Related Experiment Video
Updated: May 9, 2026
10:13
A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks
Published on: April 28, 2023
75As NMR study of overdoped CeFeAsO0.8F0.2
Damian Rybicki1, Thomas Meissner, Grant V M Williams
1Faculty of Physics and Earth Science, University of Leipzig, Leipzig, Germany. rybicki@physik.uni-leipzig.de
Summary
Nuclear magnetic resonance (NMR) reveals distinct arsenic sites in CeFeAsO0.8F0.2 superconductor above 39 K. Findings suggest electronic phase segregation or charge/spin density variations, not superconducting vortices.
Area of Science:
- Condensed Matter Physics
- Materials Science
- Solid-State Chemistry
Background:
- Iron pnictide superconductors exhibit complex electronic properties.
- Understanding doping effects is crucial for superconductor development.
- Previous studies suggested vortex effects on arsenic NMR shifts.
Purpose of the Study:
- To investigate the electronic structure of overdoped CeFeAsO0.8F0.2 using (75)As NMR.
- To clarify the origin of observed NMR signals above the superconducting transition temperature.
- To explore potential mechanisms behind the observed spectral features.
Main Methods:
- (75)As nuclear magnetic resonance (NMR) spectroscopy.
- Temperature-dependent NMR shift and spin-lattice relaxation rate measurements.
- Analysis of spectral features including quadrupole splittings.
Main Results:
- Two distinct (75)As NMR sites with different shifts were observed up to 100 K, above the superconducting transition temperature (39 K).
- A significantly enhanced spin-lattice relaxation rate suggests hyperfine coupling from magnetic Ce to As.
- Low-temperature spectra revealed As ions with two different quadrupole splittings.
Conclusions:
- The observed NMR shifts are attributed to hyperfine coupling from magnetic Ce, not superconducting vortices.
- Findings are consistent with electronic phase segregation due to varying F doping.
- An alternative explanation involves correlated spatial charge and spin density variations.
Related Concept Videos
01:15NMR Spectroscopy: Chemical Shift Overview
The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
For instance, the proton...
For instance, the proton...
01:04¹³C NMR: ¹H–¹³C Decoupling
The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
A broadband decoupling technique is used to simplify these complex, sometimes overlapping, signals. Broadband decoupling relies on a...
01:20¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)
When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
01:14NMR Spectroscopy of Aromatic Compounds
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range. Consider...
01:02¹H NMR: Interpreting Distorted and Overlapping Signals
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
01:062D NMR: Homonuclear Correlation Spectroscopy (COSY)
Homonuclear correlation spectroscopy, or COSY, is a 2-dimensional NMR technique that provides information about coupled protons. Typically, the geminal and vicinal coupling are observed. For example, consider the COSY spectrum of ethyl acetate, where its 1D proton NMR spectrum is plotted along the vertical and horizontal axes with their corresponding chemical shift scale. Three spots on the diagonal corresponding to the three peaks in the 1D proton spectrum are called diagonal peaks. The COSY...