- Email: [email protected]
Magnetic resonance and structural study of the cluster fluoride Nb6F15
ARTICLE IN PRESS
Physica B 381 (2006) 47–52 www.elsevier.com/locate/physb
Magnetic resonance and structural study of the cluster fluoride Nb6F15 R. Knolla, J. Sokolovskib, Y. BenHaimb, A.I. Shamesa, S.D. Gorena,, H. Shakeda, J.-Y. The´potc, C. Perrind, S. Cordierd a Department of Physics, Ben Gurion University of the Negev, P.O. Box 653, 84105 Be’er Sheva, Israel Department of Material Engineering Ben Gurion University of the Negev, P.O. Box 653, 84105 Be’er Sheva, Israel c Laboratoire d’Organome´tallique et Catalyse: Chimie et Electrochimie Mole´culaire, Institut de Chimie, UMR 6509 CNRS-Universite´ de Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France d Laboratoire de Chimie du Solide et Inorganique Mole´culaire, Institut de Chimie, UMR 6511 CNRS-Universite´ de Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France b
Received 28 October 2005; received in revised form 12 December 2005; accepted 15 December 2005
Abstract In this work, we report updated structural data, obtained by single-crystal X-ray diffraction, on the binary fluoride Nb6F15 as well as measurements of 19F NMR over the temperature range 100–300 K, and EMR measurements over the temperature range 4–300 K. Nb6F15 is built up from Nb6 Fi12 Fa6 octahedral cluster units sharing all apical fluorine ligands to form linear Nb–Fa–Nb interunit bridges (Fi: inner fluorine located in edge-bridging position, Fa: apical fluorine located in terminal position). The 19F NMR results showed two resonance lines. One line corresponds to the inner fluorine Fi in the (Nb6F12)3 cluster core. The second 19F NMR line corresponds to the apical Fa. The shift of the NMR signal from Fi depends on the temperature and exhibits a Curie type behavior, while the position of the NMR signal from Fa is temperature independent. EMR measurements show a single Lorentzian line. The temperature dependence of the inverse of the intensity of the EMR line shows a Curie–Weiss behavior and an onset to an antiferromagnetic (AF) order at 5 K. r 2006 Elsevier B.V. All rights reserved. Keywords: Octahedral cluster; Niobium fluoride; Crystal structure; X-ray diffraction;
1. Introduction Many ordered solid-state compounds reported in the literature are based on transition metal clusters as building blocks instead of individual metal atoms, for instance octahedral cluster halides of transition elements built up from M6L18 unit [1]. In metallic cluster compounds, the number of valence electrons per cluster (VEC) depends on the charge transfers between cluster, cations and ligands. As illustrated by the examples of LuNb6Cl18 [2,3] or Nb6F15 [4], both characterized by 15e/Nb6, these clusters exhibit magnetic behavior for odd VEC values. The crystal structure of the latter was reported as early as 1965 [5]. It has a BCC structure (space group.: Im3m, Corresponding author. Tel.: +972 8 6461171; fax: +972 8 6461171.
E-mail address: [email protected] (S.D. Goren). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.253
19
F NMR; EMR; Magnetic order
a ¼ 8.1878(2) A˚). The niobium atoms are linked together to form an Nb6 octahedra characterized by 12 Nb–Nb bonds. Within the unit, 12 fluorine ligands are located in edge-bridging positions (Fi) whilst six supplementary fluorine atoms are located in terminal position (Fa) to build a Nb6 Fi12 Fa6 cluster unit (Fig. 1). These cluster units are interconnected in the three directions by sharing the six apical fluorines forming linear NbFaNb bridges along the axes of the cubic unit cell. Owing to the I-lattice, it results in two interpenetrating simple cubic (SC) lattices of cluster units related to each other by a 12 12 12 translation (Fig. 2). In this work, we report the results of 19F NMR and EMR studies, as well as updated structural data on Nb6F15. It will be observed that only the inner fluorines show the paramagnetic and antiferromagnetic (AF) ordering of the Nb6 Fi12 cluster core.
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R. Knoll et al. / Physica B 381 (2006) 47–52
Fig. 1. Representation of the Nb6 Fi12 Fa6 cluster unit. The ellipsoids are represented at the 50% probability level. The Nb6 cluster is edge-bridged by 12 inner and 6 apical fluorine ligands noted Fi and Fa, respectively.
encapsulated in an evacuated silica ampoule and subsequently placed in a furnace. The temperature was slowly brought up to 750 1C within 2 days and remained there for 2 days. Then the furnace was switched off and the sample was taken off after complete cooling. The X-ray powder pattern of the resulting light-brown product did not reveal the presence of any impurity. However, it appears that it may slowly decompose in NbO2F under ambient atmosphere or when stored in glass container. No single crystal could be obtained from such preparations even when the preparation was slowly cooled down. Indeed, iodine was introduced within the preparation as crystallizing agent in the form of Nb3I8. A pellet was prepared from Nb (94 mg), NbF5 (178 mg) and Nb3I8 (385 mg; synthesized according to the procedure reported in Ref. [6]) and heated in the same conditions as in the aforementioned procedure but slowly cooled down to room temperature within 2 days. It resulted in a crystalline powder and single crystals of Nb6I11 and Nb6F15. Energy-dispersive spectrometry (EDS) analysis did not evidence the presence of any traces of iodine in the single crystal used for structural determination. 2.2. Crystal structure determination of Nb6F15
Fig. 2. Schematic representation of the Nb6F15 structure. The Nb6 clusters of the two interpenetrating SC lattices are represented by large dark and bright circles. The small circles represent the apical fluorines Fa. For clarity, the inner fluorine Fi is not shown.
