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Vanadium oxides V2O5 and NaV2O5 under high pressures: Structural, vibrational, and electronic properties
Journal of Alloys and Compounds 317–318 (2001) 103–108
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Vanadium oxides V2 O 5 and NaV2 O 5 under high pressures: Structural, vibrational, and electronic properties a, a a,b a c a I. Loa *, A. Grzechnik , U. Schwarz , K. Syassen , M. Hanfland , R.K. Kremer b
a ¨ Festkorperforschung ¨ , D-70569 Stuttgart, Germany Max-Planck-Institut f ur ¨ Chemische Physik fester Stoffe, D-01187 Dresden, Germany Max-Planck-Institut f ur c European Synchrotron Radiation Facility, F-38043 Grenoble, France
Abstract The effect of pressure on the physical properties of V2 O 5 and NaV2 O 5 has been studied by high-resolution angle-dispersive X-ray powder diffraction, Raman spectroscopy, and optical reflectivity measurements. We present a comparative study of the structural, vibrational and electronic properties of both compounds under pressure. The compression of V2 O 5 and NaV2 O 5 is highly anisotropic with the stacking directions of the layered structures being the soft axes. A structural phase transition to a three-dimensionally linked structure occurs at |7 GPa in V2 O 5 as compared to |25 GPa in NaV2 O 5 . This indicates that intercalation of Na results in a higher stability of the V2 O 5 layers under pressure. In either case the V coordination changes from 5- to 6-fold. Substantial red shifts of the optical excitation spectra and changes of the vibrational properties accompany the phase transitions. 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadium oxide; High pressure; Phase transition; X-ray powder diffraction; Raman spectroscopy; Optical reflectivity
1. Introduction V2 O 5 -based systems have since long been studied and are widely used in industry for selective oxidations, ammoxidation of hydrocarbons, and selective reduction of NO x . The open structure of V2 O 5 favors molecular and ionic intercalations. More recently, NaV2 O 5 has attracted considerable interest after it was proposed to be a second realization of an inorganic spin-Peierls system [1–3]. The observed phase transition at a critical temperature of 34 K to a nonmagnetic ground state accompanied with a lattice distortion is, however, currently discussed as a secondary effect to a charge-ordering process [4–6]. Fig. 1a sketches the crystal structure of NaV2 O 5 at ambient conditions (space group Pmmn, Z 5 2) [7]. Double chains of edge-sharing distorted VO 5 square pyramids are running along the b-direction. The chains are corner-linked within the ab layers. Sodium is intercalated in between sheets which are stacked along the c direction. V2 O 5 adopts a very similar structure [8]. The main structural distinction is a larger layer spacing of NaV2 O 5 (Dc /c¯ 5 9%) and *Corresponding author. Tel.: 149-711-689-1469; fax: 149-711-6891010. E-mail address: [email protected] (I. Loa).
Fig. 1. (a) Crystal structure of NaV2 O 5 at ambient conditions (space group Pmmn). (b) ‘Idealized’ V2 O 5 structure.
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01404-3
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related differences of oxygen z-positions. The ‘idealized’ V2 O 5 structure based on regular VO 6 octahedra is depicted in Fig. 1b. There have been a few reports on the properties of V2 O 5 under high pressure. Electrical resistance measurements by Minomura and Drickamer [9] in 1963 revealed a phase transition of V2 O 5 at 10.5 GPa with a large increase in resistance. Later, the crystallization of amorphous splatcooled V2 O 5 under pressure was studied by Suzuki et al. [10], who also synthesized a second modification, b-V2 O 5 . This modification was also obtained by Volkov et al. [11] in a high-pressure and high-temperature synthesis. Recently, Grzechnik investigated a pressure-induced phase transition of V2 O 5 at 7–10 GPa by Raman spectroscopy [12]. Changes of the vibrational properties were attributed to 5to 6-fold coordination change of the V ions. The physical properties of NaV2 O 5 at high pressures have recently been studied extensively: Structural properties [13,14], lattice dynamics [15], the optical excitation spectrum [16], and the effect of pressure on the temperature-induced spinPeierls / charge-ordering transition [17] were investigated. We present here a comparative study of the structural, vibrational and optical properties of V2 O 5 and NaV2 O 5 at high pressures. The two compounds exhibit many similarities and a comparison helps to gain a better understanding of these materials. The compounds differ with respect to the formal oxidation state of V ions which is 5 1 in V2 O 5 compared to 4 1 / 5 1 in the mixed-valence NaV2 O 5 .
