Chapter 9 - Characterisation Flashcards
What are soft x-ray absorption spectroscopy (sXAS) used for?
Tool to probe the electronic states through excitation of core-level electrons to unoccupied states above the Fermi level.
3d-states of TM and p-electrons of anions can be studied.
What are some properties of soft X-ray absorption spectroscopy (sXAS)?
- Extremely surface sensitive
- Different modes with different probing strength (1 - 500 nm, Auger < Partial electron yield < Total electron yield < Total fluorescense yield)
What are some limitations with soft X-ray absorption spectroscopy (sXAS)?
- Requires UHV -> limits operando capabilities
What is x-ray diffraction (XRD) used for?
To probe the average crystal structure:
- lattice parameter, phase, strain, atomic positions, site occupancy, texture, stacking faults
What is pair distribution function (PDF)?
A total scattering technique. Looks at Bragg peaks and diffuse scattering.
Yields information about the local structure. Interatomic distances, coordination numbers.
Covers mid-range structure between XRD and EXAFS.
What is hard x-ray absorption spectroscopy (XAS) used for?
Looking at valence state changes during electrochemical reactions (reveals charge compensation)
- Looking at coordination environment
- Looking at local structural changes (bond length and degree of disorder).
What can scanning/transmission x-ray microscopy (STXM/TXM) be used for?
- Evaluation of morphology and micro-structure evaluation in micro/meso/macro scale (microcracking, particle fracture, tortuosity)
- Chemical information (elemental / chemical mapping, revealing concentration gradient, oxidation states)
What are some factors that complicate operando neutron experiments?
- High incoherent scattering cross section of elements in the battery (especially hydrogen).
- Lithium, while having good contrast in neutron experiments, also is a good neutron absorber which reduces signal.
- Amount of material needed for experiments
What are some great advantages of neutrons?
- High sensitivity to lithium
- High contrast between neighbouring elements in the periodic table
- High penetrating power allowing whole battery to be imaged
How can we study the motion of atoms with neutron scattering?
By looking at the inelastic scattering. This is where neutrons start or cancel oscilliations in the atoms, creating phonons or magnons and thus loosing energy.
Wavelength and energy are comparable to typical atomic spacing and vibrational energy.
How are neutrons at a neutron facility generated?
Through a fission process of e.g. 235 U
What are some fundamental properties of neutrons?
- Is neutral
- Has magnetic moment
- Has spin
- Has energies similar to vibrational modes
- Has wavelengths similar to atomic spacings
How does the scattering cross section of x-rays vary with Z?
Increases with increasing Z (scatters of electron cloud)
How does the scattering cross section of neutrons vary with Z?
Randomly - dependent on isotopes for same element as well.
What does the neutron scattering cross section depend on?
Atomic element, isotope and nuclear spin state.
