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π Electron Effects on Chemical Shift: Overview

An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0, resulting in...
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Crystal Field Theory
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Electronic correlation effects in the Cr2GeC Mn+1AXn phase.

Maurizio Mattesini1, Martin Magnuson

  • 1Departamento de Física de la Tierra, Astronomía y Astrofísica I, Universidad Complutense de Madrid, E-28040 Madrid, Spain. mmattesi@fis.ucm.es

Journal of Physics. Condensed Matter : an Institute of Physics Journal
|December 11, 2012
PubMed
Summary

The hexagonal Cr2GeC system exhibits compensated antiferromagnetic ordering due to Ge-mediated super-exchange. Accurate electronic structure calculations are crucial for understanding this Cr-based MAX phase material.

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Area of Science:

  • Condensed Matter Physics
  • Materials Science
  • Computational Materials Science

Background:

  • The Cr2GeC system is a hexagonal MAX phase material with potential applications.
  • Understanding its magnetic and electronic properties is key to its technological utilization.
  • Strongly correlated d electrons in Cr atoms often require advanced theoretical treatment.

Purpose of the Study:

  • To investigate the magnetic properties, electronic band structure, and Fermi surfaces of hexagonal Cr2GeC.
  • To determine the ground-state spin ordering and the mechanisms governing it.
  • To elucidate the electronic transport properties and the role of electron correlation.

Main Methods:

  • Utilized generalized gradient approximation (GGA) and GGA + U methods for electronic structure calculations.
  • Computed effective U values using the augmented plane wave (APW) scheme and constrained density functional theory (DFT).
  • Performed topological analysis of Fermi surfaces to assess electronic transport characteristics.

Main Results:

  • Identified a compensated antiferromagnetic spin ordering as the ground state of Cr2GeC.
  • Revealed Ge-mediated super-exchange coupling responsible for spin distribution in Cr-C networks.
  • Observed asymmetrical carrier-type distribution in the hexagonal lattice, impacting transport properties.
  • Structural properties calculated showed good agreement with experimental data.

Conclusions:

  • The GGA + U method accurately describes the ground-state magnetic ordering of Cr2GeC.
  • Ge-mediated super-exchange plays a critical role in the material's magnetic behavior.
  • Accurate treatment of strongly correlated Cr d electrons is essential for interpreting the properties of this MAX phase.