2. Experimental The structural properties of V2 O 5 and NaV2 O 5 under pressure were studied at ambient temperature by highresolution angle-dispersive X-ray powder diffraction at the European Synchrotron Radiation Facility (ESRF Grenoble, beamline ID9). Monochromatic radiation of l ¯ 45 pm was used and the diffraction patterns were recorded on image plates. The images were integrated using the program FIT2D [18] to yield intensity vs. 2u diagrams. For pressure generation a diamond anvil cell (DAC) was used. Nitrogen served as pressure medium to ensure nearly hydrostatic conditions for both compounds. One series of experiments on V2 O 5 was performed with paraffin oil. Results obtained with the two different pressure media were fully consistent, such that we can rule out any intercalation reactions. The DAC was rocked by 638 to improve the powder averaging. In all experiments presented here, pressures were measured by the ruby luminescence method using the calibration of Ref. [19]. Raman spectra of NaV2 O 5 single crystals at ambient temperature were recorded in quasi-backscattering configuration using a triple-grating spectrometer. An argon-ion laser was used for excitation at 514.5 nm. Experimental details are described in Ref. [15]. Optical reflectivity spectra of V2 O 5 and NaV2 O 5 in the energy range 0.6–4.0
eV were measured at pressures up to 11 and 38 GPa, respectively, using a micro-optical setup described in Ref. [20]. The samples were in direct contact with one anvil of the DAC such that the absolute reflectance R d at the diamond–sample interface could be determined without interference from the pressure medium. CsCl was used as soft solid pressure medium between the sample and the second diamond anvil. Commercially available V2 O 5 powder (Aldrich) was used. For the optical experiments small single crystals were grown from a melt of this powder. Crystals of NaV2 O 5 were grown by a self-flux method from a 5:1:1 mixture of NaVO 3 , V2 O 3 , and V2 O 5 in Pt crucibles in flowing Ar atmosphere [17,21]. The NaV2 O 5 powder sample was prepared by grinding a single crystal at liquidnitrogen temperature.
3. Structural properties Fig. 2 illustrates the pressure dependences of the lattice parameters of V2 O 5 and NaV2 O 5 . In either case, the compression is highly anisotropic with the soft direction being along the c-axis. In case of V2 O 5 , the b and c dimensions shorten by 0.5 and 8%, respectively, up to 5.5 GPa. In this pressure range the a-axis expands by 0.5%. A least-squares fit of the Birch relation [22] to the volume vs. pressure data yields a bulk modulus B0 5 5062 GPa (with ˚3 the zero-pressure unit-cell volume fixed at V0 5 177.92 A and the zero-pressure derivative of the bulk modulus set to B9 5 12). NaV2 O 5 exhibits a similarly anisotropic compression and anomalous a- and b-axis compressibility. At zero pressure the c dimension is 10% larger compared to V2 O 5 . Its compression Dc /c 0 5 12% at 5.5 GPa is significantly higher than that of V2 O 5 . The inset in Fig. 2b illustrates that the a and b directions show anomalous compression with regions of negative compressibilities. The volume data up to 30 GPa yield B0 5 2463 GPa and B9 5 1263 ˚ 3 [13]. with a zero-pressure volume of 196.2 A The diminishing difference between the b and c lattice parameters observed for both substances implies a continuous change of the V coordination from square pyramidal towards octahedral. The main effect of pressure is to move the apex oxygen O1 into a more symmetrical position between V ions belonging to neighboring layers. This is illustrated in Fig. 3 which shows the intralayer V5 O1 and interlayer V? ? ? O1 distances. Diffraction diagrams of V2 O 5 above 6 GPa show additional reflections that are not compatible with the Pmmn space group. The new peaks gain intensity with increasing pressure, while some features of the low-pressure phase vanish gradually. This indicates either a phase mixture or a distortion / modulation of the low-pressure structure. Consistent with the observed extinctions at 7 GPa is a structure of P2 1 2 1 2 symmetry. It relates to the
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Fig. 2. Lattice parameters of (a) V2 O 5 and (b) NaV2 O 5 as a function pressure. Note that the pressure scales are different in (a) and (b). The inset depicts the relative changes of a and b for NaV2 O 5 . Solid lines are guides to the eye. The vertical dashed line in (a) indicates the onset pressure of a structural transition in V2 O 5 .
Pmmn structure with a tripled b parameter. However, pronounced structural disorder leading to severe anisotropic peak broadening accompanies the transition and inhibits a conclusive analysis. These difficulties may be overcome by thermal annealing at high pressures. A pressure-induced reversible structural phase transition has also been observed for NaV2 O 5 [13,14]. Under nearly
hydrostatic pressure conditions (N 2 pressure medium) this transition starts at 24 GPa and is completed at 35 GPa. The high-pressure phase was proposed to be a slightly distorted monoclinic variant [13] of the ‘idealized’ V2 O 5 structure shown in Fig. 1b. The transition occurs at significantly lower pressures if other pressure media are used [13,14], but results in the same high-pressure phase [15]. Experi-
Fig. 3. Pressure dependence of the intra- and interlayer V–O1 distances of (a) V2 O 5 and (b) NaV2 O 5 .
