characterisation of solid state pt 2

what are advantages of powder XRD?

(purity, sample prep, changes, equipment)

establishes phase purity

don’t need to grow large single crystals

can monitor structural changes

lab equipment usually sufficient

what does x-ray scattering factors depend on?

what does this mean about pXRD compared to single crystal XRD? 3

depends on number of electrons around atom or ion

• light atoms (H, Li) hard to locate

• hard to discriminate between atoms with similar numbers of electrons (periodic table/ox states)

• heavy atoms dominate diffraction

issues are worse for pXRD

what can you do if no structural model is available?

what is an issue and how can you overcome it

single crystal structure determination is needed

• hard to obtain big enough crystals

• can use synchrotron radiation

what is the de Broglie relationship

any beam of moving particles will display wave properties according to

λ = h / p

p is momentum

how can you control wavelength of neutrons? give example of technique

what wavelength is needed for neutron diffraction?

by controlling their velocity

e.g. pass fast neutrons through moderators like D₂O to slow them

wavelength of ~1Å

is a high or low flux of neutrons needed for diffraction?

what are 2 possible sources

high flux needed

• high flux nuclear reactor

• spallation sources where neutrons are produced by bombarding metal target with high energy protons (generated by synchrotron)

what in a crystal scatters the neutrons? how is this different from x ray diffraction?

neutrons scattered by atomic nuclei

x rays scattered by electrons

why are bond lengths more accurate from neutron than x ray diffraction?

x ray estimates where the centre of the atom is based on electron density (elliptical electron cloud)

neutron determines location of nuclei

do neutron scattering factors depend on size as with XRD?

how do they change for different atoms/isotopes? light/heavy atoms? Bragg angles?

do not depend on size factors

• different for all atoms, and even different isotopes of same atom

• relatively similar between light and heavy so light atoms scatter as well as heavy

• do not decrease at high Bragg angles

how does charging lithium battery affect structure?

what can pXRD do/not do? what other technique could you use?

causes loss of Li⁺ from LiCoO₂ to give Li_xCoO₂

can monitor whether phase or structural changes are occurring during delithiation

cannot determine x in Li_xCoO_2, could be determined by neutron diffraction

how do neutrons interact with magnetic moment of atom

what is atomic magnetic moment caused by?

neutrons have a spin so have a magnetic moment

• can interact with magnetic moment of atom

atomic magnetic moment caused by alignment of electron spins

how can neutron diffraction be used for magnetic moments in structure? what is determined?

magnetic scattering of polarised neutrons from ordered magnetic material

gives magnetic Bragg reflections

• can find magnitude and direction of magnetic moments

disadvantages of neutron diffraction 2

expensive

neutrons have low flux - need very large crystals for single crystal studies. longer collection times than X-ray. large banks of detectors often used

show diagram of atom under bombardment of incident electron beam

how are electrons scattered? what is emitted? show K,L,M

what forms electron diffraction pattern?

what machine is used?

elastically scattered electrons can form diffraction pattern if sample is crystalline

transmission electron microscope used

what do scattering factors depend on for electron diffraction? why

scattering factors depends on atomic number (as with XRD)

interact with both electron density and nuclei

is small or large sample needed for electron diffraction? why?

very high scattering efficiency

only v small sample needed

2 advantages of electron diffraction over XRD/neutron

much more sensitive - only trace amounts of sample needed

can focus on different regions of sample as electron microscope used

2 disadvantages of electron diffraction

restricted to indexing - determining unit cell parameters and miller indices of reflection

full structure solution / refinement is possible but specialist instrumentation is very new and rare

electron diffraction diagram

show L - what is it?

show r - what is it?

show angle

what is the relationship between 2θ, r and L?

what approximation is made?

combine 2θ = r/L and Bragg’s law to give electron diffraction equation

what approx is made?

what does wavelength of electrons depend on?

accelerating voltage thus instrument used

what electrons from electron microscopy (bombardment) are used for imaging?

transmitted, secondary and/or backscattered electrons

what does scanning electron microscopy show?

images of surface of sample , with depth

what does transmission electron microscopy show?

how is image formed? what does this mean about the sample? resolution?

image formed by beam of electrons passing through sample so sample must be thin

images are projection through crystal so look 2D

high resolution, down to near atomic scale - shows lattice

what is transmission electron microscopy good for 5

seeing periodic nature of materials (lattice)

composite materials

showing faults in the lattice structure

giving structural information

chemical analysis

what is an issue with transmission electron microscopy?

