Determining Electron Spin-Transfer Mechanisms in Paramagnetic

Article pubs.acs.org/JPCC

Determining Electron Spin-Transfer Mechanisms in Paramagnetic Ba2YMO6 (M = Mo, Re, Ru) Double Perovskites by 89Y and 137Ba MAS NMR Spectroscopy Vladimir K. Michaelis,†,§ Brandon J. Greer,† Tomoko Aharen,‡,∥ John E. Greedan,‡ and Scott Kroeker*,† †

Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada Department of Chemistry, McMaster University, Hamilton, Ontario, L8S 4M1, Canada

‡

S Supporting Information *

ABSTRACT: The spin-transfer mechanisms of three B-site ordered double perovskites, Ba2YMO6 (M = Mo, Re, and Ru), were determined using variable-temperature 89Y and 137Ba magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. On the basis of symmetry-compatible overlap and trends observed in the MAS NMR, spin polarization was determined to be the dominant mechanism at the Y sites and spin delocalization to be the operative mechanism at the Ba sites. The MAS NMR spectra display 89Y and 137Ba chemical shifts that far exceed those of other paramagnetic solids, appearing at up to −6402 and +12 200 ppm, respectively. Detection of additional resonances in 89Y MAS NMR indicates a small degree of cation site mixing in two cases.

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room temperature.6−8 During the course of these studies, we observed 89Y NMR peak positions well outside the known chemical shift range for diamagnetic compounds9 which increased according to the number of unpaired electrons on the neighboring B′ cation [Mo5+ (t2g1, S = 1/2), Re5+ (t2g2, S = 1), or Ru5+ (t2g3, S = 3/2)]. This inspired us to investigate the spintransfer mechanisms in these paramagnetic systems and to expand on the original work by including 137Ba MAS NMR. NMR spectroscopy of solids containing stoichiometric quantities of paramagnetic ions is relatively rare due to the experimental challenges posed by unpaired electrons. Electron− nuclear interactions can shorten spin−spin relaxation times (T2), broadening peaks and degrading signal-to-noise ratios, while the Fermi contact interaction can cause significant deviations from typical chemical shift positions. Nevertheless, valuable structural information has been obtained from NMR of paramagnetic proteins, 10,11 battery materials, 12−16 coordination compounds,17−20 and organometallic complexes.19,21,22 Recent increases in magic-angle spinning speeds facilitate spectral acquisition of paramagnetic solids by more effective averaging of the electron−nuclear dipolar interactions, thereby opening up the field to broader investigation. Yttrium-89 appears to be an attractive nucleus for NMR as it is spin-1/2 with 100% natural abundance and generally possesses a small chemical shielding anisotropy easily averaged by MAS.

INTRODUCTION Solid-state compounds with the perovskite structure are ubiquitous in natural minerals and materials chemistry. Common applications include microwave dielectric ceramics1 and solidoxide fuel cells.2 The geometric arrangement of the cations within this structure can give rise to interesting magnetic properties3−5 which have recently been studied in a series of Bsite ordered double perovskites, Ba2YMO6 (M = Mo, Re, and Ru), forming a class of geometrically frustrated antiferromagnetic materials at low temperatures.6−8 These systems have facecentered-cubic symmetry (Fm3m) with six-coordinate Y, M, and O sites, and a 12-coordinate Ba site (Scheme 1). 89 Y magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to measure the cation ordering at Scheme 1. Fragment of the Cubic Ba2YMO6 (M = Mo, Re, and Ru) Double-Perovskite Structure

Received: August 17, 2012 Revised: October 17, 2012 Published: October 17, 2012 © 2012 American Chemical Society

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Figure 1. 89Y MAS NMR of (a) Ba2YMoO6, (b) Ba2YReO6, (c) Ba2YRuO6 double perovskites at 318 K (νMAS = 20−22 kHz); the inset shows cation disorder (ref 6) for Ba2YMoO6: δ(Y6Mo) = −1160 ppm, δ(Y5Mo,1Y) = −1124 ppm. (d) Temperature dependence of the observed chemical shifts.

into its direct dependence on spin density and its inverse temperature dependence; the remainder of the equation can be combined into a constant (M = 23.5 × 106 ppm K au−1) following the work of Zhang et al.,22 yielding a simple straightline expression which can be plotted against T−1 to obtain ραβ:

However, long spin−lattice relaxation times (T1) and a low gyromagnetic ratio (−1.31 × 107 rad T−1 s−1) make it fairly unreceptive and unpopular.9,23 In the present case, the paramagnetism of these double perovskites reduces T1 and significantly improves the ease of spectral acquisition. Barium135/137 are traditionally difficult to probe by NMR since both are found in low natural abundance (6.6% and 11.3%, respectively), are spin-3/2 with moderate quadrupolar interactions (16.0 and 24.5 fm2, respectively), and suffer from low gyromagnetic ratios (2.67 and 2.99 × 107 rad T−1 s−1, respectively). The present compounds have Ba located in a cubic environment where the electric field gradient is zero, yielding a sharp resonance. Acquisition of NMR signals from both nuclides at the moderately high field of 14.1 T brings them into the range of accessibility. The common occurrence of these cations in solid-state chemistry and the paucity of NMR studies of paramagnetic solids warrants a thorough investigation of the interaction between electron and nuclear spins. We use variabletemperature 89Y and 137Ba MAS NMR to determine the electron spin densities and spin-transfer mechanisms for this series of isostructural double perovskites with an increasing number of unpaired electrons (from one to three).

δf =

⎛ S(S + 1) ⎞⎛ (3 cos2 θ − 1) ⎞ ⎟⎜ δpc = −μB 2 ⎜ ⎟(g − g⊥) ⎝ 3kT ⎠⎝ r3 ⎠

9kTao3

ραβ

(3)

where θ is the angle of the electron−nuclear dipolar vector relative to Bo and r is the “distance” between the electron and nucleus. This term becomes zero when the g-tensor is isotropic, for instance, when located at a point of cubic symmetry or S = 1/2. The net effect of these additive interactions is to shift the observed peak position from the conventional diamagnetic chemical shift (δdia):19

THEORETICAL BACKGROUND Unpaired electrons interact with nuclear spins through the Fermi contact and the pseudocontact mechanisms.19 The former arises from the electric current induced by unpaired electron spin density at the nucleus, while the latter is a direct dipolar interaction between the unpaired electron and the nucleus. As such, the Fermi contact mechanism is mediated by the electrons in the intervening bonds and is formally analogous to indirect spin−spin coupling: δf =

(2)

[Note that, in eq 1, the constant ao represents the Bohr radius (5.29177 × 10−11 m) which is required for determining the “M” value, as defined. The M value specified in Zhang et al. does not include this term.] The pseudocontact interaction operates through space and depends additionally on the anisotropic g-tensor:

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μo μB 2 ge 2(S + 1)

M(S + 1) ραβ T

δobs = δdia + δf + δpc

(4)

where δf and δpc are also sometimes referred to as the hyperfine shift (δhf).

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MATERIALS AND METHODS Synthesis. Three double perovskites of compositions Ba2YMoO6, Ba2YReO6, and Ba2YRuO6 were synthesized by conventional solid-state reactions and characterized as previously described.6−8 Nuclear Magnetic Resonance. Solid-state NMR spectroscopy was done with a Varian UNITYInova 600 (14.1 T) spectrometer. 89Y (vL = 29.39 MHz) and 137Ba (vL = 66.67 MHz) NMR spectra were collected with a 3.2 mm doubleresonance (H/F-X) Varian-Chemagnetics MAS probe using 22 μL fill volume (3.2 mm o.d.) ZrO2 rotors. Acquisition using

(1)

where μo is the vacuum permeability, μB is the Bohr magneton, ge is the g value for a free electron, S is the electron spin quantum number, k is the Boltzmann constant, ao is the Bohr radius, T is the temperature in K, and ραβ is the electron spin density at the nucleus of interest. The Fermi contact interaction can be distilled 23647

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Bloch decay24,25 was performed using variable-temperature conditions ranging from 15 °C (288 K) to 119 °C (392 K) and corrected for frictional heating using the 207Pb chemical shift of solid Pb(NO3)2.26,27 Spectra were acquired using 0.3−0.6 s recycle delays, 50 000−450 000 coadded transients, and spinning frequencies between 20 and 22 kHz (±0.1%). 89Y MAS NMR data were acquired using a π/4 pulse (νrf = 42 kHz) and referenced using 2 M Y(NO3)3 at 0.0 ppm. 137Ba MAS NMR spectra were collected using a π/6 pulse (νrf = 38 kHz) and referenced using 1 M Ba(NO3)2 at 0.0 ppm.9,23,28

