Materials and Nanomaterials Flashcards
In ionic structures are anions or cations bigger?
Anions are MUCH BIGGER!
Which are more mobile in an ionic structure, cations or anions?
Anions are much bigger and cations fill the interstitial sties; cations are much more mobile
TRUE or FALSE? The solid lines in diagrams of solid state structures are the bonds between atoms
FALSE - the lines are coordination not bonds - although many ionic solids show some covalent characteristics
What is the overall charge of a bulk solid
0 - there is no net charge. Charge can be distributed heterogeneously
At what temperature does the perfect ionic crystal exist at?
At absolute zero - 0 K
Above absolute zero, what will every solid contain - why is this thermodynamically favourable?
Solids will contain defects - increased entropy is thermodynamically favourable
What does the number of defects depend on?
The temperature and the energy required to generate the defects
What are the two types of INTRINSIC defects and what is the difference between them?
1) Schottky defects: a missing cation and anion pair
2) Frenkel defects: an ion (usually cation) moves into an interstitial site
What is the equation for calculating the proportion of defects?
n/N = exp(-ΔH / 2 k T)
n = number of defects per unit volume
N = number of anion and cation sites per unit volume
Do higher charge density ions tend to have more or less defects compared to lower charge dense ions?
Higher charge density ions are harder to move and therefore have a larger ΔH. Plugging into the equation shows that there are fewer defects
Name the EXTRINSIC defect and give a brief description
Doping: the introduction of different ions into a structure with the aim to change the properties of the solid.
How does doping with P into an Si lattice increased conductivity?
P has an extra electron (grp 5) than Si (grp 4) leading to a new valence band being introduced closer to the conduction band. The decrease in the band gap means easier excitation of electrons to the conduction band (from the valence band) and greater conductivity
What do defects result in?
Non-stoichiometry and solid solutions
Why is non-stoichiometry common for transition metals?
As transition metals have a range of oxidation states
What is non-stoichiometry?
(Usually of solids) Deviation from the ideal ratio of atoms, usually with as fractions (or d.p.)
What structure is TiOx?
Rock salt
How does the unit cell size change as x increases for TiOx?
As x increases the unit cell volumes decreases
TRUE of FALSE? For TiOx a change in x signifies a change in the number of oxygen atom vacancies?
FALSE - its just a change in the ratio of oxygen:titanium atoms. Could be due to more titanium vacancies
What is a solid solution?
A crystalline solid of 2 or more atoms that repeat to form a structure
What are the two types of solid solutions?
1) Substitutional - where a new atom replaces an existing atom (e.g. by doping)
2) Interstitial - where atoms are added to the structure
Give an example of a substitutional solid solution
Al2O3 + Cr2O3 –> Al(2-x)Cr(x)O3, when x = 0.05 we get ruby (which is used in lasers)
Give an example of an interstitial solid solution
C into Fe –> FeCx. C occupies interstitial sites and gives steel
Above absolute zero, atoms/ions respond to external stimuli . Name 4 different external stimuli
- Electric field
- Magnetic field
- Pressure
- Temperature
What are the components of a rechargeable lithium ion battery?
- 2x Electrodes
- Electrolyte (medium for ions to move)
What is intercalation and why is it important for lithium ion batteries?
The process of inserting an ion into a layered material. Important for the charging/discharging cycle.
What structural features of graphite and LiCoO(2) make for good use in lithium ion batteries?
They have layered structures that allow for intercalation and deintercalation (addition and removal of Li+ ions) and allow for movement of Li+ ions
What is polymorphism?
The solid state equivalent of isomerism - TiO2 can have 3 different structures depending on temperature and pressure
Why should intercalation/deintercalation not result in big structural changes?
Leads to mechanical stress and possible fracture - performance loss
What is the importance of having a secondary layer between the electrolytes and the electrodes?
Very oxidising/reducing environment - stops degradation of electrolyte and growth of dendrites (which could lead to ignition of flammable electrodes
Why, to obtain bulk polarisation, should a material be a non-centrosymmetric crystal?
Otherwise dipoles cancel each other out
What are the 3 types of compounds that respond to external stimuli to give a bulk polarisation? What do each respond to?
- Ferroelectrics: dipoles respond to electric field
- Pyroelectrics: dipoles respond to heat (or photons) and convert heat to electrical energy.
- Piezoelectrics: dipoles respond to pressure and convert kinetic energy to electrical energy
What should 2 qualities should good ferroelectric have?
- A high dielectric strength (not breakdown at high voltages and becoming conducting)
- A low dielectric loss (no loss of electrical energy as heat or as alternating electric field)
What structure is BaTiO(3)?
Perovskite
What happens to the Ti atoms in BaTiO(3) when heated below 120c?