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ments with nitrogen, methanol–ethanol, KCl, and CsCl indicate that shear forces occurring in the solid pressure media are the crucial parameter for lowering the transition pressure. It remains unclear, however, why the transition starts at pressures as low as 13 GPa in the X-ray diffraction experiment by Ohwada et al. [14]. For V2 O 5 we found no significant influence of the pressure medium. The onset of the phase transition was always observed at 6–7 GPa. The effect of Na intercalation is, in the first place, to move the layers of V2 O 5 further apart. It weakens the interlayer interaction as the compressibility along the stacking direction is significantly enlarged in NaV2 O 5 . This effect, however, apparently diminishes under pressure. The c-axis compressibility of NaV2 O 5 changes noticeably around 10 GPa. At this pressure, the layer spacings of V2 O 5 and NaV2 O 5 become nearly equal [cf. Fig. 2b]. In addition, intercalation of Na stabilizes the layered structure under pressure; it apparently impedes the collapse of the V2 O 5 sublattice into a denser structure up to higher pressures (|24 GPa for NaV2 O 5 compared to |6 GPa in the case of V2 O 5 ). It also seems to hinder the occurrence of structural disorder which accompanies the structural transition in V2 O 5 but is far less pronounced in NaV2 O 5 . The latter effect may be visualized such that the Na ions, being located in the voids between the VO pyramids / octahedra, impede a gliding of the V2 O 5 planes and hence the development of stacking faults along the c direction. This view is supported by the observation that the pro-
nounced anisotropic peak broadening in V2 O 5 affects the (001) reflections most.
4. Vibrational properties Fig. 4a displays Raman spectra of NaV2 O 5 taken at ambient temperature for nearly hydrostatic pressures up to 17 GPa (methanol / ethanol pressure medium). With increasing pressure, substantial shifts of the phonon peaks were observed; the peaks at 530 and 972 cm 21 exhibit a pronounced softening. Fig. 4c and d depict the shifts of various phonon frequencies with respect to the 0.7-GPa values for the pressure range 0–17 GPa (low-pressure phase). All of the phonon frequencies exhibit a pronounced nonlinear pressure dependence in the low-pressure range up to | 10 GPa. The 972-cm 21 peak shows a remarkable softening of more than 50 cm 21 at 15 GPa. This mode was assigned to the out-of-plane V–O1 stretching mode [23,24]. Its softening reflects a strengthening of the layer interaction and the evolution of the pyramidal coordination of V towards an octahedral one. The increasing influence of the interlayer V? ? ? O1 bond effectively weakens the intralayer V5 O1 bond, leading to the observed softening. Comparison of these results for NaV2 O 5 with those of Grzechnik for V2 O 5 [12] shows that, qualitatively, the pressure dependences of most phonons are comparable for both materials. Specifically, the peaks at 972, 303, and 684 cm 21 and the corresponding ones in V2 O 5 (992, 310, and
Fig. 4. (a) Raman spectra of NaV2 O 5 at ambient temperature in the low-pressure phase (pressure medium methanol–ethanol). (b) Raman spectra of NaV2 O 5 in the low- and high-pressure phase in comparison (pressure medium KCl). (c, d) Pressure-induced shifts of the various phonon frequencies of NaV2 O 5 with respect to the 0.7 GPa values. Solid symbols represent data obtained for increasing pressure, open symbols for decreasing pressure.
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702 cm 21 ) exhibit the largest pressure-induced shifts in either compound; moreover the first mode softens and the latter two harden under pressure. Major deviations occur only for the mode at 453 cm 21 of NaV2 O 5 . This phonon exhibits the largest positive pressure shift at low pressures ( , 5 GPa) of 6 cm 21 / GPa whereas the corresponding mode at 483 cm 21 in V2 O 5 softens at low pressure ( 2 2 cm 21 / GPa). This discrepancy is likely related to the fact that in NaV2 O 5 this phonon mode is strongly coupled [16] to an excitation at 650 cm 21 [see the broad Raman band in the 2-GPa spectrum in Fig. 4]. The broad band probably is of electronic origin and related to the mixed valence in NaV2 O 5 . In V2 O 5 , in contrast, this excitation has not been observed. Near 25 GPa the Raman spectra of NaV2 O 5 change fundamentally, reflecting the structural phase transition. Fig. 4b shows spectra of the low- and high-pressure phase in comparison. At 27 GPa, the phase transition is not yet fully completed, as remnants of spectral features of the ambient-pressure phase are still visible. For example, the shoulder of the prominent peak at 830 cm 21 corresponds to the V–O1 stretching mode of the low-pressure structure. All modes observed in the high-pressure phase shift linearly to higher frequencies with increasing pressure. This is consistent with the notion that the layered structure transforms into a three-dimensionally linked one at high pressures. In V2 O 5 the phase transition at 7 GPa is also associated with fundamental changes to the Raman spectrum [12]. From the disappearance of the 992-cm 21 peak characteristic for the V=O1 stretching mode and the appearance of new peaks in the V–O stretching region,
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similar to the changes in NaV2 O 5 , a five-to-six coordination change has been concluded.
