Synthesis, structural approach and electronic properties of V18O45, (N2C6H14)6: a new organically templated vanadium oxide exhibiting V2O5 layer topology

Solid State Sciences 4 (2002) 285–288 www.elsevier.com/locate/ssscie

Synthesis, structural approach and electronic properties of V18 O45, (N2 C6 H14)6 : a new organically templated vanadium oxide exhibiting V2 O5 layer topology Mickael Sicard a , Antoine Maignan a , Didier Riou b,∗ a Laboratoire CRISMAT-ISMRa UMR 6508, 6 Boulevard du Maréchal Juin, 14050 Caen cedex, France b Institut Lavoisier UMR CNRS 8637, Université de Versailles St Quentin, 45 Avenue des États-Unis, 78035 Versailles cedex, France

Received 19 November 2001; accepted 22 November 2001

Abstract V18 O45 , (N2 C6 H14 )6 was hydrothermally synthesized in the form of thin platelets. Its structural approach was investigated by single crystal X-ray diffraction (non-centrosymmetric P21 (No 4) monoclinic space group with a = 10.7713(3) Å, b = 11.2697(3) Å, c = 29.7630(9) Å, β = 93.924(1)◦ , V = 3604.4(2) Å3 , Z = 2). V18 O45 , (N2 C6 H14 )6 exhibits a lamellar structure built up from the stacking of vanadium oxide slabs between which the diprotonated 1,4-diazabicyclo[2.2.2]octane organic cations are intercalated. The oxide layers are topologically similar to those encountered in the parent vanadium pentaoxide V2 O5 but exhibiting here a mixed valence VIV /VV with a ratio equal to 2. The electronic conductivity measurements performed on the crystals show that the resistivity curves are described by an Arrhenius law with an activation energy of 0.16 eV.  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Hydrothermal synthesis; Vanadium oxides; Conductivity measurements

1. Introduction The interest for vanadium oxide compounds is essentially associated to their potential applications as cathode materials for lithium batteries or as catalysts [1]. In fact, the three usual oxidation states for vanadium (III, IV, V) associated to their correlated respective coordinations lead to a particularly rich crystallochemistry. Vanadium oxides are found mixed or single valences, with topologies from discrete species (polyanionic phases) to 3D-frameworks, inserting or not mineral or organic cations. A recent paper of Zavalij and Wittingham [2] gives an almost exhaustive review of all these phases and furthermore proposes some notation rules to classify them. The use of organic templates was more recently developped correlatively to the emergence of the hydrothermal pathway, allowing a large increase of the vanadium oxide family, recently reviewed by Zubieta et al. [3]. In this way, using 1,4-diazabicyclo[2.2.2]octane (hereafter noted DABCO) as templating agent, Nazar et al. de* Correspondence and reprints.

E-mail address: [email protected] (D. Riou).

scribed a lamellar structure with V3 O7 stoichiometry [4] a few years ago. Working with the same system, we describe here the synthesis and the structure determination of V18 O45 , (DABCO)6 ; a new VIV /VV vanadium oxide exhibiting layers with V2 O5 stoichiometry.

2. Experimental 2.1. Synthesis V18 O45 , (N2 C6 H14 )6 was hydrothermally synthesized from a mixture of V2 O5 (Aldrich Chemical, 99.6%), 1,4diazabicyclo[2.2.2]octane (Aldrich Chemical, 98%), fluorhydric acid (Prolabo RP Normapur, 48%) and deionized water in the molar ratio 1 : 2 : 2 : 80. The mixture (initial pH ≈ 4) was sealed into a teflon lined steel Parr autoclave, heated at 453 K for 24 hours then slowly cooled to room temperature during one day. The final pH was close to 7. The resultant product was washed with water then dried in air. The title compound was obtained pure (yield ≈ 50%) in the form of thin black platelets (Fig. 1).

1293-2558/02/$ – see front matter  2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 1 ) 0 1 2 4 7 - X

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Fig. 1. Scanning electron micrograph of the V18 O45 , (N2 C6 H14 )6 crystals (performed with a Jeol SM-5800LV microscope).

After verification by EDX analysis that no fluorine was incorporated to the structure, attempts to synthesize the title compound without adducting HF were performed: they always failed. It is assumed that HF favours the dissolution of the vanadium pentoxide and allows to adjust the acidity of the medium. 2.2. Chemical analysis Thermogravimetric analysis was performed under oxygen flow using a TGA2050 TA Instruments apparatus with a heating rate of 5◦ min−1 . The TG curve shows an unique weight loss occurring between 280 and 420 ◦ C which corresponds to the combustion of the organic cations. The observed value (29.2%) is in good agreement with the theoretical one (29.5%). The calcined product was identified by XRD as a mixture of an amorphous phase and V2 O5 . The mixed valence VIV /VV was confirmed by poten5+ by Fe2+ ). The tiometric titration (V4+ by MnO− 4 and V measured vanadium contents are 29.6(5)% and 11.0(5)% for V4+ and V5+ , respectively, versus 26.3% and 13.2% expected. The IR spectrum of V18 O45 , (N2 C6 H14 )6 crushed in a KBr pellet was recorded on a Nicolet Magna-IR 550 in the range 300–1700 cm−1 . It provides the principal following bands: 1607 and 1475 cm−1 for ammonium functions, 1420 cm−1 for the deformation of C–H bonds, 1320 cm−1 for C–Ntert vibration, 980 and 767 cm−1 for V=O and V–O–V linkages, respectively. Density measurements were performed with a micromeritics multipycnometer operating under He flow. 2.3. Conductivity measurements Several large crystals (typical size 2 × 1 × 0.2 mm3 ) have been extracted from the batch for the transport measurements. The largest dimensions correspond to the (a, b) plane whereas the short one corresponds to the c axis direc-

