Spinel Structure and Fe3O4 Applications

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Questions and Answers

What primarily contributes to surface topographic contrast in SEM?

Backscattered electrons (BSEs)

X-rays generated from specimens

Primary beam electrons

Secondary electrons (SEs) (correct)

Which electrical issue is commonly associated with examining nonconductive surfaces in SEM?

Inadequate electron beam focus

Surface charging (correct)

Low energy electron interactions

High-voltage discharge

What is the effect of using higher electron beam energy in SEM?

It reduces the amount of secondary electrons observed.

It enhances surface detail resolution.

It provides information about the bulk material. (correct)

It increases the surface charging effect.

What causes compositional contrast in SEM images?

Atomic number of specimen atoms Signup and view all the answers

How can surface charging in SEM be effectively reduced?

By coating the specimen with a conductive film Signup and view all the answers

What is the correct formula representation for normal spinel?

(A)8a[B^2]16dO4 Signup and view all the answers

What is the Crystal Field Stabilization Energy (CFSE) of Fe2+ in the octahedral site?

-4 Signup and view all the answers

Which of the following statements about Fe3+ cations is true?

Fe3+ has equal stabilization in both octahedral and tetrahedral sites. Signup and view all the answers

How does particle size affect the line broadening observed in XRD analysis?

Line broadening increases with decreasing particle size. Signup and view all the answers

What is the purpose of small angle X-ray scattering (SAXS) in XRD applications?

To determine particle inhomogeneities of the order of 1 – 100 nm. Signup and view all the answers

What is the significance of the diffraction angle (2θ) in a Transmission Electron Microscope (TEM)?

It must be very small (≤1°) for diffraction patterns to occur. Signup and view all the answers

Which beam combination is essential for generating phase contrast in TEM?

The transmitted beam and the diffracted beam. Signup and view all the answers

What outcome results from the recombination of transmitted and diffracted beams in TEM?

An interference pattern with periodic dark-bright changes. Signup and view all the answers

How does the diffraction pattern form on the back-focal plane when the transmitted beam is parallel to a crystallographic axis?

All diffraction points from the same crystal zone form a pattern. Signup and view all the answers

Why is phase contrast often referred to as high-resolution transmission electron microscopy (HRTEM)?

It delivers the highest resolution of lattice and structure images for crystalline materials. Signup and view all the answers

What structure does MnO possess?

Sodium chloride structure Signup and view all the answers

At what temperature does the magnetic ordering of Mn2+ ions in MnO break down?

293 K Signup and view all the answers

What is the effective magnification of an SEM with a probe size of 10 nm?

20,000× Signup and view all the answers

What additional information can SEM obtain using an X-ray energy-dispersive spectrometer (EDS)?

Chemical composition Signup and view all the answers

What type of electrons are produced through elastic scattering in SEM?

Backscattered electrons Signup and view all the answers

What percentage of energy do backscattered electrons typically retain from incident electrons?

60-80% Signup and view all the answers

What is one of the advantages of scanning electron microscopy (SEM) in terms of image depth of field?

Provides a three-dimensional appearance Signup and view all the answers

How is the magnification of SEM determined compared to transmission electron microscopy (TEM)?

By the ratio of screen size to specimen size Signup and view all the answers

Which equation relates the full width at half maximum (β) to the crystallite size (τ) and the diffraction angle (θ)?

β = λ / τ cos θ Signup and view all the answers

What is represented by the variable 'k' in the relation β = k ε (sin θ / cos θ)?

The constant related to microstrain Signup and view all the answers

How can the combined effects of crystallite size and microstrain on β be separated?

By plotting β cos θ versus sin θ Signup and view all the answers

What does the differentiation of the equation nλ = 2d sin θ provide insight into?

The change in lattice spacing with microstrain Signup and view all the answers

What does the term β cos θ relate to in the context of XRD?

Crystallite size Signup and view all the answers

Which application involves assessing changes in the lattice parameter with temperature?

Determination of thermal expansion coefficient Signup and view all the answers

What type of information can be determined about crystal structures using XRD with single crystals?

Unknown structures and various structural details Signup and view all the answers

Which factor influences line broadening due to nonuniformity in the sample?

