Solid‐state NMR characterization of ⁶⁹Ga and ⁷¹Ga in crystalline solids

Abstract

Gallium model systems containing four- and six-coordinate gallium sites have been investigated using solid-state NMR. Measurement of the isotropic chemical shift and electric field gradient (EFG) have been performed at 9.4 T on alpha-Ga2O3, beta-Ga2O3, LiGaO2, NaGaO2, KGaO2, Ga2(SO4)3, and LaGaO3 using a variety of techniques on both NMR active nuclei (69Ga and 71Ga) including static, high speed magic-angle spinning (MAS), satellite transition (ST) spectroscopy, and rotor-assisted population transfer (RAPT). The chemical shift is found to correlate well with the coordination number, with four-coordinate gallium having values of approximately 50 ppm and six-coordinate gallium having values near 225 ppm (referenced to 1 M gallium nitrate solution). The magnitude of the EFG is found to be correlated to the distortion of the gallium polyhedra, with the strained systems having EFGs of 3 x 10(21) Vm(-2) or more, while the less strained systems have values of 1.5 x 10(21) Vm(-2) or less. A plot of chemical shift versus EFG suggests that solid-state NMR of gallium oxyanions can be more discriminating than liquid state NMR chemical shifts alone.

Broadband MAS NMR spectroscopy in the low-power limit

“…In all cases the sample temperature was approximately 320 K. A recycle delay of 100 ms was used for all 6 Li experiments while 10 ms was used for 7 Li experiments. Further experimental details are given in each figure caption.…”

“…Spin dynamics simulations were carried out at an external field strength corresponding to a 1 H Larmor frequency of 500.196 MHz with powder averaging achieved by simulating 109 (α,β) pairs according to the Lebedev octant scheme [42]. The pulses were applied to a single 6 Li or 7 Li nucleus using chemical shift anisotropy parameters corresponding to the fitted experimental parameters with δ iso = 0 for convenience. More specific simulation details are provided in the figure captions.…”

“…Before taking into account the number of spins present, this means that the sensitivity relative to 1 H for 7 Li is ∼5.9%, and only a miniscule ∼0.3% for 6 Li! Add to this the low natural abundance of 6 Li (7.59%) and considering that the signal-to-noise ratio grows only by the square root of the number of co-added transients, we see that it is obviously crucial to maximize the strength of the signal in each scan in order to collect a spectrum in a reasonable amount of time for low-abundance, low-gamma nuclei.…”

“…A spin echo experiment, π 2 -τ echo -π, must typically be used to acquire a flat baseline, however, conventional RF π pulses often have bandwidths limited to around 100 kHz using typical maximum pulse powers available in MAS probes. It is possible to overcome bandwidth issues by using a shorter refocusing pulse, for instance a Another approach to solve this issue could be the use of frequency stepping [5,6,7,8] to collect multiple spectra at different frequency offsets and sum them to yield a spectrum without truncation due to pulse imperfections or limited probe bandwidth. This technique is suitable for highly abundant and sensitive spins such as 1 H and 31 P, however in practice it is time consuming for sensitive studies of low-γ, low-abundant nuclei.…”

“…This material exhibits a large SA due to coupling between 6,7 Li spins and unpaired electrons in the Fe II and Mn II metal centers. We use both the 6 Li (γ6 Li = 6.27 MHz/T) and 7 Li (γ7 Li = 16.55 MHz/T) nuclei in order to examine the effect of γ on the pulse performance at MAS rates ranging from 40 kHz to 111 kHz.…”