tion along which the planes of organic molecules are intercalated. The transport geometry has been chosen to probe the in-plane (ρab ) and out-of-plane (ρc ) resistivities. Four electrical contacts of indium per crystal have been ultrasonically deposited. On one hand, for ρab measurements the current injections are made by contacts deposited on two opposite (a(or b), c) faces whereas the voltage is measured on a (a, b) large surface, the four contacts being aligned. On the other hand, for the ρc measurements purpose, some crystals are contacted with two pairs of contacts, each of them being deposited on two opposite (a, b) surfaces so that the contacts are face to face. The current is injected through the (a, b) surfaces and the voltage taken between the two (a, b) surfaces by the two remaining contacts. For both in-plane (ρab ) and out-of-plane (ρc ) resistivity measurements, four crystals have been selected. The temperature dependence of the resistance is ensured by means of a Quantum Design system which allows to vary T in the range 1.8–400 K and maximum resistance of 106 to be measured. The resistivity were calculated by using the crystals dimensions and the distance between voltage contacts. 2.4. Structure determination The structure of V18 O45 , (N2 C6 H14 )6 was characterized from single crystal X-ray diffraction. A thin platelet was sticked on a glass fiber, then the intensities were collected with a Siemens Smart diffractometer equipped with a CCD detector and working with the monochromatized Mo Kα wavelength (λ = 0.71073 Å). The lattice parameters were first determined from three sets of 15 frames then refined during the data collection with all the intensities I > 10σ (I ). The distance between the crystal and the detector of 4.5 cm allowed a data collection up to 2θ = 60◦ . The frames covering one hemisphere were registered with a scan width of 0.3◦ (ω) and an exposure time of 30 s. The unique condition 0k0, k = 2n was consistent both with the P21 /m (No 11) and P21 (No 4) monoclinic space groups. All the attempts performed in the centrosymmetric space group have failed and only the non-centrosymmetric space group led to an acceptable solution applying the direct methods of S HELX - TL program. The vanadium atoms were first located, then the remaining atoms were deduced from subsequent Fourier difference syntheses. The H atoms were located using geometrical constraints. A semi-empirical absorption correction specific to CCD detector was applied using the S ADABS programm (G. Sheldrick, unpublished). At the last stage of calculation, all the atoms except H were anisotropically refined. The rather high values of the reliability factors must be correlated to the quality of the data. Although the size of the crystals is large, they always grow by stacking of thin platelets and it becomes difficult to isolate one monocrystalline species. Consequently, it is clear that this study just deals with a structural approach of V18 O45 , (N2 C6 H14 )6 which will need to be improved by a better structure resolution. (Maybe in a centrosymmetric

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Fig. 3. Projection along [0 0 1] of one vanadium oxide layer of V18 O45 , (N2 C6 H14 )6 . Fig. 2. Projection of the structure of V18 O45 , (N2 C6 H14 )6 along [1 0 0] showing the alternation of vanadium oxide layers with organic layers. The flower shape of the DABCO cations is obtained by the superimposition of two organic molecules as drawn in the insert. Table 1 Principal crystallochemical data for V18 O45 , (N2 C6 H14 )6 Chemical formula Formula weight (g mol−1 ) Crystal system Space group a (Å) b (Å) c (Å) β (deg) V (Å3 ) Z d calc (g cm−3 ) d meas (g cm−3 ) Crystal size (mm3 ) Colour Collected intensities I
2σ (I ) R int R1 (Fo ) wR(Fo2 )

V18 O45 , (N2 C6 H14 )6 2322 monoclinic P21 (No 4) 10.7713(3) 11.2697(3) 29.7630(9) 93.924(1) 3604.4(2) 2 2.140 2.216(8) 0.540 × 0.180 × 0.040 black 26525 20065 0.0406 0.0877 0.3081

space group including a mirror at y = 1/4 as it looks like observed on Fig. 2). The principal crystallochemical data are summarized in Table 1, the atomic coordinates and the principal interatomic distances are available on request to the authors.