5. Optical excitation spectra Fig. 5 depicts optical reflectivity spectra of V2 O 5 and NaV2 O 5 for various pressures. In both cases only minor changes are observed up to the respective transition pressure. At the phase transitions, however, there are substantial red shifts of the near-UV bands which we attribute to V–O charge-transfer excitations. Furthermore, in the vicinity of the 1 eV band characteristic of NaV2 O 5 , there is a substantial spectral redistribution of oscillator strength towards the infrared range [16]. The near-IR transition of NaV2 O 5 is most likely an intervalence band corresponding to a V 41 –V 51 electron transfer. It reflects the mixed-valence situation in NaV2 O 5 and its red shift under pressure could be a precursor of an insulator to metal transition. Fig. 5b also illustrates the effect of a nonhydrostatic pressure medium on the phase transition in NaV2 O 5 . CsCl was used for this experiment as a soft solid pressure medium. The phase transition occurs at a significantly reduced pressure of 15 GPa compared to an onset of the transition at 24 GPa under nearly hydrostatic conditions. Such a dependence on the pressure medium has not been observed for V2 O 5 . Near 6 GPa the reflectance of V2 O 5 in the visible spectral range rises which originates from an increase of the optical absorption. In fact, V2 O 5 is transparent and
Fig. 5. Optical reflectivity spectra of (a) V2 O 5 and (b) NaV2 O 5 at ambient temperatures and pressures up to 11 and 20 GPa, respectively (pressure medium CsCl). R d denotes the absolute reflectance at the diamond–sample interface.
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orange in color at atmospheric pressure and becomes opaque in the high-pressure phase. This may explain the observation that the overall Raman intensity decreases strongly at the phase transition [12]. Increasing absorption reduces the effective Raman-scattering volume (smaller penetration depth) which directly affects the Raman scattering intensity.
6. Conclusions V2 O 5 and the intercalated NaV2 O 5 show strongly anisotropic compression. Most of the initial volume change is accommodated by a reduction of the c dimension, i.e. along the stacking direction of the layered structures. In addition, they exhibit anomalous compressibilities along directions within the layers, with regions of negative linear compressibility. Intercalation of Na stabilizes the structure of V2 O 5 under high pressures. A structural phase transition to a three-dimensionally linked structure occurs at |25 GPa in NaV2 O 5 compared to |7 GPa in V2 O 5 . The V coordination changes from 5- to 6-fold and the high pressure phases of both compounds are suggested to be distorted or modulated variants of the idealized V2 O 5 structure. Pronounced structural disorder accompanies the transition in V2 O 5 but is less severe for NaV2 O 5 . Substantial red shifts of the optical excitation spectra and changes to the vibrational properties occur at the phase transitions.
[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
References [24] [1] M. Isobe, Y. Ueda, J. Phys. Soc. Jpn. 65 (1996) 1178. [2] Y. Fujii, H. Nakao, T. Yosihama, M. Nishi, K. Nakajima, K.