A suitable single crystal of the Nb6F15 was mounted on a Nonius KappaCCD X-ray area-detector diffractometer with MoKa radiation (l ¼ 0.71073 A˚) (Centre de diffractome´trie de l’Universite´ de Rennes 1, France). Diffraction intensities were collected at room temperature, and once the data processing was performed by the KappaCCD analysis softwares [7] the cubic a cell parameter was refined to 8.1878(2) A˚. The lattice, according to the observed systematic extinctions, was I-centered. A spherical absorption correction was applied [8]. The atomic positions were found by a direct method using the SIR 97 [9] program, and refined anisotropically. Structural refinements by least-squares techniques and subsequent Fourier difference syntheses were performed using the SHELXL-97 program [10]. 2.3.
19
F NMR measurements
2. Sample preparation and experimental procedure 19
2.1. Synthesis and characterisation of Nb6F15 Resublimed niobium pentafluoride was provided by the Institute of High Purity Substances of Nizny Novgorod (Russia). The X-ray powder diffraction pattern of NbF5, recorded in an airtight and watertight cell on a Philips X’Pert Pro diffractometer equipped with a X’celerator detector (CoKa radiation), did not reveal the presence of any impurity. A stoichiometric mixture of niobium (0.5 g, Ventron m2N8) and NbF5 was weighted, ground, formed as a pellet under inert atmosphere, and introduced into a niobium container (Plansee). After welding, the tube was
F NMR experiments were performed on a powdered sample. NMR data were measured using a Tecmag Apollo spectrometer. The experiments were done in a magnetic field of 8.0196 T of an Oxford Instrument superconducting magnet. The frequencies of the NMR spectral lines were measured relative to the F resonance in diamagnetic CF3COOH. The absorption spectra were obtained by Fourier transforming the sum of 160 echoes following a 16pulse sequence to suppress the ringing artifacts [11]. The length of the 901 pulse was 2.1 ms. Since the spectrum was too broad to be fully excited by a single pulse, the exciting frequency was swept and the spectra were added according to the procedure outlined by Clark et al. [12].
ARTICLE IN PRESS R. Knoll et al. / Physica B 381 (2006) 47–52
49
2.4. EMR measurements 6000 5000 4000 Counts
EMR experiments, performed on the same powdered sample as for 19F NMR, were done using a Bruker EMX220 EPR spectrometer in the X-band (n 9.4 GHz) equipped with an Oxford Instruments ESR 900 cryostat over a temperature range 4–300 K. Processing of EMR spectra (which comprises of differentiation, integration, determination of peak-to-peak and integrated intensities, line widths and g-factors, etc.) was done using Bruker WIN-EPR software.
3000 2000
x
1000
x
3. Experimental results
0 10
20
x
30
40
50
60
2θ
The Nb6F15 crystallographic data were reported in the literature in 1965 [5]. At that time, the X-ray diffraction techniques did not enable to obtain anisotropic displacements and accurate bond lengths. The present work results of a single crystal X-ray diffraction, refined anisotropically yielded an improved accuracy and are reported in Table 1. Further details of the crystal structure investigation can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax: (49) 7247-808-666; e-mail: crysdata@fiz.karlsruhe.de) on quoting the depository numbers CSD-415950.
Fig. 3. XRD recorded with a Cu-Ka radiation, of a powdered sample of Nb6F15 that was used in the magnetic resonance measurements measured. Lines marked with were identified as belonging to the decomposed product NbO2F.
80
60
Intensity (a.u.)
3.1. Structural results
3.2. NMR results
40
20
0
Table 1 Refined atomic positional parameters and the isotropic equivalent displacement factors for Nb6F15 Atom
Wyckoff position
x
y
z
Ueq (A˚2)
Nb Fi Faa
12e 24h 6b
0 0 0
0 0.2513(2) 0
0.24133(5) 0.2513(2) 1/2
0.0089 0.0121(9) 0.0141(12)
The space group Im3m was used in the refinement. Refined lattice parameters a ¼ 8.1878(2) A˚. Intraunit atomic distances: dNbNb ¼ 2.7944(6) A˚; d iNbF ¼ 2:0589ð17Þ A˚; ˚ d aa NbF ¼ 2:1179ð4Þ A. The closest distance between two clusters belonging to the two frameworks is 7.0906(2) A˚ (from center to center of the Nb6 octahedra).
-200
-100
0
100
200
300
Frequency (kHz) Fig. 4. Room temperature 19F NMR spectrum of a powdered sample of Nb6F15 obtained with a repetition rate of 1 pulse every 2 s. The narrow line in the middle of the spectrum corresponds to fluorine NMR line from the decomposed product NbO2F.
