1]. Consequently they find wide range of
applications covering catalysis, gas sensors, phosphors,
switching devices, [1,2] and also in rechargeable batteries
3
3
3
compounds consist of corner-sharing VO tetrahedra
4
3
ꢀ
forming infinite chains of (VO4) . Interestingly, a-NaVO3
was reported to exhibit pressure-induced amorphization
(PIA) at around 6 GPa [12,13], while the structurally
related compound LiVO3 showed a series of structural
transitions, with no PIA during compression upto 20 GPa
[14].
[
3]. Vanadium oxide-based compounds show rich variety of
phenomena. Owing to the strong electron correlation, the
b-vanadium bronzes have shown many interesting beha-
vior such as charge ordering [4], pressure-induced super-
conductivity [5], etc. Orthovanadates of the type M (VO )
3
4 2
(
M: Ca, Sr, Ba) have excellent luminescence properties
Compared to other metavanadates, high pressure
behavior of NaVO appears to be complex. There exists
which are enhanced when doped with suitable rare-earth
element, and find application as laser host materials [6].
Metavanadates of type AVO (A: Li, Na, K, Rb, Cs, NH )
have been reported to show ferroelectric and pyroelectric
properties [7]. Basic research on the stability and phase
3
another polymorph, b-NaVO , which transforms irrever-
3
sibly to the a-form at temperature above 405 1C [15].
Structural similarities and differences of the two poly-
morphs may be briefly summarized as follows. a-NaVO3
crystallizes in a monoclinic structure (Cc) and is ferro-
3
4
Ã
electric at ambient conditions [16,17]. b-NaVO is orthor-
hombic (Pnma) at ambient conditions and is made up of
Corresponding author.
E-mail address: rekhar@barc.gov.in (R. Rao).
3
0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2007.07.023

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R. Rao et al. / Journal of Solid State Chemistry 180 (2007) 2824–2829
2825
infinite double chains parallel to the b-axis [18]. The two
polymorphs have comparable volumes at ambient condi-
per unit cell. Factor group analysis predicts 57 optic
branches whereas only 22 distinct Raman modes have been
experimentally observed [12]. The Raman spectrum of
a-NaVO3 at ambient conditions in the present studies
agrees well with that reported [12,15]. Six lattice modes
3
˚
tions; the volume per formula unit is 69.7 A for a-NaVO
3
3
˚
[
13], and 69.18 A for b-NaVO [19]. The main difference
3
between the two is that in a-NaVO the vanadium atom
3
ꢀ
1
has four-fold coordination while in b-NaVO , it has five-
appear in the spectral range 60–150 cm . Based on the
comparison of the spectrum of crystalline compound and
that of the aqueous solution, the Raman bands in the
3
fold oxygen coordination. High pressure behavior of both
the polymorphs has been investigated using spectroscopic
ꢀ
1
techniques [12,19]. a-NaVO has been reported to exhibit
spectral range 200–380 cm were identified to be due to
VO rocking, twisting and chain deformation vibrations
3
two transitions at pressures around 4.3 and 6GPa. Based on
the excessive broadening and weakening of the Raman bands,
it was concluded that the new high pressure phase above
2
[15]. In the present studies, all the Raman modes appearing
ꢀ
1
at frequencies above 90 cm
could be followed as a
6
GPa is an amorphous phase [12]. Amorphization was found
function of pressure. Fig. 1 shows the Raman spectra of
a-NaVO3 at various pressures. Frequencies of all the
Raman modes increase monotonically with increasing
pressure upto 5.6 GPa. Fig. 2 shows the pressure depen-
to be irreversible from pressures of about 17 GPa and
recrystallization of a-phase was reported upon heating the
recovered amorphous sample [20]. Subsequent high pressure
mid-infrared measurements suggested that the phase above
ꢀ
1
dence of mode frequencies in a-NaVO . The 225 cm
3
6
atoms may have higher coordination [21]. On the other hand,
GPa could be a disordered phase where the vanadium
mode appears as a single broad band up to about 2.5 GPa,
beyond which the second component at higher frequency
separates out. Similar behavior is noted for the Raman
spectroscopic studies on b-NaVO up to 16.6 GPa indicated
3
ꢀ
1
three phase transitions at pressures 1.4, 4.4 and 8.0 GPa, with
no signatures of any disorder in the high pressure phases [19].