yttrium-containing oxides. The slope of the line provides a spin density, ραβ, at the Y3+ cation of −0.0139(1) (Table 1). 137 Ba MAS NMR (at 288 K) reveals a single resonance at 2405 ppm (Table 1, and shown in Figure 2a for T = 318 K), shifted about 1800 ppm outside the known range for diamagnetic oxides (−450 to 800 ppm).9,29 The temperature dependence of the signal is similar in magnitude but opposite in sign to that of 89Y, resulting in a spin density of +0.0166(1) at Ba and δdia = 349 ppm (Figure 2d, Table 1). The different signs are a consequence of different mechanisms of spin transfer between the electron and nuclear spins, as discussed in further detail below. Whereas two distinct peaks were observed for 89Y NMR due to 3% Y/Mo site mixing in this sample,6 no such effects are apparent in the 137Ba NMR spectrum despite its next-nearest-neighbor proximity to both cations. Cation disorder (due to site mixing) generates a nonzero electric field gradient which, when coupled with the large quadrupole moment of 137Ba, produces a very broad peak with low intensity. In fact, NMR observation of 137Ba is generally hampered by any deviation from cubic symmetry, resulting in extremely broad signals which are difficult to observe at normal magnetic fields without sensitivity enhancement techniques.29,30 Combined with the low level of site mixing (≈3%), any 137Ba resonance associated with a noncubic environment would be expected to be well below detection limits. Ba2YReO6, S = 1. The presence of Re5+ in the B′ site adds a second unpaired electron to the system. Correspondingly, the 89 Y and 137Ba MAS NMR signals are observed to be shifted even further from their “normal” diamagnetic ranges (Figures 1b and 2b for T = 318 K). At 288 K, the 89Y chemical shift appears at about −2450 ppm, while the 137Ba chemical shift is 4450 ppm, both nearly twice those of the Mo5+ analogue with one unpaired electron. Since this is an S = 1 spin system, both the Fermi contact and pseudocontact interactions contribute to the observed peak positions; however, the cubic symmetry at B′ constrains the g-tensor to be isotropic, forcing the pseudocontact interaction to zero. Spin density values calculated from the slopes are −0.0122(3) for 89Y and +0.0138(3) for 137Ba, comparable to those in Ba2YMoO6. Extrapolation to infinite temperature yields the diamagnetic components of the shifts to be −483 and 2197

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RESULTS AND DISCUSSION Ba2YMoO6, S = 1/2. Figure 1a shows the 89Y MAS NMR spectrum of Ba2YMoO6 at 318 K. At 288 K, the two peaks, Table 1. Observed Chemical Shifts (δobs) at 288 K and Measured Electron Spin Density (ραβ) Terms from 89Y and 137 Ba MAS NMR

Ba2YMoO6 Ba2YMoO6a Ba2YReO6 Ba2YRuO6 Ba2YRuO6b a

δobs, 89Y (ppm)

ραβ, 89Y (a.u.)

δobs, 137Ba (ppm)

ραβ, 137Ba (a.u.)

−1390(5) −1335(5) −2450(5) −6402(5) −5610(5)

−0.0139(1) −0.0128(1) −0.0122(3) −0.0211(1) −0.0190(1)

2405(10) n.a. 4450(10) 12200(10) n.a.

0.0166(1) n.a. 0.0138(3) 0.0212(1) n.a.

Y(O−Mo)5(O−Y). bY(O−Ru)5(O−Y).

representing Y(O−Mo)6 and Y(O−Mo)5(O−Y), as previously described,6 appear at −1390 and −1335 ppm, respectively, well outside the known oxide chemical shift range for 89Y NMR (ca. 60−340 ppm),16 indicating that other electronic interactions are present. The presence of a single unpaired electron at Mo5+ restricts the paramagnetic interaction to the Fermi contact term, which can be conveniently measured by variable-temperature NMR. Figure 1d exhibits the temperature dependence of the major peak, showing a linear increase in the signal frequency with increasing temperature. Extrapolation to “infinite temperature” yields an estimate of the diamagnetic isotropic chemical shift, δdia = 318 ppm, which is within the expected range for diamagnetic

Figure 2. 137Ba MAS NMR of (a) Ba2YMoO6, (b) Ba2YReO6, (c) Ba2YRuO6 double perovskites at 318 K (νMAS = 20−22 kHz). (d) Temperature dependence of the observed chemical shifts. 23648

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Figure 3. Diagrammatic representations of the spin polarization and delocalization spin-transfer mechanisms to Y and Ba in Ba2YMO6 double perovskites after Carlier et al. (ref 15) (see text).