Kakurai, Y.U. Masahiko Isobe, H. Sawa, J. Phys. Soc. Jpn. 66 (1997) 326. T. Chatterji, K.D. Lib, G.J. McIntyre, M. Weiden, R. Hauptmann, C. Geibel, Solid State Commun. 108 (1998) 23. A.N. Vasil’ev, V.V. Pyradun, D.I. Khomskii, G. Dhalenne, A. Revcolevschi, M. Isobe, Y. Ueda, Phys. Rev. Lett. 81 (1998) 1949. H. Seo, H. Fukuyama, J. Phys. Soc. Jpn. 67 (1998) 2602. P. Thalmeier, P. Fulde, Europhys. Lett. 44 (1998) 242. H.G. von Schnering, Y. Grin, M. Kaupp, M. Somer, R.K. Kremer, O. Jepsen, Z. Kristallogr. 213 (1998) 246. R. Enjalbert, J. Galy, J. Acta Crystallogr. C42 (1986) 1467. S. Minomura, H.G. Drickamer, J. Appl. Phys. 34 (1963) 3043. T. Suzuki, S. Saito, W. Arakawa, J. Non.-Cryst. Solids 24 (1977) 355. V.L. Volkov, V.G. Golovkin, A.S. Zaynulin, G. Yu, Izv. Akad. Nauk SSSR Neorg. Mater. 24 (1988) 1836. A. Grzechnik, Chem. Mater. 10 (1998) 2505. I. Loa, K. Syassen, R.K. Kremer, U. Schwarz, M. Hanfland, Phys. Rev. B 60 (1999) R6945. K. Ohwada, H. Nakao, Y. Fuji, N. Isobe, Y. Ueda, J. Phys. Soc. Jpn. 68 (1999) 3286. I. Loa, K. Syassen, R.K. Kremer, Solid State Commun. 112 (1999) 681. I. Loa, U. Schwarz, M. Hanfland, R.K. Kremer, K. Syassen, Phys. Stat. Sol. (b) 215 (1999) 709. R.K. Kremer, I. Loa, F.S. Razavi, K. Syassen, Solid State Commun. 113 (2000) 217. A. Hammersley, Computer program FIT2D, ESRF, Grenoble, France, 1998. H.K. Mao, J. Xu, P.M. Bell, J. Geophys. Res. 91 (1986) 4673. ˜ K. Syassen, in: Semiconductors and Semimetals, Vol. 54, A.R. Goni, Academic, New York, 1998, p. 248. M. Ueda, C. Kagami, Y. Ueda, J. Cryst. Growth 181 (1997) 314. F. Birch, J. Geophys. Res. 83 (1978) 1257. ´ M.J. Konstantinovic, ´ R. Gajic, ´ V. Popov, Y.S. Raptis, Z.V. Popovic, A.N. Vasil’ev, M. Isobe, Y. Ueda, Solid State Commun. 110 (1999) 381. M.N. Popova, A.B. Sushkov, S.A. Golubchik, B.N. Mavrin, V.N. Denisov, B.Z. Malkin, A.I. Iskhakova, M. Isobe, Y. Ueda, ZhETF 115 (1999) 2170, preprint cond-mat / 9807369.
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Vanadium oxides V2 O 5 and NaV2 O 5 under high pressures: Structural, vibrational, and electronic properties a, a a,b a c a I. Loa *, A. Grzechnik , U. Schwarz , K. Syassen , M. Hanfland , R.K. Kremer b
a ¨ Festkorperforschung ¨ , D-70569 Stuttgart, Germany Max-Planck-Institut f ur ¨ Chemische Physik fester Stoffe, D-01187 Dresden, Germany Max-Planck-Institut f ur c European Synchrotron Radiation Facility, F-38043 Grenoble, France
Abstract The effect of pressure on the physical properties of V2 O 5 and NaV2 O 5 has been studied by high-resolution angle-dispersive X-ray powder diffraction, Raman spectroscopy, and optical reflectivity measurements. We present a comparative study of the structural, vibrational and electronic properties of both compounds under pressure. The compression of V2 O 5 and NaV2 O 5 is highly anisotropic with the stacking directions of the layered structures being the soft axes. A structural phase transition to a three-dimensionally linked structure occurs at |7 GPa in V2 O 5 as compared to |25 GPa in NaV2 O 5 . This indicates that intercalation of Na results in a higher stability of the V2 O 5 layers under pressure. In either case the V coordination changes from 5- to 6-fold. Substantial red shifts of the optical excitation spectra and changes of the vibrational properties accompany the phase transitions. 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadium oxide; High pressure; Phase transition; X-ray powder diffraction; Raman spectroscopy; Optical reflectivity
1. Introduction V2 O 5 -based systems have since long been studied and are widely used in industry for selective oxidations, ammoxidation of hydrocarbons, and selective reduction of NO x . The open structure of V2 O 5 favors molecular and ionic intercalations. More recently, NaV2 O 5 has attracted considerable interest after it was proposed to be a second realization of an inorganic spin-Peierls system [1–3]. The observed phase transition at a critical temperature of 34 K to a nonmagnetic ground state accompanied with a lattice distortion is, however, currently discussed as a secondary effect to a charge-ordering process [4–6]. Fig. 1a sketches the crystal structure of NaV2 O 5 at ambient conditions (space group Pmmn, Z 5 2) [7]. Double chains of edge-sharing distorted VO 5 square pyramids are running along the b-direction. The chains are corner-linked within the ab layers. Sodium is intercalated in between sheets which are stacked along the c direction. V2 O 5 adopts a very similar structure [8]. The main structural distinction is a larger layer spacing of NaV2 O 5 (Dc /c¯ 5 9%) and *Corresponding author. Tel.: 149-711-689-1469; fax: 149-711-6891010. E-mail address: [email protected] (I. Loa).
Fig. 1. (a) Crystal structure of NaV2 O 5 at ambient conditions (space group Pmmn). (b) ‘Idealized’ V2 O 5 structure.