Above 120c the Ti atoms are in Oh geometry (which is centrosymmetric) and so there is no bulk polarisation. Below 120c the Ti atoms are displaced along 1 axis as the TiO(6) octahedra tilt slightly. This leads to a dipole.
Why does a structure distort?
For a compound of containing several elements the ideal preferences of each element is unlikely to be accommodated (e.g. preferred coordination/geometry) which introduces strain. Enough strain and the structure distorts
What is the standard empirical formula for a perovskite structure?
ABO(3)
How can distortion be estimated?
Using a tolerance factor
What is the standard empirical formula for a rock salt structure?
AB (1:1 ratio)
For the ideal structure, what is the tolerance factor (t)?
t = 1
what is the range that the tolerance factor (t) can take for a distorted perovskite structure? And outside of this range what happens to the structure?
0.85 < t < 1.06 - Outside this range a non-perovskite structure is adopted
Why, above 120c, is BaTiO(3) in a cubic perovskite structure, and why below 120c is the structure distorted?
Above 120c the TiO(6) tetrahedra thermally expand and creates enough chemical pressure to support a cubic perovskite structure.
Below this temperature the reduced thermal motion cannot compensate for the strain and the structure distorts (octahedra tilt)
How can the bulk polarisation of a ferroelectric (e.g. BaTiO3) be removed and/or reversed?
By the application of a an opposing electric field
How do pyroelectrics work?
Thermal expansion/contraction of the crystal lattices causes a change in the size of the dipoles and bulk polarisation.
Give an example of a pyroelectric material and explain how it works
ZnO - the ZnO(4) tetrahedra point in the same direction giving a bulk polarisation that cannot be reversed by an electric field
What is the output of a pyroelectric?
A voltage/current due to temperature changes
How do piezoelectrics work?
Electrical charges develop on opposite crystal phases (resulting in a voltage) in response to mechanical stress.
What is a common structure in a piezoelectric?
Tetrahedral groups that distort under stress - they dont have centres of symmetry and give rise to non-centrosymmetric structures.
What happens when a piezoelectric is placed in an alternating field?
When placed in an electric field strain is developed (it moves). In an alternating field the crystal vibrates (used in watches)
What is the difference between S spin and I spin?
S refers to the electron spin quantum number, whereas I refers to the spin angular momentum of a nucleus
What is the equation for calculating the magnetic dipole moment of a transition metal (μ)?
μ = g (sqr root(S(S+1)) where g ~ 2
Why are heavier transition metals often strongly magnetic materials?
They have large spin-orbit coupling giving large magnetic dipole moments
What is the magnetic susceptibility (χ)?
A measure of how magnetic a material is - varies with temperature and the applied field
What is paramagnetism?
Unpaired electrons are weakly attracted to a magnetic field. Paramagnetism is very temperature dependant
What happens to the dipoles in a paramagnet in a strong magnetic field (H) OR at low temperature?
Dipoles begin to align parallel to one another resulting in a bulk magnetic moment
What is antiferromagnetism?
As temperature decreases the magnetic dipoles align anti-parallel leading to no bulk magnetic moment
What is the Neel temperature?
The temperature at which an antiferromagnet stops displaying antiferromagnetic characteristics, and start paramagnetic behaviour
What is ferromagnetism?
The magnetic ion dipoles align parallel in the absence of an applied magnetic field
What is ferrimagnetism?
Magnetic ion dipoles of different atoms align anti-parallel but DO NOT cancel each other out leading to bulk magnetism
What is the Curie temperature?
Below the Curie temperature the material will display ferromagnetism (or ferrimagnetism) and above the material will display paramagnetism
What is superexchange?
Anions (O,S,F) mediate magnetic exchange between magnetic metal cations
How does superexchange lead to antiferromagnetism in rock salt structures?
Two eg electrons (containing unpaired electrons) on the metal cation overlap with filled p-orbitals on an anion that contain oppositely aligned electrons. This leads to the two metal cations having electrons that are oppositely aligned (which is antiferromagnetism) and no bulk polarisation
Why, above the Neel temperature is antiferromagnetism not seen?
The thermal energy is greater than the superexchange interaction and the magnetic dipoles no longer align anti-parallel (leading to paramagnetism)
What is needed for a structure to display ferrimagnetism?
Two different types of ions (either different elements or different oxidation states)
Why are ferrimagnets important in industry?
As they have the same properties as ferromagnets but are electrically insulating
What is the empirical formula for an spinel structure? What is the oxidation state of each metal?
AB(2)X(4) - A(II) and B (III)
For a spinel structure AB(2)X(4) which metal is in the tetrahedral geometry and which is octahedral?
A(II) is tetrahedral and B(III) is octahedral
For an inverse spinel structure AB(2)X(4) which metal is in the tetrahedral geometry and which is octahedral?