Very different high pressure behavior of the two polymorphs
leads to several questions such as what exactly is driving PIA
band at 256 cm above 3 GPa. It may be mentioned that
in some of the crystal orientations, the broad band at
ꢀ
1
225 cm (typical width of about 20 cm ) appears clearly
ꢀ1
ꢀ
1
as two modes centered around 222 and 231 cm , of width
in a-NaVO . PIA is generally understood as arising due to
3
kinetic hindrance of equilibrium transitions [22]. In this
context, it would be of interest to know what is the stable
high pressure crystalline form of NaVO . The present work is
3
6
3
4.4 GPa
X 20
aimed at understanding the nature of high pressure phases
in a-NaVO , both the intermediate crystalline phase
2
1
3
5
4
3
2
1
0
(4.3–6.0 GPa) and the subsequent amorphous phase, as
200
400
*
600
800
*
1000
reported in the earlier work [13], in relation to the behavior
observed in other alkali meta vanadates.
*
13.0
0.5
2
. Experimental details
*
1
Single crystals of a-NaVO were grown by slow cooling
3
7.2
the melt of Na CO and V O from 630 1C at the rate of
2
3
2
5
X 2
1
1C/h in a platinum crucible. Crystals were plate-like
6.0
yellowish brown in color. Raman scattering measurements
at high pressure were carried out using a diamond anvil cell
X 5
(DAC), Diacell Products, UK, Model B-05 with octagonal
5.6
diamonds 400 mm diameter culet. A 4:1 methanol–ethanol
mixture was used as pressure transmitting medium. Pressure
inside the cell was measured using the standard ruby
fluorescence technique. Raman spectrum of unoriented single
4.4
3.2
crystal bits of NaVO from inside the DAC was excited using
3
5
32 nm line of power ꢁ15 mW. Scattered light was analyzed
0.9
using a home built 0.9 m single monochromator, coupled
with a super notch filter and detected by a cooled CCD
1 -
0
4
GPa
800
(
Andor Technology). Entrance slit was kept at 50 mm, which
200
400
600
1000
ꢀ
1
gives a spectral band pass of 3 cm .
-1
Raman Shift (cm )
3
Fig. 1. Raman spectra of a-NaVO at different pressures. Spectrum in the
3
. Results
ꢀ1
range 60–460 cm have a scale factor of 5 up to 5.6 GPa, and a factor of 2
thereafter. Inset shows the appearance of weak peaks above 4.4 GPa
indicated by arrows. *Indicates the modes that increase in intensity above
9 GPa.
At ambient conditions, a-NaVO3 exists in a non-
centrosymmetric [15] structure with four formula units

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2826
1
000
4
4
3
3
2
2
1
1
50
00
50
00
50
00
50
00
9
9
8
8
50
00
50
00
6
6
5
5
50
00
50
00
0
4
8
12
0
4
8
12
Pressure (GPa)
ꢀ
1
. Note that the two modes around 800 cm have a weak negative pressure
Fig. 2. (a, b) Pressure dependencies of mode frequencies in a-NaVO
dependency in the g-phase.
3
ꢀ
1
0 and 11 cm , respectively, even at ambient conditions.
2
sities of the modes in chain deformation region
ꢀ
1
(350–420 cm ) are noted to gradually change and above
Raman measurements in higher alkali vanadates by Adams
and Fletcher [10] had shown more number of bands in this
spectral range at low temperatures, partly due to line
narrowing of the otherwise broad Raman bands. Appear-
ance of new modes in the pressure range 2.5–3.5 GPa
ꢀ
1
3.9 GPa, the mode at 377 cm
appears with strongest
intensity among the set of four bands. It may be mentioned
that in some of the high pressure runs, the band at
ꢀ
1
345 cm appears with a lesser intensity compared to that
ꢀ
1
(
overlapping modes owing to different pressure dependen-
Fig. 2) is understood as due to gradual separation of the
at 360 cm even at ambient conditions. Further decrease
in intensity of the other three modes around 3.9 GPa results
in only one of the mode in this region to be observed at
higher pressures. It is difficult to comment whether these
changes in relative intensities and increased separation of
some of the overlapping modes are due to any phase
transition, as these are rather gradual changes and appear
over a range of pressure, from 2.5 to 3.9 GPa. Intensities of
these new weak peaks (inset in Fig. 1) do not show increase
upon further pressurization. Possible origin of these peaks
will be discussed in the subsequent section.
