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Lattice Dynamics of β-V2O5: Raman Spectroscopic Insight into theAtomistic Structure of a High-Pressure Vanadium PentoxidePolymorphR. Baddour-Hadjean,M. B. Smirnov,K. S. Smirnov,V. Yu Kazimirov,J. M. Gallardo-Amores,U. Amador,M. E. Arroyo-de Dompablo,and J. P. Pereira-Ramos†Institut de Chimie et Matériaux Paris-Est, GESMAT, UMR 7182 CNRS et Université Paris-Est Créteil, 2 rue Henri Dunant, 94320 ‡Fock Institute of Physics, Petrodvorets, Ul'yanovskaya st., Saint Petersbourg 198904, Russia §Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 CNRS et Université Lille1 - Sciences et Technologies, 59655 Villeneuve d'Ascq, France ⊥Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna 141980, Russia¶Laboratorio de Altas Presiones, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain∥Departamento de Química, Universidad San Pablo-CEU, 28668 Boadilla del Monte, Spain▽MALTA Consolider Team, Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain ABSTRACT: We report here the Raman spectrum and lattice dynamics study ofa well-crystallized β-V2O5 material prepared via a high-temperature/high-pressure(HT/HP) route, using α-V2O5 as the precursor. Periodic quantum-chemicaldensity functional theory calculations show good agreement with the experimentalresults and allow one to assign the observed spectral features to specific vibrationalmodes in the β-V2O5 polymorph. Key structure−spectrum relationships areextracted from comparative analysis of the vibrational states of the β-V2O5 and α-V2O5 structures, and spectral patterns specific to the basic units of the two V2O5phases are proposed for the first time. Such results open the way for the use ofRaman spectroscopy for the structural characterization of vanadium oxide-basedhost lattices of interest in the field of lithium batteries and help us to greatlyunderstand the atomistic mechanism involved in the α-to-β phase transition ofvanadium pentoxide.
The first HP modification of V2O5 (prepared at P = 4−6 GPa Pentavalent vanadium-based frameworks attract much attention and T = 650 °C) was reported by Suzuki et al.Later, Volkov because of their outstanding structural flexibility and their et al.reproduced the same HP phase of V2O5, named the β chemical and physical properties suitable for catalytic and phase. Making use of the results of quenching experiments at electrochemical applications.−The ambient-pressure form of 600 °C and between 3.5 and 9 GPa, Volkov et determined vanadium pentoxide (α-V by X-ray diffraction (XRD) measurements that the structure of 2O5) has a layered structure with orthorhombic symmetry (space group Pmmn) consisting of the β phase is tetragonal. In situ Raman spectroscopic experiments by performed in the pressure range 5 square pyramids sharing edges and corners.The structure is well adapted to the reversible incorporation of guest Li+ ions, 7−10 GPa and at room temperature (RT) revealed qualitative and since the 1970s, the α-V changes in the Raman spectrum of the V 2O5 polymorph is recognized as an 2O5 sample, especially attractive material for applications in electrochromic thin film in the V−O stretching region. The author concluded that a new devices and as a cathode in lithium batteries because of its high phase appeared and coexisted with the α-V2O5 starting energy density and retention capacity upon cycling.The material; the phase was found to return to the starting valence state of V atoms is known to be extremely sensitive to orthorhombic material upon pressure release. A few years later, the chemical environment, and such chemical versatility may Loa et performed similar RT experiments by employing lead to a variety of structures containing different building units synchrotron radiation for detecting structural changes. The and exhibiting tunable physical properties. In this regard, high- authors observed a pronounced structural disorder under pressure/high-temperature (HP/HT) synthesis routes arepromising ways to obtain novel V2O5 polymorphs, particularly for potential electrochemical applications.
Published: February 23, 2012 2012 American Chemical Society dx.doi.org/10.1021/ic202651b Inorg. Chem. 2012, 51, 3194−3201 Inorganic Chemistry elevated pressure and suggested that the correct determination atomistic pattern of the vibrational modes was not presented, of the new phase necessitates the simultaneous application of and as a consequence, no structure−spectrum relationship was In 2001, two different groupspublished their findings on This paper reports the results of a combined experimental the V2O5 phases investigated at HT/HP, and both reported the and computational study of the β-V2O5 polymorph. A well- existence of the β-VO5 phase with orthorhombic symmetry.