30 1/2 pulse/sec
Intensity (a.u.)
Fig. 3 gives the XRD of the powdered sample employed in the measurements showing all the lines of Nb6F15 and a small amount of impurity corresponding to NbO2F. Fig. 4 gives the room temperature 19F NMR spectrum of Nb6F15. The narrow NMR line in the middle of the spectrum corresponds to this impurity [13]. The two outside lines correspond to the two inequivalent fluorine sites Fi and Fa. In order to resolve the two fluorine lines, the repetition rate of the exciting rf pulses was varied. (Fig. 5). It can be clearly seen that the left peak retains its intensity down to a repetition rate of 100 pulses/s (10 ms delay between two pulses), indicating that it has a much shorter spin-lattice
-300
10 pulses/sec
20
10
333 pulses/sec
0 -300
-200
-100
0
100
200
300
Frequency (kHz)
Fig. 5. The 19F NMR spectra of Nb6F15 are shown for different repetition rates of the excitation pulses. The spectra decreases monotonically for repetition rates of 333, 200,100, 50, 20,10, 4, 2, 1.75, 1, 23, and 12 pulse/s.
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relaxation time (of the order of a few milliseconds) than the spin-lattice relaxation of the NMR line on the right (of the order of a few hundreds milliseconds). Fig. 6 shows three spectra. Spectrum (a) was obtained with a fast repetition rate of 100 pulse/s, where only the left NMR line is observed. Spectrum (b) was measured with a repetition rate of 12 pulse/s, where the full NMR spectrum is observed. Subtraction of these two spectra gives spectrum (c). The ratio of the areas of spectra (a) and (c), Ia/Ic ¼ 4.03, is in excellent agreement with the ratio of the number of inner F i atoms to the apical Fa ones as determined by the structure, which can be written as Nb6 Fi12 Fa6=2 . Thus we assign the left NMR line to the inner fluorine atom and the right NMR line to the apical one. This result allows one to study the two NMR lines separately, first by using a fast repetition to excite the left line and then using the subtraction method to obtain the right line. The inner fluorine gives a broad asymmetric NMR line typical of an
220 200
Frequency Shift (kHz)
50
180 160 140 120 100 80 2
4
6
8
10
14
Fig. 8. The inverse temperature dependence of frequency shift of the left peak of the NMR line (inner fluorine) showing a Curie behavior.
ν = 9.479431 GHz 300000
T = 295 K
5
EMR Intensity (a.u.)
(b)
Intensity (a.u.)
12
-1
1000/T(K )
200000
(a) (c)
100000
0
-5
250 0
300
350
400
450
Magnetic Field (mT) -200
0
200
Fig. 9. The room temperature EMR spectrum of Nb6F15.
Frequency (kHz)
Normalized Intensity (a.u.)
Fig. 6. (a) The saturated spectrum. (b) The full, unsaturated spectrum. The third, weaker line (c) is equal to the difference, (b)(a).
75K 125K 175K 225K 295K
0.8
0.4
axially symmetric shift, while the other NMR line corresponding to the apical fluorine is much narrower and almost symmetric. Fig. 7 presents the 19F NMR spectra taken at several temperatures between 75 and 300 K. It can be clearly seen that the position of the NMR line corresponding to the inner fluorine shows temperature dependence while the position of the resonance line coming from the apical Fa is practically temperature independent. Fig. 8 shows that the dependence of the shift of the NMR line of the inner fluorine Fi on the reciprocal temperature is linear, demonstrating a Curie-like behavior. 3.3. EMR results
0.0 -400
-300
-200
-100 0 100 Frequency (kHz)
200
300
Fig. 7. The normalized fluorine NMR spectra of Nb6F15 taken at several temperatures between 72 and 295 K.
A single Lorentzian-shaped EMR line was observed within the whole temperature range (Fig. 9). The room temperature EMR line has a g-value of 1.93570.001, and a line width of 5.4770.01 mT. On lowering the temperature gradual changes in all EMR spectra parameters were
ARTICLE IN PRESS R. Knoll et al. / Physica B 381 (2006) 47–52
observed. The g-factor increases up to 1.939 and the line width decreases to 4.1 mT. The temperature dependence of double integrated intensity (DIN) of the EMR line confirms the Curie–Weiss magnetic behavior of the sample with an onset of a AF phase at 6 K (Fig. 10). Another Nb6 cluster compound, LuNb6Cl18, with a VEC value of 15, also showed a transition to an AF state by squid measurements [2,3]. 4. Discussion Ogliaro et al. [14] used density functional theory to calculate the electronic structure of ½M6 Li12 La6 n (M ¼ Nb, Ta and L ¼ Cl, Br). The calculations showed that the valence electrons are located on a set of four energy levels with the symmetry t2g, a1g, t1u and a2u. All these levels exhibit an M–M bonding and an M–Li antibonding character. The a2u highest occupied molecular orbital (HOMO) level does not contain any La contribution. For
14 0.09
12
0.06
1/DIN (a.u.)
