J. Phys. Chem. C 2010, 114, 12125–12132
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Sc Spectroscopy of Solids: Interpretation of Quadrupole Interaction Parameters and Chemical Shifts Marı´a D. Alba,*,† Pablo Chain,† Pierre Florian,‡ and Dominique Massiot‡ Instituto Ciencia de los Materiales de SeVilla-Dpto. Quı´mica Inorga´nica, Consejo Superior de InVestigaciones Cientı´ficas-UniVersidad de SeVilla, AVenida Americo Vespucio, 49. 41092 SeVille, Spain, and CNRS-CEMHTI, 1D AVenue de la Recherche Scientifique, 45071 Orleans Cedex 2, France ReceiVed: April 23, 2010; ReVised Manuscript ReceiVed: June 11, 2010
The aims of the present study is to describe for the first time the 45Sc MAS NMR spectra of X2-Sc2SiO5 and C-Sc2Si2O7, to combine the spectroscopic information with the structures published from diffraction data, and to propose a rational interpretation of the chemical shifts and quadrupolar parameters. For that purposed, we have correlated the experimental quadrupole coupling parameters of 45Sc determined for a number of scandium compounds to those found by a simple electrostatic calculation and we have found that the isotropic chemical shift of the 45Sc is linearly correlated to the shift parameter, calculated by bond-valence theory. We also show that a simple point charge calculation can approximate the electric field gradient to a sufficiently good approximation that it provides a valuable mean to assign the NMR spectra. Introduction 1
Scandium is found in a variety of materials, such as alloys, inorganic materials,2,3 ferroelectric relaxors and ceramics.4 However, the structures of scandium compounds have not been explored to the extent of other transition metals due to its rarity and difficulties in separation, and, thus, the relatively high cost of materials. Recently, an increasing number of compounds have been synthesized and characterized by single-crystal X-ray diffraction and other methods.5 The gyromagnetic ratio of 45Sc is comparable to that of 13C, and the natural abundance is 100%, predicting high sensitivity; however, the former is a quadrupolar nucleus with a spin quantum number of 7/2 and a quadrupolar moment of -0.22 × 1028/m2, leading to complications of quadrupolar broadening.6 Despite the potential, no many reports on solid-state 45Sc NMR have appeared,7 however, the knowledge of the relationship between observed NMR parameters of 45Sc and local structure is of increased interest in the literature.8 The electric field gradient (EFG) at the nuclear site characterized by the quadrupole coupling constant, CQ, and asymmetry parameter, ηQ, of quadrupolar nuclei in solids have been studied extensively because these parameters give information about the electronic environment of the nucleus and are therefore valuable for the exploration of structure and dynamics. Thus, we studied 45Sc NMR for several stoichiometric, ordered scandium-containing oxides and silicates to propose correlation between the 45Sc NMR parameters and the scandium local coordination environment using our data and those already presented in the literature for other compounds.2,9–12 A simple model that correlates local symmetry with the electric field gradient would be helpful with the interpretation of NMR spectra in quadrupolar nuclei. After investigating the use of global geometric parameters describing the Sc polyhedron distortion, we examine in the present study the experimental quadrupole coupling parameters of 45Sc determined for a number * To whom correspondence should be sent. E-mail:
[email protected]. † Universidad de Sevilla. ‡ CNRS-CEMHTI.