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related differences of oxygen z-positions. The ‘idealized’ V2 O 5 structure based on regular VO 6 octahedra is depicted in Fig. 1b. There have been a few reports on the properties of V2 O 5 under high pressure. Electrical resistance measurements by Minomura and Drickamer [9] in 1963 revealed a phase transition of V2 O 5 at 10.5 GPa with a large increase in resistance. Later, the crystallization of amorphous splatcooled V2 O 5 under pressure was studied by Suzuki et al. [10], who also synthesized a second modification, b-V2 O 5 . This modification was also obtained by Volkov et al. [11] in a high-pressure and high-temperature synthesis. Recently, Grzechnik investigated a pressure-induced phase transition of V2 O 5 at 7–10 GPa by Raman spectroscopy [12]. Changes of the vibrational properties were attributed to 5to 6-fold coordination change of the V ions. The physical properties of NaV2 O 5 at high pressures have recently been studied extensively: Structural properties [13,14], lattice dynamics [15], the optical excitation spectrum [16], and the effect of pressure on the temperature-induced spinPeierls / charge-ordering transition [17] were investigated. We present here a comparative study of the structural, vibrational and optical properties of V2 O 5 and NaV2 O 5 at high pressures. The two compounds exhibit many similarities and a comparison helps to gain a better understanding of these materials. The compounds differ with respect to the formal oxidation state of V ions which is 5 1 in V2 O 5 compared to 4 1 / 5 1 in the mixed-valence NaV2 O 5 .
2. Experimental The structural properties of V2 O 5 and NaV2 O 5 under pressure were studied at ambient temperature by highresolution angle-dispersive X-ray powder diffraction at the European Synchrotron Radiation Facility (ESRF Grenoble, beamline ID9). Monochromatic radiation of l ¯ 45 pm was used and the diffraction patterns were recorded on image plates. The images were integrated using the program FIT2D [18] to yield intensity vs. 2u diagrams. For pressure generation a diamond anvil cell (DAC) was used. Nitrogen served as pressure medium to ensure nearly hydrostatic conditions for both compounds. One series of experiments on V2 O 5 was performed with paraffin oil. Results obtained with the two different pressure media were fully consistent, such that we can rule out any intercalation reactions. The DAC was rocked by 638 to improve the powder averaging. In all experiments presented here, pressures were measured by the ruby luminescence method using the calibration of Ref. [19]. Raman spectra of NaV2 O 5 single crystals at ambient temperature were recorded in quasi-backscattering configuration using a triple-grating spectrometer. An argon-ion laser was used for excitation at 514.5 nm. Experimental details are described in Ref. [15]. Optical reflectivity spectra of V2 O 5 and NaV2 O 5 in the energy range 0.6–4.0
eV were measured at pressures up to 11 and 38 GPa, respectively, using a micro-optical setup described in Ref. [20]. The samples were in direct contact with one anvil of the DAC such that the absolute reflectance R d at the diamond–sample interface could be determined without interference from the pressure medium. CsCl was used as soft solid pressure medium between the sample and the second diamond anvil. Commercially available V2 O 5 powder (Aldrich) was used. For the optical experiments small single crystals were grown from a melt of this powder. Crystals of NaV2 O 5 were grown by a self-flux method from a 5:1:1 mixture of NaVO 3 , V2 O 3 , and V2 O 5 in Pt crucibles in flowing Ar atmosphere [17,21]. The NaV2 O 5 powder sample was prepared by grinding a single crystal at liquidnitrogen temperature.
3. Structural properties Fig. 2 illustrates the pressure dependences of the lattice parameters of V2 O 5 and NaV2 O 5 . In either case, the compression is highly anisotropic with the soft direction being along the c-axis. In case of V2 O 5 , the b and c dimensions shorten by 0.5 and 8%, respectively, up to 5.5 GPa. In this pressure range the a-axis expands by 0.5%. A least-squares fit of the Birch relation [22] to the volume vs. pressure data yields a bulk modulus B0 5 5062 GPa (with ˚3 the zero-pressure unit-cell volume fixed at V0 5 177.92 A and the zero-pressure derivative of the bulk modulus set to B9 5 12). NaV2 O 5 exhibits a similarly anisotropic compression and anomalous a- and b-axis compressibility. At zero pressure the c dimension is 10% larger compared to V2 O 5 . Its compression Dc /c 0 5 12% at 5.5 GPa is significantly higher than that of V2 O 5 . The inset in Fig. 2b illustrates that the a and b directions show anomalous compression with regions of negative compressibilities. The volume data up to 30 GPa yield B0 5 2463 GPa and B9 5 1263 ˚ 3 [13]. with a zero-pressure volume of 196.2 A The diminishing difference between the b and c lattice parameters observed for both substances implies a continuous change of the V coordination from square pyramidal towards octahedral. The main effect of pressure is to move the apex oxygen O1 into a more symmetrical position between V ions belonging to neighboring layers. This is illustrated in Fig. 3 which shows the intralayer V5 O1 and interlayer V? ? ? O1 distances. Diffraction diagrams of V2 O 5 above 6 GPa show additional reflections that are not compatible with the Pmmn space group. The new peaks gain intensity with increasing pressure, while some features of the low-pressure phase vanish gradually. This indicates either a phase mixture or a distortion / modulation of the low-pressure structure. Consistent with the observed extinctions at 7 GPa is a structure of P2 1 2 1 2 symmetry. It relates to the
I. Loa et al. / Journal of Alloys and Compounds 317 – 318 (2001) 103 – 108
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Fig. 2. Lattice parameters of (a) V2 O 5 and (b) NaV2 O 5 as a function pressure. Note that the pressure scales are different in (a) and (b). The inset depicts the relative changes of a and b for NaV2 O 5 . Solid lines are guides to the eye. The vertical dashed line in (a) indicates the onset pressure of a structural transition in V2 O 5 .