10
0.03
0.00
8
0
5
10
15
51
obvious symmetry reasons, the molecular orbital diagram of units must be derived from those of ½M6 Li12 La6 n with some changes in the positions of the energy levels owing to the higher electronegativity of fluorine compared to those of Cl and Br. In Nb6F15, the VEC is 15, meaning that one unpaired electron is located on the a2u HOMO level. This unpaired electron (which carries a magnetic moment) makes the cluster magnetic. The 19F NMR line shift observed in the present work are temperature dependent and independent for Fi and Fa, respectively. While the temperature dependence of the Fi spectra is expected, the temperature-independent spectra of the Fa is somewhat surprising. Let us elaborate on this fact. The temperaturedependent EMR result (Fig. 10) shows an onset of an AF order at 6 K. Consider the two interpenetrating SC lattices (Fig. 11). All the Fa are located at intersection points of cube edges of one lattice with the cube faces of the other lattice. It is easy to see that the magnetic field at these points will vanish, due to symmetry, in the case of each SC lattice is AF (Fig. 11b) and will not vanish for ferromagnetic SC lattices (Fig. 11a). The experimental finding of temperature independence of the Fa NMR spectra is consistent with the AF SC lattices and inconsistent with the ferromagnetic SC lattices. A similar situation has been recently reported [15] in the case of 17O NMR measurements in the charged ordered paramagnetic phase of Pr0.5Ca0.5MnO3.
20
5. Conclusion
6 4 2 0 0
50
100
150
200
250
300
Temperature (K) Fig. 10. The temperature dependence of the inverse intensity of the EMR line (obtained by a double integration) of Nb6F15, showing a Curie–Weiss behavior and the onset (inset) of an AF transition at 6 K.
(a)
We have determined updated structural data of Nb6F15, obtained by single-crystal X-ray diffraction (XRD). The temperature dependence of the intensity of the EMR showed a Curie–Weiss behavior with an onset to an antiferromagnetic (AF) order at 6 K. This temperature dependence is duplicated by the Fi NMR line shift, whereas, the Fa NMR line shift remains temperature independent. This behavior is interpreted as cluster paramagnetism and AF above and below 6 K, respectively.
(b)
Fig. 11. Representations of high symmetry collinear AF structures considered for Nb6F15. Each circle represents a lattice point (or Nb6F12 cluster). The Fas (not shown) are positioned at points of intersection of cube edge of one SC lattice with the cube face of the other SC lattice. The white and black circles represent magnetic moments parallel and anti-parallel to the magnetic axis (MA), respectively. (a) Two SC ferromagnetic lattices coupled antiferromagnetically. (b) Two SC AF lattices. The symmetry of the actual magnetic structure depends on the direction of the MA. It is tetragonal ((a),(b)) or trigonal ((a),(b, left)) for MA along the cube edge or along the cube diagonal, respectively.
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Acknowledgments We acknowledge the ‘‘Centre de Microscopie Electronique a` Balayage et Microanalyse’’ of Rennes 1 University for EDS analyses and the ‘‘Fondation Langlois’’ for its financial support. References [1] C. Perrin, J. Alloys Compounds 262–263 (1997) 10. [2] S. Ihmaı¨ ne, C. Perrin, O. Pen˜a, M. Sergent, J. Less-Common Met. 137 (1988) 323. [3] O. Pen˜a, S. Ihmaı¨ ne, C. Perrin, M. Sergent, Solid State Commun. 74 (1990) 285. [4] J.G. Converse, R.E. McCarley, Inorg. Chem. 9 (1970) 1361. [5] H. Scha¨fer, H.G. Schnering, K.-J. Niehues, H.G. Nieder-Vahrenholtz, J. Less-Common Met. 9 (1965) 95.
[6] H.J. Meyer, J.D. Corbett, Inorg. Chem. 30 (1991) 963. [7] Nonius, COLLECT. DEZO. SCALEPACK. SORTAV: KappaCCD Program Package, Nonius BV, Delft, The Netherlands, 1999. [8] C.W. Dwiggins Jr, Acta Crystallogr. A 31 (1975) 146. [9] G. Gascarano, A. Altomare, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, D. Siliqi, M.C. Burla, G. Polidori, M. Camalli, Acta Crystallogr. A 52 (1996) C–79. [10] G.M. Sheldrick, SHELXL-97: Program for the Refinement of Crystal Structure, University of Go¨ttingen, Go¨ttingen, 1997. [11] C. Kunwar, G.L. Tumer, E. Oldfield, J. Magn. Reson. 69 (1986) 124. [12] W.G. Clark, M.E. Hanson, F. Lefloch, Rev. Sci. Instrum. 66 (1995) 2453. [13] R Knoll, A. I. Shames, S.D. Goren, unpublished results. [14] F. Ogliaro, S. Cordier, J.-F. Halet, C. Perrin, J.-Y. Saillard, M. Sergent, Inorg. Chem. 37 (1998) 6199. [15] A. Yakubovskii, A. Trokiner, S. Verkhovskii, A. Gerashenko, D. Khomskii, Phys. Rev. B 67 (2003) 064,414.