of scandium compounds in light of those found by a simple electrostatic “point charge” calculation. Therefore, the aims of the present study are the followins: (i) to describe, for the first time, the 45Sc MAS NMR spectra of X2-Sc2SiO5 and C-Sc2Si2O7; (ii) to combine the spectroscopic information with the structures published from diffraction data; and (iii) to give a rational interpretation of the chemical shifts and quadrupolar parameters. Experimental Section Materials. Sc2O3 (99.9%, Aldrich) was used as purchased and Sc2SiO5 and Sc2Si2O7 were prepared by the sol-gel method.13 In the case of Sc2SiO5, Sc2O3 was dried at 850 °C overnight and 0.022 mol were weighted and dissolved in 3 mL 6 N nitric acid at 70 °C. Equimolar TEOS (5 mL, dissolved in 30 mL ethanol) was added to the rare earth solution. Acidity of the above solution was adjusted by slowly adding 6-7 mL 2 N ammonium nitrate solution with stirring up to pH ) 1.5. To obtain 1 g of Sc2Si2O7, 2.208 mg of Sc(NO3)3 · H2O (1 mol) were weighted and dissolved in 5 mL of ethanol by stirring. Besides, 1.76 mL of tetraorthomethilsilicate (TEOS) (1 mol) was diluted in an amount of ethanol three times the volume of TEOS. TEOS solution was slowly added to the scandium nitrate solution with gentle stirring and light heating until gel is formed. In both cases, the excess of ethanol in the gel was eliminated at 70 °C until a transparent gel appeared, which was dried in an oven at 65 °C overnight. Nitrate was eliminated by calcination at 500 °C for 1 h at a heating rate of 1 °C · min-1. The powder obtained was calcined at a heating rate of 5 °C · min-1 up to 1050 °C for 6 h and at 1500 °C for 24 h (Sc2SiO5) or directly to 1500 °C for 24 h (Sc2Si2O7) and slowly cooled down to room temperature. Methodology. The powder X-ray diffraction data were collected in the CITIUS X-ray laboratory (University of Seville) by using a Bruker D8 Advance diffractometer with CuKR1 radiation (λ ) 1.5405 Å) and a graphite monochromator. The diffractograms were obtained from 10 to 120° 2θ with a step of 0.02° and a counting time of 10 s. The crystalline phase identification was carried out by using the computer program
10.1021/jp1036525 2010 American Chemical Society Published on Web 06/28/2010
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Figure 1. XRD patterns of (a) Sc2O3, (b) X2-Sc2SiO5, and, (c) C-Sc2Si2O7. O ) Sc2O3 (PDF 84-1880), C ) low-cristobalite (PDF 71-0785)
“X’Pert HighScore”.14 Figure showing structural details were made using the ATOMS software.15 45 Sc MAS experiments were carried out in the CEMHTI (Conditions Extreˆmes et Mate´riaux: Haute Tempe´rature et Irradiation, Orle´ans) on a Bruker Avance WB 750 MHz (17.6 T) spectrometer operating at 182.22 MHz. Powdered samples were packed into zirconia rotors and spun at 30 kHz in a double resonance 2.5 mm Bruker probe. The 45Sc chemical shifts were referenced relative to a 1 M scandium chloride solution with 3% HCl added. MAS experiments were obtained at a radio frequency field νrf of 30 kHz using 0.5 µs pulse width (i.e., smaller than π/24 for quantitativity16). The spin-lattice relaxation time T1 was estimated to be 10 to 15 s with a saturation-recovery experiment and the recycle delays were set accordingly.17 A spectral width of 1.7 MHz was used to avoid folding the spinning sidebands back into the spectrum, baseline correction being applied manually afterward to remove the baseline oscillation induced by the 4.5 µs dead time. The 45Sc 3Q MQMAS18,19 NMR experiments were performed spinning at 30 kHz ((3 Hz) and using a Z-filtered pulse sequence.20 Excitation and mixing were done at νrf ) 150 kHz with pulse lengths of 1.4 and 0.5 µs, respectively, the selective π/2 pulse being applied at νrf ) 15 kHz, and the recycle delay being 5 s. To avoid sidebands in the indirect dimension the t1 time increment was set to the rotor period 33.3 µs21 and typically 90 t1 increments were needed. The spectra were simulated using the DMFit program22 handling finite spinning speeds and mixed quadrupolar/CSA interaction in MAS experiments as well as direct simulation of the 2D MQMAS experiments. Results Figure 1 displays the X-ray powder pattern of Sc2O3, Sc2SiO5, and Sc2Si2O7. The the Sc2O3 pattern (Figure 1a) indicated that all crystalline phase corresponded to the scandium oxide (PDF 84-1880). The Sc2SiO5 pattern (Figure 1b) corresponded to the polymorph X2-Sc2SiO5 (PDF 48-1612) but minor reflection have been detected to correspond to Sc2O3 (PDF 84-1880). Finally, the Sc2Si2O7 pattern (Figure 1c) showed mainly reflexions of the polymorph C-Sc2Si2O7 (PDF 72-779) and some minors reflections of low-cristoballite (PDF 71-0785).