Pmmn structure with a tripled b parameter. However, pronounced structural disorder leading to severe anisotropic peak broadening accompanies the transition and inhibits a conclusive analysis. These difficulties may be overcome by thermal annealing at high pressures. A pressure-induced reversible structural phase transition has also been observed for NaV2 O 5 [13,14]. Under nearly
hydrostatic pressure conditions (N 2 pressure medium) this transition starts at 24 GPa and is completed at 35 GPa. The high-pressure phase was proposed to be a slightly distorted monoclinic variant [13] of the ‘idealized’ V2 O 5 structure shown in Fig. 1b. The transition occurs at significantly lower pressures if other pressure media are used [13,14], but results in the same high-pressure phase [15]. Experi-
Fig. 3. Pressure dependence of the intra- and interlayer V–O1 distances of (a) V2 O 5 and (b) NaV2 O 5 .
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ments with nitrogen, methanol–ethanol, KCl, and CsCl indicate that shear forces occurring in the solid pressure media are the crucial parameter for lowering the transition pressure. It remains unclear, however, why the transition starts at pressures as low as 13 GPa in the X-ray diffraction experiment by Ohwada et al. [14]. For V2 O 5 we found no significant influence of the pressure medium. The onset of the phase transition was always observed at 6–7 GPa. The effect of Na intercalation is, in the first place, to move the layers of V2 O 5 further apart. It weakens the interlayer interaction as the compressibility along the stacking direction is significantly enlarged in NaV2 O 5 . This effect, however, apparently diminishes under pressure. The c-axis compressibility of NaV2 O 5 changes noticeably around 10 GPa. At this pressure, the layer spacings of V2 O 5 and NaV2 O 5 become nearly equal [cf. Fig. 2b]. In addition, intercalation of Na stabilizes the layered structure under pressure; it apparently impedes the collapse of the V2 O 5 sublattice into a denser structure up to higher pressures (|24 GPa for NaV2 O 5 compared to |6 GPa in the case of V2 O 5 ). It also seems to hinder the occurrence of structural disorder which accompanies the structural transition in V2 O 5 but is far less pronounced in NaV2 O 5 . The latter effect may be visualized such that the Na ions, being located in the voids between the VO pyramids / octahedra, impede a gliding of the V2 O 5 planes and hence the development of stacking faults along the c direction. This view is supported by the observation that the pro-
nounced anisotropic peak broadening in V2 O 5 affects the (001) reflections most.
4. Vibrational properties Fig. 4a displays Raman spectra of NaV2 O 5 taken at ambient temperature for nearly hydrostatic pressures up to 17 GPa (methanol / ethanol pressure medium). With increasing pressure, substantial shifts of the phonon peaks were observed; the peaks at 530 and 972 cm 21 exhibit a pronounced softening. Fig. 4c and d depict the shifts of various phonon frequencies with respect to the 0.7-GPa values for the pressure range 0–17 GPa (low-pressure phase). All of the phonon frequencies exhibit a pronounced nonlinear pressure dependence in the low-pressure range up to | 10 GPa. The 972-cm 21 peak shows a remarkable softening of more than 50 cm 21 at 15 GPa. This mode was assigned to the out-of-plane V–O1 stretching mode [23,24]. Its softening reflects a strengthening of the layer interaction and the evolution of the pyramidal coordination of V towards an octahedral one. The increasing influence of the interlayer V? ? ? O1 bond effectively weakens the intralayer V5 O1 bond, leading to the observed softening. Comparison of these results for NaV2 O 5 with those of Grzechnik for V2 O 5 [12] shows that, qualitatively, the pressure dependences of most phonons are comparable for both materials. Specifically, the peaks at 972, 303, and 684 cm 21 and the corresponding ones in V2 O 5 (992, 310, and
Fig. 4. (a) Raman spectra of NaV2 O 5 at ambient temperature in the low-pressure phase (pressure medium methanol–ethanol). (b) Raman spectra of NaV2 O 5 in the low- and high-pressure phase in comparison (pressure medium KCl). (c, d) Pressure-induced shifts of the various phonon frequencies of NaV2 O 5 with respect to the 0.7 GPa values. Solid symbols represent data obtained for increasing pressure, open symbols for decreasing pressure.