A(II) is octahedral and B(III) is both tetrahedral AND octahedral
TRUE OR FALSE? Materials with spinel structures can contain a mix of both normal and inverse structure types?
True (e.g. 80% normal and 20% inverse)
What are the three preferences for a normal vs inverse spinel structure?
1) Electrostatic - M(III) prefers octahedral so M(II) goes into tetrahedral
2) Ion size - larger cations prefer octahedral geometry
3) Crystal Field Stabilisation Energies
Is an oxide lattice a weak field or high field ligand?
Weak field - small Eg/T2g gap so HS complexes are possible
TRUE or FALSE? The 2nd and 3rd row d-block metals are always low spin
TRUE - only 1st row can be high spin and often are in with weak field ligands (like in an oxide lattice)
How can the magnetic moment of ferrimagnets be estimated?
Use μ = gS to calculate the magnetic moment (μ) for each ion. The maximum magnetic moment (μ(sat)) of the ferrimagnet is the vector sum of the individual ion moments. μ(oct) - μ(tet) = μ(sat)
Why is iron (a ferromagnet) with a high Curie temperature, not magnetic at room temperature?
Due to domains (regions of strongly aligned ions) - without an external magnetic field each domain is not parallel and so no bulky magnetisation.
Do hard magnets have large or small remnant magnetisation? What does this mean for the magnet?
Hard magnets have large remnant magnetisation which means they remain magnetised after the removal of an external magnetic field
What are the two remarkable properties of superconductors?
1) Zero electrical resistance at low temperatures
2) Ejects the external magnetic field from its volume (Meissner effect)
Which superconductors have been shown to have high critical temperatures?
Cuperates
What are superconductors used for and why?
To generate very large magnetic fields without heating up
Why are cuperates good superconductors?
CuO2 layers separated by charge reservoir layers that control the average oxidation state of Cu.
The critical temperature increases with oxygen content
How are fullerides synthesised?
Intercalation of electropositive metals into C60.
How do fullerides act as superconductors?
C60 is reduced to C60(n-). The orbitals of neighbouring fullerides overlap forming metallic bonds of conductivity. Cooling shows superconductivity.
What is a phonon?
Quantised lattice vibrations
What is a Cooper pair?
Interaction between a phonon and electron allow electrons to overcome repulsion and become attracted to each other
Describe how Cooper pairs are formed
An electron moving through a lattice causes a disturbance. This causes the lattice to vibrate and a positive pocket to be formed around the electron. Another electron is attracted to this pocket and forms a weak bind with the first electron
How does resistivity arise, and why is there no resistivity in superconductors?
Usually phonons scatter electrons which is why resistivity increases with temperature. In a superconductor Cooper pairs are not scattered and therefore there is no resistance.
TRUE or FALSE? Nanoparticles are spherical?
FALSE - they are a range of shapes each with unique energy and electronic structure
Why do the surfaces of nanoparticles tend to be reactive?
Lower coordination = higher energy = greater reactivity
What are gold alloy particles good at?
Good at oxidation reactions at low temperatures
What does x-ray diffraction give? What are the pros and cons?
Maps electron density, only gives an average structure, difficult to detect low electron density elements
What happens to the peak width in x-ray diffraction as temperature increases?
peak width decreases
What is the equation of calculating particle size in x-ray diffraction?
τ = (k x λ) / (β x cos θ)
where β is full width half maximum and θ is the Bragg angle in radians
How do you convert from degrees to radians?
1 degree = π/180
Why is electron diffraction used instead of x-ray diffraction for smaller objects?
Electrons have a smaller wavelength than photos and therefore give a higher resolving power
What is the equation for calculating the minimum resolving distance?
d = λ / (2n sinθ)
where n is the refractive index
What is scanning electron microscopy (SEM) and what is it used for?
- Analysis of back scattered electrons from the surface
- Gives morphology and particle size of surface
- Often used with EDS
What is energy dispersive spectroscopy (EDS) and what is it used for?
- Excitation of inner electrons of atoms
- Gives an elemental map
What are the similarities and differences between transmission electron microscopy and SCANNING transmission electron microscopy?
- Both give high quality images
- Both give internal structure, morphology, defects, local and crystal structure
- STEM gives elemental composition too
What does electron energy loss spectroscopy (EELS) give?
- Oxidation state
- Chemical bonding
What is scanning tunnelling microscopy (STM) used for?
- Gives surface structure and composition
- Allows atom manipulation
What is atomic force microscopy (AFM) and what is a benefit of it?
- A tip tracks along the surface
- Gives surface structure and composition
- Functionality can be built into tip
- Can measure conductivity, electrochemistry and magnetisation
What can NMR and EPR be used for in solid state chemistry?
- NMR gives local structure and dynamics
- EPR gives local structure and defects