state),31−33 previously thought to represent “extreme” chemical shifts. The measured 137Ba chemical shift is located 0.7 MHz outside of the expected chemical shift range for bariumcontaining oxides. Clearly, the presence of three unpaired electrons on Ru5+ induces an enormous Fermi contact shift. Variable-temperature NMR experiments yield spin densities of −0.0211(1) and +0.0212(1) and imply diamagnetic shifts of −2080 and 7945 ppm by extrapolation for 89Y and 137Ba, respectively. That these are outside the expected range for diamagnetic compounds is likely due at least in part to the long extrapolations. However, the fact that the deviations from the known diamagnetic range increase with the number of unpaired electrons suggests that other factors are at play. A contribution from the Knight shift associated with conducting electrons cannot be discounted strictly on the basis of the existing NMR data; however, the relaxation measurements required to confirm this would be prohibitively time-consuming. The magnetic properties are consistent with insulating behavior.6−8 As previously shown, the presence of a minor 89Y peak attributable to Y(O−Ru)5(O−Y) indicates cation site mixing of about 1%.8 The greater chemical shift dispersion between this and the main Y(O−Ru)6 peak, relative to the small peak separation in Ba2YMoO6, is due to the additional unpaired electrons in this compound. No evidence of cation disorder is present in the 137Ba NMR spectrum due to quadrupolar broadening in the mixed site (vide supra). Spin-Transfer Mechanisms. In crystalline transition metal oxides, the mechanism by which unpaired electron spin density is transferred from the metal to the nucleus under observation can be predicted by the geometry and electron occupancy.15 Spin delocalization occurs when orbitals of compatible symmetry on

Figure 4. Experimentally determined spin density values at Y and Ba for the Ba2YMO6 double perovskites [M = Mo (S = 1/2), Re (S = 1), Ru (S = 3/2)] as a function of Pauling electronegativity (χP) scaled by ionic radius.

ppm, respectively. There is no evidence of cation mixing in this system based on the single 89Y NMR signal.7 Ba2YRuO6, S = 3/2. The S = 3/2 Ru5+ analogue exhibits the largest room-temperature chemical shifts reported for 89Y and 137 Ba. At −6402 ppm and 12 200 ppm (for T = 288 K), respectively (shown in Figures 1c and 2c for T = 318 K), these are far greater than the δiso(89Y) = −1250 ppm and δiso(137Ba) = 2563 ppm found in the superconductor Ba2YCu3O7 (paramagnetic 23649

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Figure 5. Experimental hyperfine shifts as a function of spin- and thermal-scaled experimental Fermi contact spin densities for 89Y (lower) and 137Ba (upper) with lines of best fit (solid) and theoretical fit (dashed). The two subsets of points (unfilled) deviating from the line in the 89Y plot are those of the Y(O−Mo)5(O−Y) site in Ba2YMoO6 and the Y(O−Ru)5(O−Y) site in Ba2YRuO6.

oxygen atom, and the Ba site, located at 90° from the M−O bond (Scheme 1).12,14,15 At yttrium in these Ba2YMO6 double perovskites, spin delocalization would involve direct polarization of the Y s orbital across the oxygen p(σ) orbital from the d(x2 − y2) orbital, which would yield a positive spin density (Figure 3d). While this mechanism is normally accepted to be dominant, the electron occupancies are t2g1, t2g2, and t2g3 for M = Mo5+, Re5+, and Ru5+, respectively; hence, the eg orbital is unoccupied and spin delocalization is minimal. Spin polarization, by contrast, originates from the occupied t2g orbital(s) on M, polarizing

the metal, oxygen, and observed element (i.e., Y or Ba) form a single spin orbital, resulting in direct spin transfer, retention of spin polarization, and a positive spin density (Figure 3, parts c and d). The spin polarization mechanism is a consequence of the exchange interaction between an unpaired electron on the metal and electrons in adjacent doubly occupied orbitals, resulting in a negative spin transfer from the paramagnetic metal (i.e., negative spin density on the s orbital of the observed nucleus) (Figure 3, parts a and b). For a metal with Oh symmetry, it is straightforward to visualize how these mechanisms will influence the contact interaction at the Y site, located at 180° from the metal across the 23650