beam induced damage of samples

• can make or break bonds with high energy electron beams

is scanning probe microscopy image formation based on? what is it not based on?

not based on optics

based on the interaction of a sharp probe with the sample surface

how does probe move in scanning probe microscopy?

probe rastered over sample to collect info from many points

• scan area in side to side fashion

what must sample be for scanning probe microscopy?

surface must be very very clean to avoid contaminants

what happens in scanning tunnelling microscopy? when does tunnelling current flow and between what?

quantum tunnelling of electrons between the sample and outermost atoms of tip

metal tip brought close enough to sample for overlap of wave functions

tunnelling current flows between tip and surface when voltage applied

what is tunnelling current sensitive to?

what resolution can be achieved in scanning tunnelling microscopy?

changes in tip surface distance

can achieve atomic resolution as a single atom probes the electron density

what happens in atomic force microscopy?

where are forces measured? what type of forces? 5

what happens to force while measured? what is used to measure?

measurement of forces between probe tip and sample

could be: van der Waals, frictional, adhesion, magnetic, electrostatic

force is magnified by flexible cantilever. use laser reflected off cantilever to measure

show diagram of atomic force microscopy

show photodetector, laser, sample, cantilever and tip

what is the difference between atomic force microscopy and scanning tunnelling microscopy? (what must sample be like)

STM - must conduct

AFM - can be conducting or non conducting surfaces

what is x-ray emission used for?

what is a negative of it

use characteristic x-ray emission spectra for chemical analysis

• doesn’t detect the lightest atoms

what does x-ray absorption give info about? what is used to generate x-rays?

what is it used for/ not used for?

info about local structure and ox states of metals

requires high energy x-rays so synchrotron used

• used for metals

• lighter atoms (H to B) are difficult / not possible to study

why is X-ray emission characteristic of elements?

different elements have different energies for electronic transitions

X-ray fluorescence

what can it determine? light atoms?

why can it be used for antiquities etc?

elemental composition and stoichiometry

can establish if element is present, compositional ratios between elements

less useful for light atoms as doesn’t detect Li or lighter

not destructive technique - can use on samples where prep is damaging

what is energy dispersive x-ray spectroscopy?

where XRF (X-ray fluorescence) is added to an electron microscope

• can study different regions of one sample. electron mapping to give spatial distribution of elements across the sample

show X-ray emission in shells

what are the different types of emission?

show X-ray absorption in shells

show ionisation and inner shell transitions

how is X-ray absorption characteristic of an element?

show abs edge

spectra shows absorption edges that depend on relative separation of atomic energy levels

xray absorption near edge structure (XANES) and extended xray absorption fine structure (EXAFS)

what radiation source? how are different energies scanned?

what do they measure? are they good for all elements?

does sample need to be crystalline?

synchrotron radiation with tuneable wavelength to scan diff energies

both measure absorption spectra near absorption edge for one particular element.

• usually metal or relatively heavy as light atoms not generally used

• sample doesn’t need to be crystalline

what is the difference between XANES and EXAFS?

XANES - absorption around absorption edge

EXAFS - oscillations of absorption coefficient on high energy side of absorption edge

what does XANES peak depend on?3

what does it probe?

ox state

surrounding ligands

nature of bonding

• probes local structure around chosen element

why might additional peak be seen?

inter-shell electron transitions

how are XANES studies used with model compounds?

compare obtained spectrum with those of model compounds with known metal oxidation states and coordination environment

what does XANES not give info about?

no info about geometrical structural info e.g. bond length

what does EXAFS study?

oscillations in absorption of xrays for particular metallic element

what are EXAFS measured oscillations caused by?

caused by interference from neighbouring atoms

• incident X-ray ionises core electron

• ejected photo electron wave scattered by neighbouring atoms

• interference between outgoing electron and back scattered ones leads to oscillations

what do oscillations of absorption coefficient depend on?

spatial relationship between absorbing atoms and its neighbours

what transformation of EXAFS gives info about atoms causing interference?

Fourier transformation

shows peaks of distance of coordination spheres

• intensity shows what elements there are and how many

what information does EXAFS give and how accurate?

what can therefore be/not be determined?

atomic number of neighbouring atoms to ±2

number of neighbouring atoms to ±1

distance from absorbing atom (bond length) to ±0.01Å, to max ~4Å

• bond lengths can be determined but not bond angles

can EXAFS be used for amorphous structures?

yes e.g. powders, glasses, gels

can give bond lengths for complicated non crystalline materials or microcrystalline materials where no good structural model exists