crystallized β-V2O5 sample was prepared at HT/HP conditions, Filonenko and performed quenching experiments and and its structure was completely characterized by XRD found the β phase up to 7.5 GPa/900 °C, which returns to the measurements. A high-quality Raman spectrum of the sample α phase at atmospheric pressure and a temperature of 400 °C.
was obtained, and DFT calculations allowed one to assign all of Kusaba et showed the presence of the β phase at 6 GPa/ the observed spectral peaks to the vibrational modes of the 500 °C from quenching experiments and at 4.5 GPa/350 °C structure. A comparison of the β-V2O5 and α-V2O5 vibrational from in situ experiments. Later, Filonenko et al.reexamined states is provided and leads to the clear identification of spectral the β phase of V features related to the presence of specific structural basic units 2O5 using XRD, neutron diffraction, and high- resolution transmission electron microscopy experiments and in the two V2O5 polymorphs.
found the structure to be monoclinic. The authors establishedthat the β-V2O5 structure is built up of infinite chains made of 2. EXPERIMENTS AND COMPUTATIONS quadruple units of edge-sharing VO6 octahedra along the b axis.
2.1. Experimental Part. A commercial (Aldrich) α polymorph of The chains are linked by sharing corners of two octahedra V2O5 was subjected to 8 GPa pressure and a temperature of 800 °C for along the c axis, which leads to a V4O10-layered composition, 1 h in a belt-type press. After the pressure and temperature were with the layers parallel to the (100) plane. Weak interactions, applied for 1 h, the vessel was quenched to RT while pressure was similar to those in MoO slowly released. Starting and resulting samples were examined by 3, hold the layers together. Preliminary electrochemical experiments showed that this new monoclinic scanning electron microscopy (SEM) on a JEOL 6400 microscope equipped with an EDAX Inc. energy-dispersive X-ray detector for 2O5 polymorph behaves as a reversible lithium intercalation compound delivering a specific capacity of 250 mAh/g at a C/ The resulting sample was also examined by XRD performed with a 3.5 rate.More recently, the V2O5 phase diagram was revisited Bruker D8 high-resolution powder X-ray diffractometer equipped with in a wide P−T range (pressures up to 29 GPa and temperatures an MBraun PSD-50 M position-sensitive detector. Monochromatic Cu up to 1500 °C), providing two HP/HT modifications of V2O5 Kα1 (λ = 1.5406 Å) radiation obtained with a germanium primary (β- and δ-V2O) having well-defined stability ranges in the monochromator was used. The treatment of the diffraction data was phase diagram.
carried out using the FullProf The structure of the β-V2O5 Structural distinctions between the α- and β-V phase reported in ref was employed as a starting model; isotropic morphs must become apparent in their vibrational spectra, and thermal parameters and constraints in V−O distances were used to the Raman spectroscopy provides a fast, reliable, and keep the number of the parameter low and to ensure stability of thefitting procedure.
nondestructive means of studying these differences. Further- The Raman spectra were measured with a LaBRAM HR 800 more, Raman spectroscopy turned out to be a very efficient tool (Jobin−Yvon−Horiba) Raman microspectrometer including edge to follow the structural changes under operation conditions for filters and equipped with a back-illuminated charge-coupled device a wide range of transition-metal oxides used as electrode detector (Spex CCD) cooled by the Peltier effect to 200 K. A He:Ne materials for lithium batteries.The Raman spectra of α-V2O5 laser (632.8 nm wavelength) was used as the excitation source. The were thoroughly studied both experimentally−and spectra were registered in the backscattering geometry with a spectral theoretically.In addition, Raman studies of the α- resolution of 0.5 cm−1. A 100× objective was used to focus the laser beam to a spot of 1 μm2 size on the sample surface. To avoid local 2O5/Li system as a pure thin film and as a composite powder electrode were carried outand were recently extended to heating of the sample, the power of the laser beam was adjusted to0.2−0.5 mW with neutral filters of various optical densities.
the β-Na0.33V2O5/Li system.