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electrons in the eg−p(σ)−s orbital and resulting in a negative spin density on the O(p)−Y(s) orbital (Figure 3b). The measured spin density is indeed negative and causes a positive shift with increased temperature for 89Y (Figure 1d, Supporting Information Figures S1−S3). The 137Ba NMR signal in this system is affected differently due to its location relative to the M−O molecular orbitals. As illustrated in Figure 3a, spin polarization is most effective from the M(eg) orbitals, which are unoccupied in the systems at hand, rendering this mechanism inoperative. Correspondingly, facile direct delocalization from the occupied M(t2g) orbital(s) to the O(p−π)−Ba(s) causes positive spin density at the Ba nucleus (Figure 3c). In accordance with this prediction, a negative chemical shift trend is observed with an increase in temperature for 137Ba. (Figure 2d, Supporting Information Figures S4−S6). In general, the contact shift of spin delocalization is greater than that of spin polarization,15 which is certainly observed for the Ba2YMO6 double perovskites given the extreme 137Ba NMR chemical shifts compared to those of 89Y. It might be intuited that the spin density at a particular nucleus should increase with S, especially considering that the metal radii involved decrease regularly across this series [r([6]M5+) = 0.75 Å (Mo), 0.72 Å (Re), and 0.70 Å (Ru)] bringing the orbitals into closer contact. However, this proves not to be the case. An alternate rationale for the spin density ordering involves electronegativity as the dominant factor in localizing the electron density on the metal. Figure 4 shows ραβ as a function of Pauling electronegativity (χP) scaled by the ionic radius, revealing a more regular trend. A likely explanation for the failure of S to simply account for ραβ is that the spin-only approximation provides a poor description of these heavy, group 6, 7, and 8 transition metal ions. One might expect spin−orbit coupling to play a major role for at least the t2g1 (Mo5+) and t2g2 (Re5+) ions. Ru5+ is a t2g3 ion for which L = 0. Evidence from the paramagnetic electronic susceptibility data can help to clarify the situation.6−8 For both Ba2YMoO6 and Ba2YRuO6, the observed effective moment is very near the spin-only value, while that for Ba2YReO6 is drastically reduced, indicating a weak role for spin−orbit coupling in the former two ions but an important role in the latter case. Furthermore, Re has an occupied shell of f orbitals which could provide other spin-transfer pathways. Despite the attractiveness of examining a continuous series within the second row, the radioactivity of technetium is a deterrent to this project. The magnitudes of the room-temperature shifts are only partly explained by the combined influence of S and ραβ (eq 1).22 The diamagnetic components of the chemical shifts for the Ru compound are unusually large, unless the pseudocontact interaction plays an unexpected role (as per eq 4). Indeed, the known chemical shift range for 137Ba is substantially larger than that of 89Y, usually attributed to its larger, more polarizable electron “cloud”, and comparable to nuclides such as 199Hg, 207 Pb, and 205Tl.19,20 Figure 5 depicts the experimental hyperfine shifts as a function of scaled spin density, (S + 1)ραβ/T, which allows representation of the variable-temperature data so that the theoretical slope of the line is equal to M (see eq 2) with an intercept through the origin. The expected trend is observed for 137Ba; however, a subset of points in an equivalent plot for 89Y deviate from the expected relationship. In fact, these stray data points are those of the second yttrium site in both Ba2YMoO6 (i.e., the Y(O− Mo)5(O−Y) species) and Ba2YRuO6 (i.e., the Y(O−Ru)5(O−Y) species), signifying a contribution from the pseudocontact

interaction that results from the lowering of the cubic symmetry upon cation substitution.

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CONCLUSIONS Y and 137Ba are rarely studied by solid-state NMR, despite their importance in many materials. However, the combination of high symmetry and unpaired electrons has facilitated the acquisition of high-quality NMR spectra which provide valuable information about local structure in a series of double perovskites. The paramagnetic interaction induced by the unpaired electron(s) amplifies the chemical shift sensitivity, leading to well-resolved peaks for minor species generated by cation disorder and enormous peak shifts which increase dramatically with each additional unpaired electron, producing the largest documented 89 Y and 137Ba NMR shifts. The uniquely high symmetry present in these compounds restricts the paramagnetic interaction to the Fermi contact term, facilitating the accurate characterization of the electron spin density at each observed nucleus. The signs of the spin densities can be rationalized in terms of electron occupancy and geometric arrangement under the spin-only approximation, showing that delocalization is responsible for the 137 Ba shifts, while the spin polarization mechanism governs spin transfer to 89Y. The magnitudes of the spin densities are surprisingly invariant to the ionic radii and the number of unpaired electrons, implying that orbital contributions and/or spin−orbit coupling play a hidden role in spin transfer. This study shows that, far from obstructing structurally informative NMR spectroscopy, paramagnetism can enhance both the accessibility and information content of spectra. 89

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ASSOCIATED CONTENT

S Supporting Information *

89

Y and 137Ba variable-temperature MAS NMR spectra for each double-perovskite sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 204-474-9335. Fax: 204-474-7608. E-mail: Scott_ [email protected]. Present Addresses §

Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, 02139, U.S.A. ∥ Department of Chemistry, University of Ottawa, Ottawa, ON, K1N 6N5, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Natural Sciences and Engineering Research Council of Canada for funding: PGSD3 (V.K.M.) and NSERC Discovery Grants (S.K. and J.E.G.). S.K. acknowledges the Canada Foundation for Innovation and the Manitoba Research Innovation Fund for Infrastructure Grants.

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