ꢀ
1
cies. Line width of the 179 cm mode gradually decreases
ꢀ
1
ꢀ1
from ꢁ22 cm at ambient pressure to 11 cm at 3.2 GPa,
accompanied by an increase in intensity.
Around 4.0 GPa, new peaks of rather weak intensity
ꢀ
1
appear at frequencies around 475, 497, 730, and 862 cm
Inset in Fig. 1 shows the new modes that appear around
GPa. Earlier high pressure Raman studies [13] had
.
4
reported subtle spectroscopic changes around 4 GPa,
relating to intensities and pressure dependence of mode
frequencies, as well as appearance of new weak bands. This
was attributed to a first order phase transition. However,
mid-infrared studies at high pressure, did not find any
noticeable spectral changes around this pressure [21]. In the
present work also, no significant changes are noted in the
pressure dependence of mode frequencies. Relative inten-
Above 5.6 GPa, the Raman spectrum shows abrupt
changes. The sharp intense bands around 955, 920, 635,
ꢀ
1
and 508 cm arising from the symmetric and antisym-
metric stretching vibrations of V–O and V–O–V, respec-
tively, show rapid decrease in intensity along with the
appearance of new intense, broad bands around 917, 810

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R. Rao et al. / Journal of Solid State Chemistry 180 (2007) 2824–2829
2827
ꢀ
1
and 791 cm . The latter two are found to be broad, of
ꢀ
1
width 20 and 40 cm , respectively, compared to those in
p-released
ꢀ
1
6
5
4
3
2
1
0
a-phase (typically ꢁ5 cm ) in this region. Intensity of
these modes grows at the expense of those of the a-phase
bands suggesting nucleation and growth of a new phase,
say g, at this pressure. In the chain deformation mode
region also, a new set of Raman modes appear (Fig. 1)
1.0
ꢀ
ꢀ
1
riding over a broad background centered around 300 cm
At pressures typically about 7 GPa, the band at 807 cm
.
1
2.0
appears as the strongest mode. Some of the strong Raman
bands characteristic of the a-phase could be detected with a
weak intensity up to the highest pressure reached in the
present investigations suggesting a rather sluggish nature of
the transition. From the Raman spectra obtained from
spatially different region of the sample, the set of bands
corresponding to the g-phase could be clearly differentiated
from the bands of a-phase. Pressure dependencies of only
the g-phase modes are shown beyond 5.6 GPa in Fig. 2.
Besides the new set of modes, pressure dependencies of
many bands show considerable decrease in the g-phase.
The excessive broadening of the stretching modes and
increased background in the chain deformation mode
region suggest that the new high pressure phase is
disordered, which could be partly due to the sluggish
nature of the a–g transition. Well-defined lattice modes
could not be observed beyond 6 GPa.
3.0
3.4
5.4
7.0
1
1 GPa
200
400
600
800
1000
-1
Raman Shift (cm )
Fig. 3. Raman spectra of NaVO in the pressure reducing cycle. Arrows
3
ꢀ
1
In the g-phase, the mode at 810 cm
has two
3
indicate the peaks identified to be due to b-NaVO .
components; both of them show weak decrease in
frequency with further increase in pressure indicative of a
gradual increase in V–O bond length evolving towards
increasing V–O coordination. Above 9 GPa, a broad band
4. Discussion
The spectral changes observed in a-NaVO3 around
6 GPa in the present work, by and large, agree well with
that reported by Shen et al. [20]. It remains to be
understood whether there is any intermediate structural
transition around 4 GPa, as concluded by Shen et al. [13].