2.2. Computational Details. The vibrational states of the two On the other hand, much less attention was paid to the β- V2O5 polymorphs were computed with the CASTEP using V2O5 structure. Up to now, the in situ recorded Raman density functional perturbation theory (DFPT).The calculations spectrum at ambient temperatures reported in ref was the were carried out within the local density approximation (LDA) to only reliable experimental information on vibrational states of DFT and employed Troullier−Martins norm-conserving pseudopo- tentials.The plane-wave energy cutoff of 35 Ha was used, and the 2O5. However, Raman peaks in this spectrum are markedly large because of strong inhomogeneous broadening due to Brillouin zone integration was done over a 2 × 4 × 2 grid of points size/strain effects of pressure gradients in the cells.In the chosen according to the Monhorst−Pack scheme in the irreducible present work, the thermal annealing under pressure is expected part of the Brillouin zone. The positions of atoms in the unit cell wereoptimized with the lattice parameters fixed at their experimental values to yield a grain coarsening and well-crystallized β-V2O5 with a prior to calculation of the vibrational spectral characteristics. This low level of remaining stress, which results in a better quality option was favored over complete geometry optimization because a test calculation of the α-V2O5 structure with optimization of both the The Raman spectra measured by Balog et al.on atomic positions and the lattice parameters, in particular, resulted in a postquench samples retrieved from HP/HT experiments did value of the c parameter that was by 14% too small compared to its not show any marked distinction from the spectra of the experimental value. This finding is in a line with that of Zhou and and is explained by the fact that the LDA fails to correctly describe 2O5 material, although the presence of HP/HT phases was inferred from the XRD experiments. Density weak interactions responsible for long V−O contacts (ca. 2.7 Å)between the layers in the structure. A recent computational study by functional theory (DFT) calculations confirmed the stability of Londero and Schröshowed that dispersion-corrected exchange- the β structureand provided a theoretical Raman spectrum correlation functionals need to be used to mimic the interplane of the material that generally agrees with the experimental spacing in the α polymorph of vanadium pentoxide.
spectra reported in ref These calculations also ascertained The Raman activity of the vibrational modes was computed within the symmetry assignment of the observed Raman peaks, but the DFPT as described in ref To obtain the Raman scattering dx.doi.org/10.1021/ic202651b Inorg. Chem. 2012, 51, 3194−3201 Inorganic Chemistry intensities, the Raman activities of the vibrational modes were an average isotropic crystallite diameter of 374(5) Å and a low multiplied by Bose−Einstein factors corresponding to the exper- level of remaining stress. Thus, the prepared β-V2O5 is a well- imental conditions (temperature and wavelength of exciting radiation).
crystallized and ordered material with low strain effects (forcomparison see, for instance, ref 3. RESULTS AND DISCUSSION Table compares the fractional coordinates of atoms in the 3.1. Structure. SEM examination of commercial α-V2O5 unit cell of the β-V2O5 structure obtained in the present study shows that it consists of large aggregates of particles with a wide with those given in ref and with the coordinates of atoms distribution in size (up to 50 μm). The HT/HP treatment optimized in the DFT calculations. The agreement with the yields a β-V2O5 sample with much smaller aggregates of literature data is good, and the differences between the two particles not exceeding 10 μm. We have checked the resulting experimental sets of coordinates are explained by the fact that sample's composition by energy-dispersive spectrometry by XRD measurements performed in the present work are less analyzing up to 15 particles. The V/O ratio was equal to 2:5 precise, especially in locating light O atoms, than the within the experimental error, which confirms the stoichio- experimental data reported by Filonenko et using neutron metric nature of HP-V2O5. Furthermore, the sample was diffraction. Nevertheless, the results of XRD measurements verified to be single phase because all of the particles showed unambiguously point to the fact that the prepared HP/HT the same composition.
sample is indeed the β phase of vanadium pentoxide. Given the The XRD pattern of the quenched sample is shown in Figure above arguments, the following discussion of the structure β- All of the diffraction peaks can be assigned to the monoclinic V2O5 makes use of the structural data reported in ref It is common to consider the structure of V2O5 polymorphs as an arrangement of VOx polyhedra. Such a representation ofthe α-V2O5 structure built of VO5 polyhedra is shown in FigureA similar view of the β-V2O5 structure in which the V atomsare considered 6-fold-coordinated is given in Figure In bothstructures, the VOx polyhedra share their edges and corners,thus forming layers that are stacked along the z direction andare held together by relatively weak interactions. This structurerepresentation, however, is not fully justified from the crystalchemistry viewpoint because it considers V−O contacts longerthan 2 Å as valence bonds.
