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Further, almost all of the observed modes have a large width. This strong broadening is due to a strong electron–phonon
coupling and/or near degeneracy of many modes with nearly the same energies.
Below the charge ordering transition the unit cell triples, keeping the same symmetry (P21/a) [6]. In this case, the number of
active phonons is 198 for each irreducible representation. At liquid-nitrogen temperature almost all modes split as a
consequence of charge ordering followed by the tripling of the unit cell. Several modes also appear from the broad structure
ranging from 550 up to 700 cm −1 . The localization of the electrons leads to a decrease in the coupling of these phonons to
electronic excitations, and they become more pronounced below the transition temperature (165 K). At the lowest
temperature used in our experiments (79 K), these modes are the most intense ones, though they are much broader than the
rest of them.
In Fig. 2 we present polarized Raman spectra of β-Sr 0.33 V 2 O 5 measured upon a pressure of 55 Kbar at two different
temperatures, 79 and 180 K. In ref. [7] room temperature polarized Raman spectra of β-Sr 0.33 V 2 O 5 micro crystals were
obtained at different pressure up to about 57 kbar. Generally, the various peaks broaden with pressure increase and show
almost linear frequency vs. pressure dependence. Substantial differences were recorded for (cc) polarization where a broad
structure, peaked at about 640 cm -1 at ambient temperature and pressure, transforms into relatively narrow peak at about 600
cm -1 by pressure increase. For crossed polarization new modes between 500 and 600 cm -1 , were observed for the highest
applied pressure. We believe that these modes were not observed at ambient conditions due to their low intensity. The (bb)
polarized (Ag symmetry modes) Raman spectra of β-Sr 0.33 V 2 O 5 did not show dramatic changes in the Raman spectra of β-
Sr 0.33 V 2 O 5 , in accordance with electrical resistivity measurements of β-Sr 0.33 V 2 O 5 along the b-axis at high pressure [5], where
no phase change was found up to 85 kbar. The above characteristics can also be observed in the spectra of fig. 2.
(cc)
55 K
79 K
180 K
Intensity (arb. units)
(bb)
(cb)
100 200 300 400 500 600
Raman shift (cm -1 )
Fig. 2. Raman spectra of β-Sr 0.33 V 2 O 5 at 55 kbar measured at temperature above (180 K) and below (79 K) the metal-toinsulator
phase transition temperature of β-Sr 0.33 V 2 O 5 at ambient pressure (165 K).
Comparison of the spectra at 55 kbar, measured above (180 K) and below (79 K) the metal-to-insulator phase transition
temperature of β-Sr 0.33 V 2 O 5 at ambient pressure (165 K), shows minor differences, except of mode narrowing due to lowering
of the temperature. This finding is in accordance with Yumagashi et al. results, that the charge order in β-Sr 0.33 V 2 O 5
collapses at relatively low pressure (15 kbar). In our case, due to relatively high hydrostatic pressure (55 kbar), the charge
ordered phase at 79 K has been collapsed and drastic changes of Raman spectra could not be expected by lowering the
temperature.
References
[1] Yamauchi T., Ueda Y. and Mori N., Phys. Rev. Lett. 89 (2002) 057002.
[2] Isobe M. and Ueda Y., Mol. Cryst. and Liq. Cryst., 341 (2000) 271.
[3] Ueda Y., Yamada H., Isobe M., and Yamauchi T., J. Alloys and Compounds, 317-318 (2001) 109.
[4] Popović Z. P., Kontos A.G., Raptis Y.S., Isobe M. and Ueda Y., J. Phys. Cond. Matter, 18 (2006) 7779.
[5] Yamauchi T., Isobe M., and Ueda Y., Solid State Sciences, 7 (2005) 874.
[6] Sallier C, Boucher F and Junod E 2003 Solid State Sci. 5 (2003) 591.
[7] Kontos A.G., Lampakis D., Raptis Y.S., Liarokapis E., Popović Z.P., Isobe M. and Ueda Y., phys. stat. sol. (b), 244
(2006) 362.
This work is supported by the bilateral Greek–Serbian collaboration ‘Joint Research and Technology Programmes, 2003–
2007’.
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