In recent years magic angle spinning-dynamic
nuclear polarization
(MAS-DNP) has developed as an excellent approach for boosting the
sensitivity of solid-state NMR (ssNMR) spectroscopy, thereby enabling
the characterization of challenging systems in biology and chemistry.
Most commonly, MAS-DNP is based on the use of nitroxide biradicals
as polarizing agents. In materials science, since the use of nitroxides
often limits the signal enhancement to the materials’ surface
and subsurface layers, there is need for hyperpolarization approaches
which will provide sensitivity in the bulk of micron sized particles.
Recently, an alternative in the form of paramagnetic metal ions has
emerged. Here we demonstrate the remarkable efficacy of Mn(II) dopants,
used as endogenous polarization agents for MAS-DNP, in enabling the
detection of 17O at a natural abundance of only 0.038%.
Distinct oxygen sites are identified in the bulk of micron-sized crystals,
including battery anode materials Li4Ti5O12 (LTO) and Li2ZnTi3O8, as
well as the phosphor materials NaCaPO4 and MgAl2O4, all doped with Mn(II) ions. Density functional theory
calculations are used to assign the resonances to specific oxygen
environments in these phases. Depending on the Mn(II) dopant concentration,
we obtain significant signal enhancement factors, 142 and 24, for 6Li and 7Li nuclei in LTO, respectively. We furthermore
follow the changes in the 6,7Li LTO resonances and determine
their enhancement factors as a function of Mn(II) concentration. The
results presented show that MAS-DNP from paramagnetic metal ion dopants
provides an efficient approach for probing informative nuclei such
as 17O, despite their low gyromagnetic ratio and negligible
abundance, without isotope enrichment.
We report coexistence of high spin Co3+ and Co2+ in ceramic Co3TeO6 using X-ray Absorption Near Edge Structure (XANES), DC magnetization, and first principles ab-initio calculations. The main absorption line of cobalt Co K-edge XANES spectra, along with a linear combination fit, led us to estimate relative concentration of Co2+ and Co3+as 60:40. The pre edge feature of XANES spectrum shows crystal field splitting of ∼1.26 eV between eg and t2g states, suggesting a mixture of high spin states of both Co2+ and Co3+. Temperature dependent high field DC magnetization measurements reveal dominant antiferromagnetic order with two Neel temperatures (TN1 ∼ 29 K and TN2 ∼ 18 K), consistent with single crystal study. A larger effective magnetic moment is observed in comparison to that reported for single crystal (which contains only Co2+), supports our inference that Co3+ exists in high spin state. Furthermore, we show that both Co2+ and Co3+ being in high spin states constitute a favorable ground state through first principles ab-initio calculations, where Rietveld refined synchrotron X-ray diffraction data are used as input.
We report observation of magneto-electric and magneto-dielectric couplings along with short range ferromagnetic order in ceramic Cobalt Tellurate (Co3TeO6, CTO) using magnetic, structural, dielectric, pyroelectric, and polarization studies. DC magnetization along with dielectric constant measurements indicate a coupling between magnetic order and electrical polarization. A strong anomaly in the dielectric constant at ∼17.4 K in zero magnetic field indicates spontaneous electric polarization, consistent with a recent neutron diffraction study. Observation of weak short range ferromagnetic order at lower temperatures is attributed to the Griffiths-like ferromagnetism. Furthermore, magnetic field dependence of the ferroelectric transition follows earlier theoretical predictions, applicable to single crystal CTO. Finally, combined dielectric, pyroelectric, and polarization measurements suggest that the ground state of CTO may possess spontaneous symmetry breaking in the absence of magnetic field.
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