As mentioned earlier, appearance of weak peaks around
475, 735 and 862 cm , along with changes in intensities of
some of the low frequency modes was taken as an
indication for a phase transition. Appearance of new
modes across a phase transition results either from
correlation splitting or due to lowering of symmetry. In
particular, loss of inversion symmetry leads to the infrared
active modes becoming Raman active in the new phase,
and such modes have poor Raman intensities. However, a-
ꢀ
1
ꢀ1
centered around 645 cm and the shoulder at 790 cm
gains in intensity. Raman bands in the chain deformation
ꢀ
1
mode region (300–450 cm ) also appear with better
intensity above this pressure. All these changes are found
to be reversible with large hysteresis. Fig. 3 shows the
Raman spectra of the compound in the pressure reducing
cycle. The changes observed above 9 GPa, namely im-
proved intensities of the V–O–V stretching and deforma-
tion modes, persist down to about 5.5 GPa, below which
these bands reduce in intensity. Spectrum resembles that of
the disordered g-phase down to about 3.4 GPa. Below this
ꢀ
1
ꢀ
1
pressure, a broad peak at around 850 cm , noted as a
shoulder towards the higher frequency side of the V–O
ꢀ
1
stretching band at 820 cm , gains in intensity. Around
GPa, the strongest Raman bands characteristic of the
a-phase start gaining in intensity, however, a few other
NaVO does not have inversion even at ambient condi-
tions, hence such a possibility is ruled out. On close
comparison of the frequencies of the new modes with those
3
2
ꢀ
1
bands such as the one around 865 and 720 cm (Fig. 3)
also appear which neither belong to a-NaVO nor the high
reported in b-NaVO , these are identified as the strong
Raman bands of b-NaVO3 [19]. It is known that in
3
3
pressure g-phase. Intensity of this broad feature is much
reduced when the pressure is completely released. Raman
spectrum of the pressure-released sample predominantly
has the characteristic lines of a-phase along with a few new
modes, marked by arrows in Fig. 3. It is noted that the
Raman intensities of the pressure-cycled sample are
very much reduced when compared to that of the starting
phase.
b-NaVO , vanadium atom has five-fold oxygen coordina-
3
tion forming a distorted trigonal bipyramid [18]; hence a
b-NaVO3 type of phase may be expected prior to
transformation to a six-fold coordinated phase. However
it is important to note that in the present studies, the
intensities of these new modes do not grow upon further
pressurization implying that it is not due to an equilibrium
phase transition. Considering the fact that there is

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R. Rao et al. / Journal of Solid State Chemistry 180 (2007) 2824–2829
2828
nucleation and growth of g-phase around 5.6 GPa, forma-
tion of traces of five-fold coordinated crystalline phase at a
slightly lower pressure is not unexpected. There are a
number of tetrahedrally coordinated framework structures
coordinated phase at high pressure is noted to be highly
impeded often resulting in an amorphous phase [29].
Spectroscopic studies of CsVO3 at high pressure have
shown evidence for partial disorder in the chain structure
[11]. Disordered nature of g-phase could be due to the
sluggish nature of transition, and partly due to co-existence
of different phases. In the present studies, it is noted that
such as SiO [23]; many of the tungstates and molybdates
2
[
24], which are reported to exhibit PIA due to highly,
impeded transition to an octahedrally coordinated struc-
ture. Experimental studies of the evolution of local
structure [25,26] as well as simulation studies [27,28] have
shown that the intermediate disordered phase is often a
mixture of four-fold (ambient), five-fold (intermediate) and
six-fold (stable high pressure phase) coordinated. Hence,
appearance of new modes of rather weak intensities around
ꢀ
1
new Raman bands in the range 350–450 cm correspond-
ing to chain deformation vibrations, gradually gain in
intensity in the high pressure phase suggesting a tendency
towards evolving into a more ordered structure. In the
ꢀ
1
lattice modes region, the peak noted around 180 cm at
13 GPa (Fig. 1) is most likely from b-phase as discussed
earlier. Another broad shoulder is noted at a lower
4
GPa is quite possibly due to formation of a small fraction
ꢀ
1
of b-NaVO as an impurity phase prior to transformation
frequency (160 cm
at 13 GPa) that is indeed a new
3
to g-phase. In the present studies, though the typical
evolution of Raman spectrum was as described earlier,
larger intensity of the b-phase was observed from some
parts of the sample around 4 GPa in a few runs. Fig. 4
shows the typical (curve a) and b-rich spectrum (curve b) at
feature; however, it is difficult to unambiguously assign it
as the lattice mode of the new high pressure phase from the
present results.