On the other hand, if considering only contacts with lengths of less than 2 Å as "true" bonds, one can highlight thesimilarities and differences in the arrangement of structuralentities in the crystal lattices of the two V2O5 polymorphs. Sucha representation of the α- and β-V2O5 structures is displayed inFigure One sees that both structures are built of [V2O5] units. Three Figure 1. Profile refinement of the XRD pattern corresponding to a types of V−O bonds with lengths of less than 2 Å can be sample prepared at a pressure of 8 GPa and a temperature of 1073 K identified in the units (Table vanadyl V−O1 bonds (d1), V− for 1 h in a belt-type press. Red circles: observed pattern. Black line: 3 bonds (d2) forming V−O3 V bridges in the xz planes, and calculated pattern. Blue line: difference between observed and calculated. The Bragg peaks are indicated by vertical bars.
2 bonds (d3) forming V−O2 V bridges oriented along the y direction. The arrangement of the [V2O5] units via the d3 unit cell previously proposed for the HP form of V bonds results in [V 2O5]∞ chains running in the y direction, and final unit cell parameters (space group P2 the chains are interconnected by V−O 2 contacts (d4) longer 7.1016(3) Å, b = 3.5668(1) Å, c = 6.2742(3) Å, and β = than 2 Å, which are shown in Figure by dashed lines. As thepoint symmetry changes from D 90.121(3)°, and these values are in very good agreement with 2h in α-V2O5 to C2h in β-V2O5, the crystallographic sites of V, O1, and O2 atoms split into two those determined in previous studies.
nonequivalent groups distinguished by subscripts a and b in The microstructural features of the sample were determined O2b and Vb O2a contacts link the by the two-step procedure proposed by Langford.We found chains of two V2O5 layers in the z direction (cf. the dashed lines Table 1. Fractional Coordinates of Atoms in the Unit Cell of the β-V2O5 Structure expt, present work DFT, present work aRp = 3.64, Rwp = 4.48, Rexp = 2.38, χ2 = 3.55, and Bragg R factor = 3.61.
dx.doi.org/10.1021/ic202651b Inorg. Chem. 2012, 51, 3194−3201 Inorganic Chemistry Figure 2. Polyhedral views in the xz projection of α-V2O5 (a) and β-V2O5 (b) structures.
in Figure Furthermore, half of the vanadyl d1 bonds in the β-V2O5 structure lose their terminal character and transforminto highly asymmetric V − O1b Vb bridges. The long Va O1b contacts shown in Figure by dotted lines connect thechains in the x direction. In addition, the V − become asymmetric.
This structural information provides an indispensable basis for the interpretation of the vibrational spectroscopic pattern ofthe V2O5 polymorphs.
3.2. Raman Spectra. Both of the α- and β-V2O5 crystal structures contain two formula units per unit cell. According tothe group symmetry analysis, 21 Raman-active phonon modesof the α- and β-V2O5 polymorphs are distributed over theirreducible symmetry representations as follows: Figure 3. Views of the α-V2O5 (a) and β-V2O5 (b and c) structureswith contacts longer than 2 Å shown as dashed and dotted lines (see α‐V O (D ): Γ the text for details). Images a and b are in the xz projection. Image c isin the xy projection.
β‐V O (C ): Γ Table 2. Characteristic Interatomic Distances (in Å) in α- and β-V2O5 According to the Experimental Data Reported in 2O5 structure, vibrational modes involving atomic displacements in the xz plane belong to A g and B2g species, and the modes combine into the A g representation in the β-V2O5 lattice. Similar symmetry distribution is valid for the out-of- plane vibrations with y displacements, which belong to B 3g species in α-V2O5 and merge into the Bg representation in The experimental Raman spectrum of the β-V shown in Figure while Figure presents the Raman microstructure of the sample because thermal annealing under spectrum of the α-V2O5 material.
The Raman spectrum of β-V pressure led to a well-crystallized β-V 2O5 exhibits a series of well- 2O5 sample with a low resolved peaks, many of which have not been observed level of remaining stress.
previously in the in situ RT/HP experimentsand havebecome evident in the present work. This is related to the The following features can be observed: dx.doi.org/10.1021/ic202651b Inorg. Chem. 2012, 51, 3194−3201 Inorganic Chemistry Table 3. Experimental and Calculated Frequencies andMaximum Atomic Amplitudes of Raman-Active PhononModes of the β-V2O5 Structure max atomic amplitudes Figure 4. Raman spectra: (a) β-V 2O5 sample prepared at a pressure of 8 GPa and a temperature of 1073 K for 1 h in a belt-type press; (b) α- 2O5 precursor.
aL1 and L2 denote translations of neighboring layers.