Physica B 381 (2006) 47–52 www.elsevier.com/locate/physb
Magnetic resonance and structural study of the cluster fluoride Nb6F15 R. Knolla, J. Sokolovskib, Y. BenHaimb, A.I. Shamesa, S.D. Gorena,, H. Shakeda, J.-Y. The´potc, C. Perrind, S. Cordierd a Department of Physics, Ben Gurion University of the Negev, P.O. Box 653, 84105 Be’er Sheva, Israel Department of Material Engineering Ben Gurion University of the Negev, P.O. Box 653, 84105 Be’er Sheva, Israel c Laboratoire d’Organome´tallique et Catalyse: Chimie et Electrochimie Mole´culaire, Institut de Chimie, UMR 6509 CNRS-Universite´ de Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France d Laboratoire de Chimie du Solide et Inorganique Mole´culaire, Institut de Chimie, UMR 6511 CNRS-Universite´ de Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France b
Received 28 October 2005; received in revised form 12 December 2005; accepted 15 December 2005
Abstract In this work, we report updated structural data, obtained by single-crystal X-ray diffraction, on the binary fluoride Nb6F15 as well as measurements of 19F NMR over the temperature range 100–300 K, and EMR measurements over the temperature range 4–300 K. Nb6F15 is built up from Nb6 Fi12 Fa6 octahedral cluster units sharing all apical fluorine ligands to form linear Nb–Fa–Nb interunit bridges (Fi: inner fluorine located in edge-bridging position, Fa: apical fluorine located in terminal position). The 19F NMR results showed two resonance lines. One line corresponds to the inner fluorine Fi in the (Nb6F12)3 cluster core. The second 19F NMR line corresponds to the apical Fa. The shift of the NMR signal from Fi depends on the temperature and exhibits a Curie type behavior, while the position of the NMR signal from Fa is temperature independent. EMR measurements show a single Lorentzian line. The temperature dependence of the inverse of the intensity of the EMR line shows a Curie–Weiss behavior and an onset to an antiferromagnetic (AF) order at 5 K. r 2006 Elsevier B.V. All rights reserved. Keywords: Octahedral cluster; Niobium fluoride; Crystal structure; X-ray diffraction;
1. Introduction Many ordered solid-state compounds reported in the literature are based on transition metal clusters as building blocks instead of individual metal atoms, for instance octahedral cluster halides of transition elements built up from M6L18 unit [1]. In metallic cluster compounds, the number of valence electrons per cluster (VEC) depends on the charge transfers between cluster, cations and ligands. As illustrated by the examples of LuNb6Cl18 [2,3] or Nb6F15 [4], both characterized by 15e/Nb6, these clusters exhibit magnetic behavior for odd VEC values. The crystal structure of the latter was reported as early as 1965 [5]. It has a BCC structure (space group.: Im3m, Corresponding author. Tel.: +972 8 6461171; fax: +972 8 6461171.
E-mail address: [email protected] (S.D. Goren). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.253
19
F NMR; EMR; Magnetic order
a ¼ 8.1878(2) A˚). The niobium atoms are linked together to form an Nb6 octahedra characterized by 12 Nb–Nb bonds. Within the unit, 12 fluorine ligands are located in edge-bridging positions (Fi) whilst six supplementary fluorine atoms are located in terminal position (Fa) to build a Nb6 Fi12 Fa6 cluster unit (Fig. 1). These cluster units are interconnected in the three directions by sharing the six apical fluorines forming linear NbFaNb bridges along the axes of the cubic unit cell. Owing to the I-lattice, it results in two interpenetrating simple cubic (SC) lattices of cluster units related to each other by a 12 12 12 translation (Fig. 2). In this work, we report the results of 19F NMR and EMR studies, as well as updated structural data on Nb6F15. It will be observed that only the inner fluorines show the paramagnetic and antiferromagnetic (AF) ordering of the Nb6 Fi12 cluster core.
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R. Knoll et al. / Physica B 381 (2006) 47–52
Fig. 1. Representation of the Nb6 Fi12 Fa6 cluster unit. The ellipsoids are represented at the 50% probability level. The Nb6 cluster is edge-bridged by 12 inner and 6 apical fluorine ligands noted Fi and Fa, respectively.
encapsulated in an evacuated silica ampoule and subsequently placed in a furnace. The temperature was slowly brought up to 750 1C within 2 days and remained there for 2 days. Then the furnace was switched off and the sample was taken off after complete cooling. The X-ray powder pattern of the resulting light-brown product did not reveal the presence of any impurity. However, it appears that it may slowly decompose in NbO2F under ambient atmosphere or when stored in glass container. No single crystal could be obtained from such preparations even when the preparation was slowly cooled down. Indeed, iodine was introduced within the preparation as crystallizing agent in the form of Nb3I8. A pellet was prepared from Nb (94 mg), NbF5 (178 mg) and Nb3I8 (385 mg; synthesized according to the procedure reported in Ref. [6]) and heated in the same conditions as in the aforementioned procedure but slowly cooled down to room temperature within 2 days. It resulted in a crystalline powder and single crystals of Nb6I11 and Nb6F15. Energy-dispersive spectrometry (EDS) analysis did not evidence the presence of any traces of iodine in the single crystal used for structural determination. 2.2. Crystal structure determination of Nb6F15
Fig. 2. Schematic representation of the Nb6F15 structure. The Nb6 clusters of the two interpenetrating SC lattices are represented by large dark and bright circles. The small circles represent the apical fluorines Fa. For clarity, the inner fluorine Fi is not shown.