Further, increase in intensity of the V–O–V band
ꢀ
1
(around 645 cm ) above 9.5 GPa indicate that the
V–O–V linkages persist in the g-phase. Shen et al. [20]
have suggested that the chain structure could be broken in
the high pressure phase. While the four-fold coordination
4
.5 GPa. This b-rich region of the sample continued to
show sharp crystalline lines, agreeing with the reported
high pressure behavior of the b–NaVO3 [19]. Pressure
dependence of these ‘impurity’ phase modes are not shown
in Fig. 2. These bands continue to be observed in the
spectrum of the pressure-cycled sample also. It may be
mentioned that Shen et al. [20] have also observed the
spectral features of b-NaVO3 along with those of the
a-phase upon heating the quenched high pressure phase.
The other aspect to be understood is the nature of
of the vanadium atom in a-NaVO may be described as
3
chains of the corner-shared VO tetrahedra, arrangements
4
of structural units in five- and six-fold coordinated
structures are different, often consisting of corner- as well
as edge-shared polyhedral units. Such a transition may not
necessarily involve breaking of chains. For example,
heating the pressure amorphized Ca (VO ) found to result
3
4 2
ꢀ
1
g-phase. Rapid decrease in intensity of the 984 cm mode,
in a crystalline monoclinic phase having three dimensional
network structure unlike the parent phase [8]. Some other
compounds such as NaV O [30], CaV O [31], etc., that
ꢀ
1
that is typical of V ¼ O, and appearance of the 810 cm
characteristic of the V–O stretching vibration strongly
suggest that the transition involves increase in vanadium
coordination to six-fold across the transition. In many
vanadium oxide-based compounds that are made up of
2
5
2
5
has corner- and edge-sharing VO5 units at ambient
conditions, also show a tendency for transformation to a
structure of higher coordination at comparable pressures.
In the case of NaV O , this transformation is understood
tetrahedral VO units, transformation to an octahedrally
4
2
5
5
4
3
2
1
(b)
(
a)
80
160
240
320
400
480
560
640
720
880
960
-1
Raman Shift (cm )
Fig. 4. Raman spectra of NaVO
3
at 4.5 GPa in different regions of the sample. The peaks marked by arrows in (b) are the intense lines (phase III) of b-
is recognized
modes continue to be present in the
NaVO (see Fig. 1 in Ref. [18]). Such a spatial inhomogeneity is noted only at high pressures, above 3.5 GPa. A larger fraction of b-NaVO
3
3
ꢀ1
ꢀ1
by the doublet structure of the 981 cm mode and a strong lattice mode at around 160 cm . These b-NaVO
disodered g-phase.
3

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R. Rao et al. / Journal of Solid State Chemistry 180 (2007) 2824–2829
2829
to be associated with the two dimensional chain structures
becoming a three dimensional network structure [30].
Considering the trend in the high pressure behavior of
other vanadates along with the present results which show
evidence for the presence of V–O–V linkages, the g-phase
appears to have corner-linked structure in such a way as to
achieve six-fold vanadium coordination, rather than
breaking of the chain structure.
traces of b-NaVO . Above 5.6 GPa, it transforms to a new
3
phase g and the spectral changes across the transition
suggest that vanadium is in octahedral coordination in the
g-phase. The transition is noted to be sluggish with co-
existence of phases observed over a large pressure range.
Excessive broadening of the internal modes and absence of
sharp lattice modes indicate that the g-phase is highly
disordered.
The high pressure behavior of a-NaVO3 may be
compared with that of the other alkali metavanadates. In
the case of LiVO , the first transition is reported to be
3
ꢀ
1
References
around 3.5 GPa, across which the intensity of the 980 cm
mode, characteristic of the V ¼ O stretching decreases
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Matter 18 (2006) R1–R4.
substantially in intensity and a new mode at around
ꢀ
1
7
50 cm
appears as the strongest band [14] in the
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[3] C.R. Martin, C.R. Sides, F. Xu, ECS Trans. 1 (2006) 13.
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Crystallogr. B61 (2005) 17–24.
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3
[
[
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61 (1992) 2386–2388.
main difference is that while the g-phase of NaVO3 is
disordered, LiVO has sharp crystalline peaks in the high
3
pressure phases suggesting the structural transition is far
less hindered in the latter. Shen et al. [14] have attributed
this difference to the smaller size of the alkali metal atom.
Similarity of the behavior in the pressure reducing cycle is
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[
[
8] A. Grzechnik, Solid State Sci. 4 (2002) 523–527.