(i) Two intense and sharp peaks in the high-frequency region at 942 and 1021 cm−1, and computed spectra and proposes the assignment of (ii) Two large peaks located at 686 and 736 cm−1, observed Raman peaks to specific vibrational modes, based (iii) A large peak with a maximum at 574 cm−1 and a wide on the results of the calculations. One can notice a systematic shoulder reaching 620 cm−1 from the high-frequency overestimation of the computed vibrational frequencies in the region above 500 cm−1, which is inherent to the level of theory (iv) Two peaks at 434 and 476 cm−1; note that only one peak used. One can also observe that the intensities of some peaks in at ca. 450 cm−1 was observed in ref the low-frequency part of the spectrum are not well (v) A number of well-resolved peaks below 400 cm−1 with reproduced. Nevertheless, a good general correspondence the most intense features at 96, 176, 230, 244, 271, 285, between the spectra allows interpretation of the measured 301, 340, and 357 cm−1. Note that only ill-defined bands spectral pattern.
with an intense peak at ca. 80 cm−1 were observed in this Now we turn to assignment of the most prominent spectral features. It is instructive to begin with the α phase of the The calculated spectrum of β-V vanadium pentoxide structure. This consideration will facilitate 2O5 is shown in Figure where it is compared with the experimental one. The spectrum the subsequent interpretation of the spectral pattern of themore complicated β polymorph. Taking into account thestructural data collected in Table one can expect to find fourlines in the high-wavenumber region of the Raman spectrum of α-V2O5 that correspond to four different bond stretchingmodes (ν) with frequency relationships ν(d ) < ν(d ) < ν(d ) < ν(d ) Furthermore, the O2 and O3 atoms are located in symmetricV−O−V bridges and, therefore, the ν(d2) and ν(d3) modessplit into symmetric (s) and antisymmetric (as) components.
Only νas(V−O−V) modes are genuine bond-stretching modesbecause the νs(V−O−V) modes are coupled with the δ(V−O−V) bending modes and thus have markedly lower frequencies.
Consequently, one can detail the above relationship as follows: Figure 5. Experimental (black) and computed (color bars) Ramanspectra of the β-V2O5 sample. Red and blue bars stand for the Ag and ν (d ) < ν (d ) < ν(d ) < ν (d ) < ν (d ) < ν(d ) Bg vibrational modes, respectively.
The experimental Raman spectrum of α-V2O5 (Figure has well agrees with the theoretical spectrum reported in ref four Raman peaks at 996, 701, 528, and 482 cm−1, which wereassigned to the ν(d1), νas (d3), ν (d4), and νs(d2) modes, Table compares the positions of the peaks in the experimental respectively(Table The missing νas(d2) mode is not dx.doi.org/10.1021/ic202651b Inorg. Chem. 2012, 51, 3194−3201 Inorganic Chemistry Table 4. Frequencies and Assignments of High-Frequency Raman-Active Phonon Modes of the α-V2O5 and β-V2O5 Structures max atomic amplitudes observed because of its low intensity. This feature is related to (iii) The Raman line at 700 cm−1 due to the νas(d3) mode in the structural peculiarity of the α-V2O5 structure, where the V− the α phase splits into two Raman lines in the β phase: V bridges are almost linear (cf. Figure and the νas(d2) the first one, ν O2a Va), is related to the line at 686 mode, being formally Raman-active, has very low Raman cm−1, and the second one, ν O2b Vb), contributes activity because of a compensation effect. A theoretical to the signal at 574 cm−1.
prediction of the frequency of this quasi-silent mode based (iv) The Raman line at 528 cm−1 corresponding to the ν(d4) on a force-field model gave a value of 848 cm−1recent DFT mode in the spectrum of α-V2O5 undergoes a downward calculations predicted higher values of or 1010 cm−1 shift to 476 cm−1 in the β phase. This line can now be Regardless the exact value of the νas(d2) wavenumber, one sees attributed to the ν(V − O2a) mode because the Va O2b that the Raman features in the high-frequency part of the contacts are too long (∼2.3 Ǻ) to give rise to a spectrum of the α-V2O5 structure can be assigned to the characteristic normal vibration. A signal due to variation variation of specific structural elements with the use of of the latter contacts is hidden in a host of low-frequency structural information and common spectroscopic wisdom. It is noteworthy that such an analysis for the low-frequency part The above results allow us to propose fingerprints permitting of the spectrum is much more difficult because the vibrational identification of the α- and β-V modes with frequencies in this region contain important 2O5 polymorphs with the help of Raman spectroscopy: contributions of angle-bending coordinates and involve thedynamics of larger structural entities.