A suitable single crystal of the Nb6F15 was mounted on a Nonius KappaCCD X-ray area-detector diffractometer with MoKa radiation (l ¼ 0.71073 A˚) (Centre de diffractome´trie de l’Universite´ de Rennes 1, France). Diffraction intensities were collected at room temperature, and once the data processing was performed by the KappaCCD analysis softwares [7] the cubic a cell parameter was refined to 8.1878(2) A˚. The lattice, according to the observed systematic extinctions, was I-centered. A spherical absorption correction was applied [8]. The atomic positions were found by a direct method using the SIR 97 [9] program, and refined anisotropically. Structural refinements by least-squares techniques and subsequent Fourier difference syntheses were performed using the SHELXL-97 program [10]. 2.3.
19
F NMR measurements
2. Sample preparation and experimental procedure 19
2.1. Synthesis and characterisation of Nb6F15 Resublimed niobium pentafluoride was provided by the Institute of High Purity Substances of Nizny Novgorod (Russia). The X-ray powder diffraction pattern of NbF5, recorded in an airtight and watertight cell on a Philips X’Pert Pro diffractometer equipped with a X’celerator detector (CoKa radiation), did not reveal the presence of any impurity. A stoichiometric mixture of niobium (0.5 g, Ventron m2N8) and NbF5 was weighted, ground, formed as a pellet under inert atmosphere, and introduced into a niobium container (Plansee). After welding, the tube was
F NMR experiments were performed on a powdered sample. NMR data were measured using a Tecmag Apollo spectrometer. The experiments were done in a magnetic field of 8.0196 T of an Oxford Instrument superconducting magnet. The frequencies of the NMR spectral lines were measured relative to the F resonance in diamagnetic CF3COOH. The absorption spectra were obtained by Fourier transforming the sum of 160 echoes following a 16pulse sequence to suppress the ringing artifacts [11]. The length of the 901 pulse was 2.1 ms. Since the spectrum was too broad to be fully excited by a single pulse, the exciting frequency was swept and the spectra were added according to the procedure outlined by Clark et al. [12].
ARTICLE IN PRESS R. Knoll et al. / Physica B 381 (2006) 47–52
49
2.4. EMR measurements 6000 5000 4000 Counts
EMR experiments, performed on the same powdered sample as for 19F NMR, were done using a Bruker EMX220 EPR spectrometer in the X-band (n 9.4 GHz) equipped with an Oxford Instruments ESR 900 cryostat over a temperature range 4–300 K. Processing of EMR spectra (which comprises of differentiation, integration, determination of peak-to-peak and integrated intensities, line widths and g-factors, etc.) was done using Bruker WIN-EPR software.
3000 2000
x
1000
x
3. Experimental results
0 10
20
x
30
40
50
60
2θ
The Nb6F15 crystallographic data were reported in the literature in 1965 [5]. At that time, the X-ray diffraction techniques did not enable to obtain anisotropic displacements and accurate bond lengths. The present work results of a single crystal X-ray diffraction, refined anisotropically yielded an improved accuracy and are reported in Table 1. Further details of the crystal structure investigation can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax: (49) 7247-808-666; e-mail: crysdata@fiz.karlsruhe.de) on quoting the depository numbers CSD-415950.
Fig. 3. XRD recorded with a Cu-Ka radiation, of a powdered sample of Nb6F15 that was used in the magnetic resonance measurements measured. Lines marked with were identified as belonging to the decomposed product NbO2F.
80
60
Intensity (a.u.)
3.1. Structural results
3.2. NMR results
40
20
0
Table 1 Refined atomic positional parameters and the isotropic equivalent displacement factors for Nb6F15 Atom
Wyckoff position
x
y
z
Ueq (A˚2)
Nb Fi Faa
12e 24h 6b
0 0 0
0 0.2513(2) 0
0.24133(5) 0.2513(2) 1/2
0.0089 0.0121(9) 0.0141(12)
The space group Im3m was used in the refinement. Refined lattice parameters a ¼ 8.1878(2) A˚. Intraunit atomic distances: dNbNb ¼ 2.7944(6) A˚; d iNbF ¼ 2:0589ð17Þ A˚; ˚ d aa NbF ¼ 2:1179ð4Þ A. The closest distance between two clusters belonging to the two frameworks is 7.0906(2) A˚ (from center to center of the Nb6 octahedra).
-200
-100
0
100
200
300
Frequency (kHz) Fig. 4. Room temperature 19F NMR spectrum of a powdered sample of Nb6F15 obtained with a repetition rate of 1 pulse every 2 s. The narrow line in the middle of the spectrum corresponds to fluorine NMR line from the decomposed product NbO2F.