3. Discussion V18 O45 , (N2 C6 H14 )6 exhibits a two-dimensional structure built up from the stacking along [0 0 1] of inorganic layers inside between organic cations are intercalated (Fig. 2). In the oxide layers, the vanadium atoms are located on 18

different crystallographic sites, all in distorted square pyramidal five-fold coordination. The V–O distances are similar on all the sites with the distribution usually encountered in VO5 square pyramids: four V–O distances in the range 1.8– 2.0 Å in the equatorial plane and one shorter (≈ 1.6 Å) corresponding to the perpendicular V=O bond. This last one is terminal. Bond valence calculations [5] confirm the absence of any cationic ordering and an electron delocalization in vanadium oxide layers. The VO5 square pyramids share their equatorial edges to build up along [1 0 0] some chains with terminal V=O bonds pointing up and down alternatively (Fig. 3). According to the rules of Zavalij and Whittingham [2], this type of chains is noted {UD}. These chains linked by the remaining free apices of their base form inorganic layers labelled (2{UD}). This symbol indicates that two adjacent chains are linked through square pyramids with the same orientation. This type of layer is encountered in V2 O5 [6] and therefore, V18 O45 , (N2 C6 H14 )6 can be considered as the result of the topotactic intercalation of DABCO cations between the layers of the parental vanadium pentoxide. Curiously, if numerous intercalated vanadium oxides are known, the alternation of organic planes with vanadium oxide layers exhibiting V2 O5 topology was never observed. According to the VIV / VV = 2 ratio, each inorganic layer globally carries a negative charge equal to 12 whose neutralization is ensured by six diprotonated DABCO cations. The six crystallographically different diamines interact with the (V18 O45 )12− layers via strong hydrogen bonds between the H atoms of the organic cations and the apical oxygen atoms of the square pyramids (d O–H < 2.35 Å). It is worth noting that in this compound, the strongest hydrogen bonds do not occur with the protons of the amino functions as usually observed in the organically templated oxides or metallophosphates. One can effectively show that the N–N axis of each DABCO cation lies in a direction parallel to the oxide slabs (Fig. 2) inducing longer N–O distances than the C–O distances (for example, in amine labelled 1, the shortest N–O

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ln(ρ) = f (T −1 ) curves which yield the same values of activation energy for both ρc (T ) and ρab (T ) in the same T range 175–400 K where an almost linear regime is observed. However, it cannot be excluded that this unexpected resistivity is an experimental artefact induced by the heating when the contact were deposited. Unfortunately, it is difficult to choose between the two hypotheses. The TEM experiments required to identify the crystal defects type are inconceivable due to the poor stability of the compound exposed to the electron beam of a microscope.

4. Conclusion Fig. 4. Resistivity curves ρ = f (T ) in the [V2 O5 ]∞ plane (a) and out of plane (b).

distances are approximatively 4.1 Å whereas the C–O distances are in the range 2.9–3.4 Å). Recently, we have reported [7] on the structure of V4 O10 , NC7 H14 : a lamellar mixed valence vanadium oxide (VV / VIV = 3) inserting quinuclidinium cations in between its (V4 O10 )− layers labelled (2{UUDD}.). This last compound presents a phase transition at low temperature (around 220 K) due to the ordering of the organic cations between the layers. This transformation induces a transient regime between two linear variations on the electronic conductivity curve σ = f (1/T ). Here, the title compound provides an usual behaviour with a variation between 175 and 400 K (Fig. 4a) fitted by an Arrhenius law with an activation energy close to 0.16 eV. At room temperature, the in-plane resistivity is approximatively 103  cm, lower by one order of magnitude than V4 O10 , NC7 H14 . A similar T dependence is obtained for the out-of-plane experiments (Fig. 4b) but with larger ρ values (ρc ≈ 700  cm versus ρab values in the range 1 to 2  cm at 400 K). This large anisotropy is consistent with the presence of the intercalated DABCO cations which prevent from a charge delocalization along c . We would thus expect the transport to be insulating along c . Indeed, small regions without intercalated molecules playing the role of shortcuts may provide pathways for the current along c . These crystal defects could be generated by the absence of some organic planes through the crystals leading to small domains with V2 O5 topology. This is consistent with the values of activation energies derived from the

Numerous organically templated vanadium oxides are described in the literature. A lot of them present pure square pyramidal structures; in this class, vanadium atoms are VIV/V mixed valence. V5+ and V4+ realizes the same square pyramidal coordinations leading to an electronic delocalization on the network of VIV/V O5 square pyramids. Astonishingly, among the different vanadium oxide with square pyramidal networks, the one of the parental V2 O5 precursor had never been observed. We show here that V18 O45 , (N2 C6 H14 )6 provides the first example of mixed valence vanadium oxide with V2 O5 topology inserting organic cations.

Acknowledgements The authors are very indebted to Dr. M. Riou-Cavellec (Institut Lavoisier, Versailles) and Dr. J. Marrot (Institut Lavoisier, Versailles) for help in microscopy and X-ray diffraction measurements, respectively.

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