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Figure 2. Bonding environment around each Sc atom of (a) Sc2O3 (Sc1 site at the top of the figure and Sc2 site at the bottom of the figure) (b) X2-Sc2SiO5 (Sc1 site at the top of the figure and Sc2 site at the bottom of the figure), and (c) C-Sc2Si2O7.
Sc2O3 crystallizes in the monoclinic system with space group Ia3.23 The two six-coordinated Sc sites (Sc1, Sc2) have a 3:1 ratio (Figure 2a). Sc2 lies on the C3 axis with equal bond distances (2.120 Å) to six oxygens, while Sc1 is in a much more disordered octahedron with a mean Sc-O distance of 2.123 Å. The X2-Sc2SiO5 structure was refined by Alba et al.24 in the spatial group I2/c. The unit cell contains a unique Si site and two equally populated and crystallographically different Sc sites (Figure 2b) with coordination numbers 6 and 7 and mean Sc-O distances of 2.132 and 2.276 Å, respectively. Finally, C-Sc2Si2O7 is isostructural with the natural-occurring mineral thorveitite. It crystallizes in the monoclinic system, with space group C2/ m. The unit cell based on structural data published by Smolin et al25 consists of a single Si site and a single Sc site; the diortho groups Si2O7 show Si-O-Si bond angles of 180° (due to the symmetry of the unit cell) and the Sc atoms are in octahedral coordination with a mean Sc-O distance of 2.124 Å (Figure 2c). In this case, the bridging oxygen is not involved in the Sc octahedron. Figure 3 shows the 3QMAS and one-pulse 45Sc MAS NMR spectra of the compounds. The triple quantum spectra of Figure 3a,b showed two different Sc sites and a single Sc site was observed in the triple quantum spectrum of Figure 3c. Those results agreed with the structural details of Sc2O3, Sc2SiO5, and Sc2Si2O7, respectively. All spectra exhibited very narrow discontinuities, showing that all samples were very well crystallized. Apart from the main peaks belonging to the 〈-1/ 2, 1/2〉 central transition, spinning sidebands of the external 〈( 3/2, ( 1/2〉 and 〈( 5/2, ( 3/2〉 were also clearly visible, spun over more than 1 MHz and presented complex lineshapes. The simulations of the Sc2O3, Sc2SiO5, and Sc2Si2O7 spectra were successfully performed by taking into account the quadrupolar interaction only and by calculating finite spinning-speed lineshapes for all transitions and, as seen in Figure 3, reproduced extremely well all sets of transitions on the 1D spectra as well as the MQMAS experiments. The resulting fitting parameters are given in Table 1, with uncertainties of (0.5 ppm for δiso, (0.01 MHz for CQ, and (0.02 for ηQ. The 45Sc MAS NMR spectrum of Sc2O3 was fitted to two signals with high quadrupolar constants, 15.37 and 23.37 MHz, in good agreement with the two different Sc crystallographic sites. Both signals showed a relative intensity of 76.7 and 23.3% in accordance with what is expected from the multiplicity of their Wyckoff positions.
45Sc
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Figure 3. 45Sc (3Q) MAS NMR and 45Sc (SP) MAS NMR of (a) Sc2O3, (b) X2-Sc2SiO5, and (c) C-Sc2Si2O7. For both experiments, the simulations are given on top of the experimental spectra. For the SP spectra, the individual components are given below the experimental spectra. Isotropic (vertical dimension) and MAS (horizontal dimension) projections are given on the side of the 3Q spectra.