Crystallite size and microstrain Signup and view all the answers

Study Notes

Spinel Structure

Eight MgAl2O4 formulas per unit cell
Oxygen forms a face-centered cubic (fcc) lattice.
Half of the octahedral sites are occupied by B ions in an alternating pattern.
One-eighth of the tetrahedral sites are occupied by A ions.
Normal spinel: (A)8a[B2]16dO4
Inverse spinel: (B)8a[AB]16dO4

Fe3O4 Spinel

Fe2+ (high spin): 3d6, t2g4eg2 (octahedral), e3t23 (tetrahedral)
Octahedral site stabilization energy (OSSE) for Fe2+ is -1.33 Dqoct, making it more stable in octahedral sites than tetrahedral sites.
Fe3+ (high spin): 3d5, t2g3eg2 (octahedral), e2t23 (tetrahedral)
Fe3+ has no preference for octahedral or tetrahedral sites.
Cation distribution in Fe3O4: (Fe3+)tet[Fe2+Fe3+]octO4, or (Fe3+)8a[Fe2+Fe3+]16dO4 (inverse)
Magnetic measurements confirm the cation distribution.

Applications of X-ray Diffraction (XRD)

Particle size analysis using XRD is effective for particles smaller than 200 nm.
Line broadening increases as particle size decreases.
Below 2-10 nm, lines become so broad that they disappear.
Small-angle X-ray scattering (SAXS) measures inhomogeneities from 1-100 nm.
Line broadening is influenced by instrumental factors (resolution of incident X-ray wavelength) and sample factors (crystallite size & microstrain).

Microstrain Determination

Scherrer equation:  =  /  cos 
Microstrain is related to line broadening through the equation: = - ( / 2) (cos / sin )
Combined effect of crystallite size and microstrain:  cos  = k sin 
Plotting  cos  vs.sin  separates crystallite size and strain effects.

Other Applications of XRD

Determining thermal expansion coefficient through changes in lattice parameter with temperature.
Assessing phase transitions at different temperatures.
Determining crystal structure with single crystals, including atom positions, space groups, bond lengths, coordination numbers, and electron distribution.

Neutron Diffraction of MnO

Neutrons have a magnetic dipole, leading to magnetic scattering.
MnO adopts a sodium chloride structure with a Neel temperature of 120 K.
At 80 K, Mn2+ ions undergo antiferromagnetic ordering, doubling the unit cell for neutron scattering but not for X-ray scattering.
Magnetic ordering breaks down at 293 K due to thermal energy.

Scanning Electron Microscopy (SEM)

Examines microscopic structure by scanning the surface.
Offers high resolution and depth of field, providing a three-dimensional view.
Magnification is determined by the ratio of display screen size to specimen area scanned.
Provides magnifications from 20× to > 100,000×.
Chemical information is obtained using an X-ray energy-dispersive spectrometer (EDS).

Signal Detection in SEM

Elastic scattering produces backscattered electrons (BSEs) which are useful for elemental composition contrast.
Inelastic scattering produces secondary electrons (SEs) which are used to create topographic contrast images.
By varying the energy from 5 to 20 keV, surface features (low keV) or bulk information (high keV) can be obtained.

Topographic Contrast in SEM

Variation in signal levels due to geometric features on the specimen surface.
Edges emit more electrons than flat surfaces.

Particle Shape and Elemental Distribution from SEM

SEM can visualize particle shape and elemental distribution.

Compositional Contrast in SEM

Arises from the dependence of BSE escape ability on the atomic number of the specimen's atoms.

Charging Effect in SEM

Surface charging occurs when examining electrically nonconductive surfaces.
Excessive electrons accumulate on the surface, leading to charged regions that deflect the electron probe.
Charging can be prevented by coating the specimen with a conductive film.

Transmission Electron Microscopy (TEM)

Bragg's law applies to electron diffraction.
Diffraction angles are small (<1°) in TEM.
Diffracted beams from crystallographic planes form a diffraction pattern on the back-focal plane of the objective lens.
When the transmitted beam is parallel to a crystallographic axis, all the diffraction points from the same crystal zone will form a diffraction pattern (reciprocal lattice) on the back-focal plane.

Diffraction Contrast in TEM
Differences in the scattering power of the specimen's atoms create contrast in the image.

Phase Contrast in TEM

Phase contrast TEM (HRTEM) offers the highest resolution for lattice and structure images of crystalline materials.
At least two electron waves with different phases are required, such as the transmitted and diffracted beams.
Interference patterns from these beams create periodic dark-bright changes on the image plane.

Phase Analysis by TEM

TEM helps analyze the phases (different crystal structures) present in a material and how they are arranged.

Conclusion

This summary provides a general overview of important concepts from the text on characterization techniques. The text offers insight into various techniques like X-ray, neutron, and electron diffraction, as well as scanning and transmission electron microscopy, focusing on their principles, applications, and underlying mechanisms.

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Description

This quiz explores the structural characteristics of spinel compounds, specifically focusing on their crystalline arrangements and cation distribution in Fe3O4. Additionally, it covers applications of X-ray Diffraction (XRD) in particle size analysis. Test your knowledge on these important concepts in material science and crystallography.