30 1/2 pulse/sec
Intensity (a.u.)
Fig. 3 gives the XRD of the powdered sample employed in the measurements showing all the lines of Nb6F15 and a small amount of impurity corresponding to NbO2F. Fig. 4 gives the room temperature 19F NMR spectrum of Nb6F15. The narrow NMR line in the middle of the spectrum corresponds to this impurity [13]. The two outside lines correspond to the two inequivalent fluorine sites Fi and Fa. In order to resolve the two fluorine lines, the repetition rate of the exciting rf pulses was varied. (Fig. 5). It can be clearly seen that the left peak retains its intensity down to a repetition rate of 100 pulses/s (10 ms delay between two pulses), indicating that it has a much shorter spin-lattice
-300
10 pulses/sec
20
10
333 pulses/sec
0 -300
-200
-100
0
100
200
300
Frequency (kHz)
Fig. 5. The 19F NMR spectra of Nb6F15 are shown for different repetition rates of the excitation pulses. The spectra decreases monotonically for repetition rates of 333, 200,100, 50, 20,10, 4, 2, 1.75, 1, 23, and 12 pulse/s.
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relaxation time (of the order of a few milliseconds) than the spin-lattice relaxation of the NMR line on the right (of the order of a few hundreds milliseconds). Fig. 6 shows three spectra. Spectrum (a) was obtained with a fast repetition rate of 100 pulse/s, where only the left NMR line is observed. Spectrum (b) was measured with a repetition rate of 12 pulse/s, where the full NMR spectrum is observed. Subtraction of these two spectra gives spectrum (c). The ratio of the areas of spectra (a) and (c), Ia/Ic ¼ 4.03, is in excellent agreement with the ratio of the number of inner F i atoms to the apical Fa ones as determined by the structure, which can be written as Nb6 Fi12 Fa6=2 . Thus we assign the left NMR line to the inner fluorine atom and the right NMR line to the apical one. This result allows one to study the two NMR lines separately, first by using a fast repetition to excite the left line and then using the subtraction method to obtain the right line. The inner fluorine gives a broad asymmetric NMR line typical of an
220 200
Frequency Shift (kHz)
50
180 160 140 120 100 80 2
4
6
8
10
14
Fig. 8. The inverse temperature dependence of frequency shift of the left peak of the NMR line (inner fluorine) showing a Curie behavior.
ν = 9.479431 GHz 300000
T = 295 K
5
EMR Intensity (a.u.)
(b)
Intensity (a.u.)
12
-1
1000/T(K )
200000
(a) (c)
100000
0
-5
250 0
300
350
400
450
Magnetic Field (mT) -200
0
200
Fig. 9. The room temperature EMR spectrum of Nb6F15.
Frequency (kHz)
Normalized Intensity (a.u.)
Fig. 6. (a) The saturated spectrum. (b) The full, unsaturated spectrum. The third, weaker line (c) is equal to the difference, (b)(a).
75K 125K 175K 225K 295K
0.8
0.4
axially symmetric shift, while the other NMR line corresponding to the apical fluorine is much narrower and almost symmetric. Fig. 7 presents the 19F NMR spectra taken at several temperatures between 75 and 300 K. It can be clearly seen that the position of the NMR line corresponding to the inner fluorine shows temperature dependence while the position of the resonance line coming from the apical Fa is practically temperature independent. Fig. 8 shows that the dependence of the shift of the NMR line of the inner fluorine Fi on the reciprocal temperature is linear, demonstrating a Curie-like behavior. 3.3. EMR results
0.0 -400
-300
-200
-100 0 100 Frequency (kHz)
200
300
Fig. 7. The normalized fluorine NMR spectra of Nb6F15 taken at several temperatures between 72 and 295 K.
A single Lorentzian-shaped EMR line was observed within the whole temperature range (Fig. 9). The room temperature EMR line has a g-value of 1.93570.001, and a line width of 5.4770.01 mT. On lowering the temperature gradual changes in all EMR spectra parameters were
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observed. The g-factor increases up to 1.939 and the line width decreases to 4.1 mT. The temperature dependence of double integrated intensity (DIN) of the EMR line confirms the Curie–Weiss magnetic behavior of the sample with an onset of a AF phase at 6 K (Fig. 10). Another Nb6 cluster compound, LuNb6Cl18, with a VEC value of 15, also showed a transition to an AF state by squid measurements [2,3]. 4. Discussion Ogliaro et al. [14] used density functional theory to calculate the electronic structure of ½M6 Li12 La6 n (M ¼ Nb, Ta and L ¼ Cl, Br). The calculations showed that the valence electrons are located on a set of four energy levels with the symmetry t2g, a1g, t1u and a2u. All these levels exhibit an M–M bonding and an M–Li antibonding character. The a2u highest occupied molecular orbital (HOMO) level does not contain any La contribution. For
14 0.09
12
0.06
1/DIN (a.u.)