TABLE 1: Crystallographic Details and Experimental 45Sc MAS NMR Spectral Parameters crystallographic datab
45
Sc MAS NMR
distortion no. crys site
coord number (%)
Sc-O mean distance (Å)
σ
|Ψ|
Sc2O3
2
X2-Sc2SiO5
2 1
2.123 2.120 2.132 2.276 2.124
0.040 0.000 0.067 0.331 0.051
2.628 1.893 2.369
C-Sc2Si2O7
6 (75%) 6 (25%) 6 (50%) 7 (50%) 6 (100%)
compounds
a
2.494
site syma
δiso (ppm)
CQ (MHz)
ηQ
%
A
2. -3 1 1 2
128.7 108.0 86.3 95.8 77.8
15.37 23.37 25.66 19.84 20.69
0.63 0.00 0.00 0.90 0.45
76.7 23.3 50.0 50.0 100.0
1.2037 1.2072 1.2268 1.2210 1.2204
International symbol of point symmetry. b References 22, 23, and 24.
For X2-Sc2SiO5, a small quantity (4.4%) of Sc2O3 was detected and accounted for by using the fitting of the experimental spectra of that compound. This small quantity of Sc2O3 was already observed by X-ray diffraction (see Figure 1b). The two signals with equal population observed in the 45Sc MAS NMR spectrum for X2-Sc2SiO5 were due to the two different Sc crystallographic sites. The site’s populations extracted from the simulation of the Sc2SiO5 spectrum were in excellent agreement with the structural data,24 which indicates a ratio of 4/4. Finally, the 45Sc MAS NMR spectrum of C-Sc2Si2O7 was fitted to a unique contribution and the fitting parameters are shown in Table 1. It could be seen that the quadrupolar coupling constant CQ is found between 15 and 26 MHz for all samples; such high values explain why even with the use of a high field and a high spinning speed one still gets spinning sidebands from the central transition. Discussion Data from the literature together with the results reported here indicate that the isotropic chemical shifts of the 45Sc NMR (Table 1 and 2) spread over a wide range from -48 to 202 ppm. Several authors have suggested a correlation between the chemical shift of quadrupolar nuclei and the coordination
number of the corresponding cations.26 As shown in Figure 4, the shift region for the 6-fold coordination overlaps heavily with the 7- and 8-fold coordination regions, a situation reminiscent of what has been observed for 23Na NMR chemical shifts in oxides.27 However, it is observed a certain correlation between the central chemical shift regions with the coordination number in such way that the region is centered at lower chemical shift when the coordination number increases. A similar trend is found in NMR data for many other cations such as 11B, 23Na, 25 Mg, 27Al, 29Si, and 89Y NMR.28 It can be interpreted as the central Sc-X bond becoming more ionic (i.e., higher electron density at the Sc site) and hence, the chemical shift decreases (i.e., increasing shielding).29 In this figure, the assignment of the NMR parameter of X2-Sc2SiO5 (open circle) was based on this assumption. If this behavior bears some similarities with the 23Na case, our set of results shows nevertheless that δiso(Sc) is not correlated to the mean Sc-O distance. It follows from the qualitative considerations presented above that an adequate description of the chemical environment of the atoms surrounding the scandium atom is necessary for a quantitative interpretation of the chemical shifts of 45Sc in structural terms. A suitable concept for that purpose is the empirical bond-valence approach in which the valence, that is, the strength of the distinct bonds formed by an atom, can be determined from the corresponding bond length.30 In this model,
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TABLE 2:
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Sc MAS NMR Spectral Parameters of Scandium Compounds 45
compounds LiScO2 NaScO2 ScPO4 ScVO4 Sc2W3O12 Sc2Mo3O12 C6H22F2N2O20P4Sc4
Sc(acac)3 Sc(TMHD)3 Sc(NO3)3.5H2O Sc(OAc)3 ScCl3 · 3H2O ScCl3 · 3THF
δiso (ppm)
Sc MAS NMR
CQ (MHz)
b
148 158.2 -48.2c -34.0 15.6 10.1 25.0 34.0 46.0 127.0 82.0 89.5 -18.5 -6.2 125.4 202.0
1.6 14.2
21.6 2.4 2.1 8.2