1 Raman lines around 900 cm−1 correspond to the Now we turn to the discussion of the vibrational states of β- stretching vibrations of vanadyl VO bonds. A number of such lines indicate the number of nonequivalent 2O5. The differences between the α- and β-V2O5 structures discussed above are obviously expected to manifest themselves vanadyl bonds. It is one for the α phase and two for the β in the vibrational spectra, and given the above structural information, one can await the following changes in the 2 The appearance of two Raman peaks at 736 and 686 spectrum of the β-V cm−1 is characteristic of the β-V 2O5 structure compared with that of the α- 2O5 polymorph. The peak at 736 cm−1 points to the presence of the (i) Because half of the V−O asymmetric V−O3 V bridge in the β-V2O5 structure.
1 bonds lose their terminal character, a new peak related to the ν(V − In the α phase, the vibrations localized within the has to appear at a lower frequency.
symmetric and quasi-linear V−O3 V bridge manifest (ii) Because of the asymmetry of the V−O − themselves as one single νs(d2) mode located at 482 cm−1, whereas in the β phase, the change of the bridge V) and νas (V−O3 V) modes have to be replaced by the ν(V − geometry leads to the appearance of two Raman-active O3) and ν(Vb O3) modes at 736 and 574 cm−1.
The second fingerprint peak at 686 cm−1 corresponds g modes related to νas(V−O2 V) vibrations must to a νas(Va O2 Va) mode of Bg symmetry, while the O2a Va) and νas(Vb O2b Vb) modes.
Analysis of the experimental Raman spectrum of β-V b) Bg mode contributes to the Raman signal at 574 cm−1. The origin of such a large frequency Figure and of the peak assignments given in Table shows splitting between the two B that the observed spectral pattern of the HP phase of vanadium g modes is worth a special comment because similar ν pentoxide indeed follows these expectations: and B3g symmetry in the α-V2O5 structure have almost (i) The spectrum of the β phase has two lines at 942 and equal frequencies of 700 cm−1. The 110 cm−1 splitting 1021 cm−1 assigned to the ν(V − O1b) and ν(Va O1a) between the Bg modes in the β structure cannot be modes, respectively, instead of one line at 996 cm−1 explained by the difference in the lengths of the V − corresponding to the ν(d1) mode in the spectrum of the O2b bonds, which are equal to 1.871 and 1.872 Å, respectively. Furthermore, both modes involve equal as(d2) and νs(d2) modes inherent to the vibrations contributions of the νas(Va O2a Va) and νas(Vb O2b of the bridging O3 atoms in the α phase become the Vb) oscillations.
O3) and ν(Va O3) modes in the β structure. The The splitting can readily be understood by considering former gives rise to the Raman line at 736 cm−1, while the structure of the β-V2O5 phase and the displacement the latter contributes to the wide spectral feature of atoms in the two modes. The structure of the centered at 574 cm−1.
vanadium oxide polymorph is characterized by short dx.doi.org/10.1021/ic202651b Inorg. Chem. 2012, 51, 3194−3201 Inorganic Chemistry O2b contacts of 2.52 and 2.49 Ǻ in the experimental Raman pattern of the β phase of vanadium pentoxide;these data and DFT calculations, respectively. Therefore, one experimental data are complemented by periodic quantum- might expect a strong influence of O−O interactions on chemical DFT calculations. A good agreement between the frequencies of modes with large vibrational observed and computed Raman spectra was demonstrated, amplitudes of the O atoms. Parts a and b of Figure and the combination of theoretical and experimental techniquesprovides valuable information, permitting a reliable assignmentof all observed spectral features. Comparative analysis of theRaman spectra and phonon states of the β-V2O5 phase withthose of the parent α-V2O5 structure has allowed us to identifyfor the first time spectral fingerprints specific to structural basicunits of the two V2O5 polymorphs. The established structure−spectrum correlations are expected to promote the use ofRaman spectroscopy for characterization of more complex β-V2O5-based structures such as β-Na0.33V2O5 bronze used as apositive-electrode material in lithium Figure 6. Displacements of O atoms in the two Bg modes with ■ AUTHORINFORMATION Corresponding Author calculated frequencies of 695 cm−1 (a) and 564 cm−1 (b) of the β- V2O5 structure. O and V atoms are shown by white and gray circles,respectively. V−O contacts longer than 2 Å are shown by dashed lines.