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[11] G.A. Kourkolis, A. Jayaraman, G.P. Espinosa, A.S. Cooper,
also striking, with the high pressure phase in LiVO3
persisting down to about 3 GPa and absence of any
J. Raman Spectrosc. 22 (1991) 57–60.
12] Z.X. Shen, C.W. Ong, S.H. Tang, M.H. Kuok, Phys. Rev. B49 (1994)
[
[
[
[
[
detectable Raman intensity upon pressure releasing [14].
Amorphization up on pressure releasing is known in some
silicate perovskites [32] and is understood as the instability
of the high pressure crystalline structure at ambient
conditions combined with a large barrier for the reverse
transformation. KVO and RbVO [10] are also reported
1
433.
13] S.H. Tang, M.H. Kuok, Z.X. Shen, C.W. Ong, J. Phys.: Condens.
Matter 6 (1994) 6565.
14] Z.X. Shen, C.W. Ong, M.H. Kuok, S.H. Tang, J. Phys.: Condens.
Matter 7 (1995) 939.
15] S. Seetharaman, H.L. Bhat, P.S. Narayanan, J. Raman Spectrosc. 14
3
3
(
1983) 403.
to show only one transition at 5.6 and 5.3 GPa, respec-
tively, not much of details are known about the spectral
changes across these transitions. CsVO shows a different
16] K. Ramani, A.M. Shaikh, B. Swaminatha Reddy, M.A. Visvamitra,
Ferroelectrics 9 (1975) 49.
[17] A.M. Shaikh, Ferroelectrics 107 (1990) 219.
3
trend in the high pressure behavior, namely the Raman
band characteristic of the V ¼ O persists in all the high
pressure phases [11] implying that in this compound, it is
possible to transform in to more compact high pressure
phases by suitable reorientation and repacking of the chain
structure. Interestingly, high pressure behavior of
b-NaVO bears similarities with that of CsVO with the
[18] V. Katsuo, E. Takayama, Acta Crystallogr. B40 (1984) 102.
[
[
[
19] Z.X. Shen, C.W. Ong, S.H. Tang, M.H. Kuok, J. Phys. Chem. Solids
5 (1994) 661.
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5 (1994) 665.
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5
5
[22] S.M. Sharma, S.K. Sikka, Prog. Mater. Sci. 40 (1996) 1.
[23] D.M. Teter, R.J. Hemley, G. Kresse, J. Hafner, Phys. Rev. Lett. 80
3
3
(
1998) 2145–2148.
V ¼ O band persisting in the high pressure phases [18]
implying the basic chain structure is retained across the
phase transitions. It may be noted that the compounds
KVO , RbVO CsVO3 and b-NaVO3 crystallize in an
[
[
24] C.A. Perottoni, J.A.H. da Jornada, Science 280 (1998) 886.
25] J.D. Jorgensen, Z. Hu, S. Teslic, D.N. Argyriou, S. Short,
J.S.O. Evans, A.W. Sleight, Phys. Rev. B59 (1999) 215.
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C.-S. Zha, Phys. Rev. B72 (2005) 024117.
3
3
orthorhombic structure while LiVO and a-NaVO crystal-
3
3
[27] R.M. Wentzcovitch, C. da Silva, J.R. Chelikowsky, N. Binggeli,
Phys. Rev. Lett. 80 (1998) 2149–2152.
lize in a monoclinic phase, and it would be interesting to
study if there is a definite correlation between the high
pressure behavior and the structure at ambient conditions.
[
[
[
28] N. Choudhury, S.L. Chaplot, Phys. Rev. B73 (2006) 094304.
29] A. Grzechnik, J. Solid State Chem. 139 (1998) 161–167.
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5
. Conclusion
[
[
31] Z.V. Popovic, V. Stergiou, Y.S. Raptis, M.J. Konstantinovic,
M. Isobe, Y. Ueda, V.V. Moshchalkov, J. Phys.: Condens. Matter
High pressure Raman spectroscopic studies on a-NaVO3
have been carried out upto 13 GPa. Spectral changes at
pressures around 4 GPa appear to be due to formation of
1
4 (2002) L583–L589.
32] M. Hemmati, A. Chizmeshya, G.H. Wolf, P.H. Poole, J. Shao,
C.A. Angell, Phys. Rev. B 51 (1995) 14841–14848.