10
0.03
0.00
8
0
5
10
15
51
obvious symmetry reasons, the molecular orbital diagram of units must be derived from those of ½M6 Li12 La6 n with some changes in the positions of the energy levels owing to the higher electronegativity of fluorine compared to those of Cl and Br. In Nb6F15, the VEC is 15, meaning that one unpaired electron is located on the a2u HOMO level. This unpaired electron (which carries a magnetic moment) makes the cluster magnetic. The 19F NMR line shift observed in the present work are temperature dependent and independent for Fi and Fa, respectively. While the temperature dependence of the Fi spectra is expected, the temperature-independent spectra of the Fa is somewhat surprising. Let us elaborate on this fact. The temperaturedependent EMR result (Fig. 10) shows an onset of an AF order at 6 K. Consider the two interpenetrating SC lattices (Fig. 11). All the Fa are located at intersection points of cube edges of one lattice with the cube faces of the other lattice. It is easy to see that the magnetic field at these points will vanish, due to symmetry, in the case of each SC lattice is AF (Fig. 11b) and will not vanish for ferromagnetic SC lattices (Fig. 11a). The experimental finding of temperature independence of the Fa NMR spectra is consistent with the AF SC lattices and inconsistent with the ferromagnetic SC lattices. A similar situation has been recently reported [15] in the case of 17O NMR measurements in the charged ordered paramagnetic phase of Pr0.5Ca0.5MnO3.
20
5. Conclusion
6 4 2 0 0
50
100
150
200
250
300
Temperature (K) Fig. 10. The temperature dependence of the inverse intensity of the EMR line (obtained by a double integration) of Nb6F15, showing a Curie–Weiss behavior and the onset (inset) of an AF transition at 6 K.
(a)
We have determined updated structural data of Nb6F15, obtained by single-crystal X-ray diffraction (XRD). The temperature dependence of the intensity of the EMR showed a Curie–Weiss behavior with an onset to an antiferromagnetic (AF) order at 6 K. This temperature dependence is duplicated by the Fi NMR line shift, whereas, the Fa NMR line shift remains temperature independent. This behavior is interpreted as cluster paramagnetism and AF above and below 6 K, respectively.
(b)
Fig. 11. Representations of high symmetry collinear AF structures considered for Nb6F15. Each circle represents a lattice point (or Nb6F12 cluster). The Fas (not shown) are positioned at points of intersection of cube edge of one SC lattice with the cube face of the other SC lattice. The white and black circles represent magnetic moments parallel and anti-parallel to the magnetic axis (MA), respectively. (a) Two SC ferromagnetic lattices coupled antiferromagnetically. (b) Two SC AF lattices. The symmetry of the actual magnetic structure depends on the direction of the MA. It is tetragonal ((a),(b)) or trigonal ((a),(b, left)) for MA along the cube edge or along the cube diagonal, respectively.
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Acknowledgments We acknowledge the ‘‘Centre de Microscopie Electronique a` Balayage et Microanalyse’’ of Rennes 1 University for EDS analyses and the ‘‘Fondation Langlois’’ for its financial support. References [1] C. Perrin, J. Alloys Compounds 262–263 (1997) 10. [2] S. Ihmaı¨ ne, C. Perrin, O. Pen˜a, M. Sergent, J. Less-Common Met. 137 (1988) 323. [3] O. Pen˜a, S. Ihmaı¨ ne, C. Perrin, M. Sergent, Solid State Commun. 74 (1990) 285. [4] J.G. Converse, R.E. McCarley, Inorg. Chem. 9 (1970) 1361. [5] H. Scha¨fer, H.G. Schnering, K.-J. Niehues, H.G. Nieder-Vahrenholtz, J. Less-Common Met. 9 (1965) 95.
[6] H.J. Meyer, J.D. Corbett, Inorg. Chem. 30 (1991) 963. [7] Nonius, COLLECT. DEZO. SCALEPACK. SORTAV: KappaCCD Program Package, Nonius BV, Delft, The Netherlands, 1999. [8] C.W. Dwiggins Jr, Acta Crystallogr. A 31 (1975) 146. [9] G. Gascarano, A. Altomare, C. Giacovazzo, A. Guagliardi, A.G.G. Moliterni, D. Siliqi, M.C. Burla, G. Polidori, M. Camalli, Acta Crystallogr. A 52 (1996) C–79. [10] G.M. Sheldrick, SHELXL-97: Program for the Refinement of Crystal Structure, University of Go¨ttingen, Go¨ttingen, 1997. [11] C. Kunwar, G.L. Tumer, E. Oldfield, J. Magn. Reson. 69 (1986) 124. [12] W.G. Clark, M.E. Hanson, F. Lefloch, Rev. Sci. Instrum. 66 (1995) 2453. [13] R Knoll, A. I. Shames, S.D. Goren, unpublished results. [14] F. Ogliaro, S. Cordier, J.-F. Halet, C. Perrin, J.-Y. Saillard, M. Sergent, Inorg. Chem. 37 (1998) 6199. [15] A. Yakubovskii, A. Trokiner, S. Verkhovskii, A. Gerashenko, D. Khomskii, Phys. Rev. B 67 (2003) 064,414.