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702 cm 21 ) exhibit the largest pressure-induced shifts in either compound; moreover the first mode softens and the latter two harden under pressure. Major deviations occur only for the mode at 453 cm 21 of NaV2 O 5 . This phonon exhibits the largest positive pressure shift at low pressures ( , 5 GPa) of 6 cm 21 / GPa whereas the corresponding mode at 483 cm 21 in V2 O 5 softens at low pressure ( 2 2 cm 21 / GPa). This discrepancy is likely related to the fact that in NaV2 O 5 this phonon mode is strongly coupled [16] to an excitation at 650 cm 21 [see the broad Raman band in the 2-GPa spectrum in Fig. 4]. The broad band probably is of electronic origin and related to the mixed valence in NaV2 O 5 . In V2 O 5 , in contrast, this excitation has not been observed. Near 25 GPa the Raman spectra of NaV2 O 5 change fundamentally, reflecting the structural phase transition. Fig. 4b shows spectra of the low- and high-pressure phase in comparison. At 27 GPa, the phase transition is not yet fully completed, as remnants of spectral features of the ambient-pressure phase are still visible. For example, the shoulder of the prominent peak at 830 cm 21 corresponds to the V–O1 stretching mode of the low-pressure structure. All modes observed in the high-pressure phase shift linearly to higher frequencies with increasing pressure. This is consistent with the notion that the layered structure transforms into a three-dimensionally linked one at high pressures. In V2 O 5 the phase transition at 7 GPa is also associated with fundamental changes to the Raman spectrum [12]. From the disappearance of the 992-cm 21 peak characteristic for the V=O1 stretching mode and the appearance of new peaks in the V–O stretching region,
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similar to the changes in NaV2 O 5 , a five-to-six coordination change has been concluded.
5. Optical excitation spectra Fig. 5 depicts optical reflectivity spectra of V2 O 5 and NaV2 O 5 for various pressures. In both cases only minor changes are observed up to the respective transition pressure. At the phase transitions, however, there are substantial red shifts of the near-UV bands which we attribute to V–O charge-transfer excitations. Furthermore, in the vicinity of the 1 eV band characteristic of NaV2 O 5 , there is a substantial spectral redistribution of oscillator strength towards the infrared range [16]. The near-IR transition of NaV2 O 5 is most likely an intervalence band corresponding to a V 41 –V 51 electron transfer. It reflects the mixed-valence situation in NaV2 O 5 and its red shift under pressure could be a precursor of an insulator to metal transition. Fig. 5b also illustrates the effect of a nonhydrostatic pressure medium on the phase transition in NaV2 O 5 . CsCl was used for this experiment as a soft solid pressure medium. The phase transition occurs at a significantly reduced pressure of 15 GPa compared to an onset of the transition at 24 GPa under nearly hydrostatic conditions. Such a dependence on the pressure medium has not been observed for V2 O 5 . Near 6 GPa the reflectance of V2 O 5 in the visible spectral range rises which originates from an increase of the optical absorption. In fact, V2 O 5 is transparent and
Fig. 5. Optical reflectivity spectra of (a) V2 O 5 and (b) NaV2 O 5 at ambient temperatures and pressures up to 11 and 20 GPa, respectively (pressure medium CsCl). R d denotes the absolute reflectance at the diamond–sample interface.
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orange in color at atmospheric pressure and becomes opaque in the high-pressure phase. This may explain the observation that the overall Raman intensity decreases strongly at the phase transition [12]. Increasing absorption reduces the effective Raman-scattering volume (smaller penetration depth) which directly affects the Raman scattering intensity.
6. Conclusions V2 O 5 and the intercalated NaV2 O 5 show strongly anisotropic compression. Most of the initial volume change is accommodated by a reduction of the c dimension, i.e. along the stacking direction of the layered structures. In addition, they exhibit anomalous compressibilities along directions within the layers, with regions of negative linear compressibility. Intercalation of Na stabilizes the structure of V2 O 5 under high pressures. A structural phase transition to a three-dimensionally linked structure occurs at |25 GPa in NaV2 O 5 compared to |7 GPa in V2 O 5 . The V coordination changes from 5- to 6-fold and the high pressure phases of both compounds are suggested to be distorted or modulated variants of the idealized V2 O 5 structure. Pronounced structural disorder accompanies the transition in V2 O 5 but is less severe for NaV2 O 5 . Substantial red shifts of the optical excitation spectra and changes to the vibrational properties occur at the phase transitions.
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