NotesThe authors declare no competing financial interest.
show atomic displacements of atoms in the Bg modesexperimentally observed at 686 and 574 cm−1, ■ ACKNOWLEDGMENTS respectively. One sees that the high-frequency mode M.B.S. gratefully acknowledges the financial support of involves displacements of O2 atoms in the opposite Université Paris Est Créteil and Université Lille 1. M.E.A.-D.
direction, whereas the atoms move in the same direction thanks the Spanish Ministry of Science for financial support in the low-frequency mode. Therefore, the O ··· under Projects MAT2007-62929 and CSD2007-00045. K.S.S.
contacts change their length in the first mode, while gratefully acknowledges the Centre de Ressources Informa- they remain unchanged in the second one, and it is just tiques of Université Lille 1 for allocation of computational this repulsive O−O interaction that accounts for the 110 cm−1 splitting of the vibrational frequencies. It isnoteworthy that a similar effect accounts for the features in the Raman spectrum of zircon.
In the case of the α-V (1) Livage, J. Chem. Mater. 1991, 3, 578.
2O5 structure, the two νas(V− (2) Chernova, N. A.; Roppolo, M.; Dillon, A. C.; Whittingham, M. S.
V) modes include antiparallel displacements of O2 J. Mater. Chem. 2009, 19, 2526.
atoms similar to those depicted in Figure and the (3) Liu, Z.; Fang, G.; Wang, Y.; Bai, Y.; Yao, K.-L. J. Appl. Phys. D: frequencies of these two modes almost coincide.
Appl. Phys. 2000, 33, 2327.
3 Finally, the origin of the large spectral band observed in (4) Enjalbert, R.; Galy, J. Acta Crystallogr. 1986, C42, 1467.
the β phase at around 600 cm−1 also merits special (5) Whittingham, M. S. Chem. Rev. 2004, 104, 4271.
consideration. This band has a maximum at 574 cm−1 (6) Murphy, D. W.; Christian, P. A.; Disalvo, F. J.; Waszczak, J. V.
and a wide high-frequency shoulder reaching 620 cm−1.
Inorg. Chem. 1979, 18, 2800.
Note that there are no Raman-active modes within this (7) Whittingham, M. S. J. Electrochem. Soc. 1976, 126, 315.
frequency interval in α-V (8) Wiesener, K.; Schneider, W.; Ilic, D.; Steger, E.; Hallmeir, K. H.; 2O5. Thus, this spectral feature could also serve as a fingerprint of the β structure.
Brackmann, E. J. Power Sources 1978, 20, 157.
(9) Delmas, C.; Cognac-Auradou, H.; Cocciantelli, J. M.; Ménétrier, According to our calculations, there are two modes of M.; Doumerc, J. P. Solid State Ionics 1994, 69, 257.
different symmetry and of almost equal Raman activity (10) Bates, J. B.; Gruzalski, G. R.; Dudney, N. J.; Luck, C. F.; within this frequency region: the Ag mode assigned to the Xiaohua, Y. Solid State Ionics 1994, 70/71, 619.
O3) vibration and the Bg as(Vb O2b Vb) mode (11) Cocciantelli, J. M.; Doumerc, J. P.; Pouchard, M.; Broussely, M.; with the atomic displacements shown in Figure The Labat, J. J. Power Sources 1991, 34, 103.
marked difference in the frequencies of these modes in (12) Suzuki, T.; Saito, S.; Arakawa, W. J. Non-Cryst. Solids 1977, 24, our calculations (A g, 581 cm−1; Bg, 564 cm−1) can explain the large width of the observed spectral band. Note that (13) Volkov, V. L.; Golovkin, V. G.; Fedyukov, A. S.; Zaynulin, Y. G.
Inorg. Mater. 1988, 24, 1568. Translated from: Izv. Akad. Nauk USSR the results of ref gave a much smaller frequency Neorg. Mater. 1988, 24, 1836.
difference between these modes (Ag, 611 cm−1; Bg, 613 (14) Grzechnik, A. Chem. Mater. 1998, 10, 2505.
(15) Loa, I.; Grzechnik, A.; Schwarz, U.; Syassen, K.; Hanfland, M.; Kremer, R. J. Alloys Compd. 2001, 317/318, 103.
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