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Raman Study of β-Sr 0.33 V 2 O 5 Micro-Crystals under Variable Temperature and
Pressure
A.G. Kontos 1 , D. Lampakis 1 ,Y.S. Raptis* 1 , E. Liarokapis 1 , Z. V. Popović 2 , M. Isobe 3 , and Y. Ueda 3
1 Physics Department, National Technical University of Athens, 157 80 Athens, Greece
2 Center for Solid State Physics and New Materials, Institute of Physics, 11080 Belgrade, Serbia
3 Institute for Solid State Physics, University of Tokyo, Kashiwa , Chiba 277- 8581, Japan
*yraptis@central.ntua.gr
The vanadium bronzes, β(β′)-A 1+(2+) xV 2 O 5 (x = 1/3; 2/3; A= Li, Na, Ag, Ca, Sr, Pb, Cu etc.) exhibit a variety of phenomena
that originate from strong electron correlations. For example, various charge and spin orderings, and even superconductivity
(β-Na 0.33 V 2 O 5 [1]) are found in this class of materials. The different phases of β-A 0:33 V 2 O 5 family, exhibit an one dimensional
metallic behavior at room temperature and undergo a metal to insulator (MI) phase transition at T MI , associated with a charge
ordering. Systems with monovalent cations (A + ) show a long-range magnetic order at T < T MI . In systems with divalent
cations (A 2+ ), no sign of long-range magnetic order is observed down to 2 K [2]. β-Sr 0.33 V 2 O 5 undergoes MI phase transition
at T MI ≈165 K, associated with a charge ordering. The phase transition has been confirmed by magnetic susceptibility, [2],
NMR, [3] and Raman scattering [4]. Electrical resistivity of this oxide, was measured under high hydrostatic pressure by
Yamauchi et al. [5]. A metallic behavior is observed only along the b-axis (chain direction), while the resistivity behavior is
semiconductive along the a- and c- axis. The resistivity along the c- and a-axis is higher about two orders than along the b-
axis. By pressure increase up to 90 kbar no sign of superconductivity was observed in all β-A 2+ 0.33V 2 O 5 .[5]. High pressure
shows a strong influence on the MI phase transition temperature, which strongly decreases with increasing pressure. It is
estimated [5] that the critical pressure, where the charge order phase completely collapses in β-Sr 0.33 V 2 O 5 , is about 15 kbar.
Single crystals of β-Sr 0.33 V 2 O 5 were grown by a flux method using SrV 2 O 6 as a flux. SrV 2 O 6 was prepared by a solid state
reaction of SrCO 3 and V 2 O 5 in air at 600 0 C. Details of the sample preparation were published in Ref. [2]. The Raman
scattering measurements have been carried out at backscattering geometry using a Jobin-Yvon T64000 triple spectrometer
with a liquid nitrogen cooled charge-coupled-device detector. The 514.5 nm laser line of an Ar + -ion laser was used as an
excitation source with a power density of 0.3 mWμm -2 upon the sample. Low temperature measurements were carried out in
an Oxfords Instrument continuous flow crystotat. Pressure was applied via a Merrill-Bassett DAC, using Si as the pressure
calibration unit and ethanol-methanol pressure medium.
1 bar
79 K (cc)
300 K
Intensity (arb. units)
(bb)
(cb)
100 200 300 400 500 600 700 800 900 1000 1100
Raman shift (cm -1 )
Fig. 1 Room temperature and liquid nitrogen temperature polarized Raman spectra of β-Sr 0.33 V 2 O 5 at ambient pressure.
Recent, room temperature x-ray measurements [6] in β-Sr 0.33 V 2 O 5, revealed the presence of a superstructure with a lattice
modulation vector q = (0, 1/2, 0). This indicates the doubling of the unit cell along the b-axis. The diffraction pattern of this
phase has been indexed using the P21/a space group. The corresponding factor analysis predicts 132 Raman active modes.
The polarized Raman spectra of β-Sr 0.33 V 2 O 5 , measured from the bc plane at room temperature and liquid-nitrogen
temperature are given in figure 1. At a first glance, we found that the number of observed modes at room temperature is much
less than those predicted. Namely, only 27 modes are observed at room temperature (20 of A g and 7 of B g symmetry).
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