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Inorganic > Slater - Solid State Chemistry > Flashcards

Slater - Solid State Chemistry Flashcards

1
Q

What 5 pieces of information can be determined by theXRD experiment?

A

Size, shape and electron density distribution within the unit cell
Type of atoms and position of atoms within the unit cell

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2
Q

What were the 7 types of crystal systems?

A 
Triclinic 
Monoclinic 
Orthorhombic 
Tetragonal
Trigonal
Hexagonal
Cubic
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3
Q

What are the four types of centring of a unit cell

A

Primative
Body centred
Face centred
Base centred

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4
Q

What is Braggs law

A

Lambda = 2dhkl sin ø

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5
Q

What is the indexing pattern for a cubic unit cell

A

1/dhkl = (h^2 + k^2 + l^2) / a^2

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6
Q

Indexing pattern for tetragonal

A

1/dhkl = (h^2 + k^2) / a^2 + l^2/ b^2

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7
Q

Indexing pattern for orthorhombic

A

1/dhkl = (h^2) / a^2 + k^2/ b^2 + l^2/ c^2

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8
Q

What equation do you use that incorporates braggs law to find a ( bond length )

A

4a^2 = lambda^2 (h^2 + k^2 + l^2) / sin ^2 ø

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9
Q

Explain thermogravemetric analysis

A

A precise microbalance used to monitor the mass whilst it is heated under a variety of atmospheres
Information about component ratios can be calculated but not their defects

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10
Q

What is the general formula for the perovskites crystal

A

ABO3 where a is the large cation and b is the small cation
Close packing spheres
Small cation occupies 1/4 of octahedral holes
Primative structure

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11
Q

What are the two groups of defects

A

Intrinsic - stoichiometric and therefore not changing overall ratio
Extrinsic - non-stoichiometric used to improve properties such as conductance

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12
Q

What are the two intrinsic defects?

A

Schottky and frenkel

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13
Q

What is the driving force for intrinsic defects?

A

Toss off between favoured entropy changes (increased disorder) and unflavoured enthalpy changes

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14
Q

What is the third law of thermodynamics

A

Crystals can have complete atomic order only at zero kelvin - at all other temperatures disorder will reduce the free energy

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15
Q

Explain schotty defects

A

Introduction of vacancies in an anion and cation site following the stoichiometry of the crystal. I.e. 1:1 removed if formula is NaCl but if MgCl2 then 1:2 removed

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16
Q

Explain frankel defects

A

Displacement of the cation or anion into an interstitial site within the lattice
Normally cation because they tend to be smaller

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17
Q

When is it not possible for schotty and frankel defects to occur

A

In non stoichiometric solids

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18
Q

Explain schotty defects in terms of entropy and enthalpy

A

Increased defects, increases entropy and increases enthalpy

Enthalpy is related to but not equal to the size of the lattice energy in schotty defects

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19
Q

Explain the frankel defects in terms of enthalpy, instability and repulsion

A

Moving a cation towards the interstitial site will move the ion closer to a neighbouring cation which will increase repulsion between the two positively charged species leading to a more unstable material
Enthalpy Change is related to energy required to move the io into a less favourable site

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20
Q

In a large crystal, how many ions do we have in a one mole lattice ?

A

Avogadros constant 6.02x10^23

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21
Q

What is the equation used to calculate the entropy change per mole of crystal changed?

A

S=k lnΩ

Where k = Boltzmann constant
S = number of statistical arrangements (typically avagadros number)

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22
Q

What eventually happens with the enthalpy entropy balance with schotty defects

A

Eventually delta g and delta h will balance out and the formation of a schotty defect will no longer be favourable

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23
Q

As you decrease the temperature, the number of defects…

A

…decreases

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24
Q

What are the most favourable intrinsic defects?

A

Schotty defects

Because a frankel defect usually involves an octahedral cation moving into a tetrahedral site which is highly endothermic and unfavourable. Partly due to size of ion

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25
Q

Which 3 examples prefer frenkel defects and why

A

AgCl, AgBr and AgI

Displacement of the Ag+ into a tetrahedral interstitial site is preferred because Ag prefers tetrahedral coordination as opposed to octahedral coordination

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26
Q

Why would frenkel defects be favoured?

A

Any vacant sites that are unoccupied by cation mean that the anions can displace from their original site and and displace to the vacant square - good for conductance

27
Q

At 800 degrees C, how many defects are there in a cube of crystal approximately 20 unit cells long (100 angstroms)

A

1

28
Q

Why is it hard to predict the number of defects at low temperatures?

A

Thermodynamic vs kinetic effects

At low temperatures the cations motion is typically frozen so defects cannot be moved

29
Q

How can you prepare a solid with high defects

A

Prepare at high temperatures and rapidly cool so the ions don’t have time to move back, leading to more defects in the solid than expected

30
Q

Why are some lattices such as TiO seen as metallic?

A

Overlap of the cations T2g orbitals forms a partially filled band of electrons therefore leaving the solid with metallic properties. The band is filling the whole structure, not truely ionic

31
Q

What are the two types of extrinsic defects

A

Aliovalent - doping vacancies with ions of different charge

Isovalent - doping with ions of the same charge

32
Q

When doping, what is the most important factor to consider?

A

Ionic size - needs to be similar to the ionic site

33
Q

When doping a crystalline solid, how is the excess charge of the positive charge balanced?

A

The structure will contain pockets of electrons in anion vacancies.
The structure will have an Imbalance in numbers of cations and anions therefore resultant charge requirements are met. The defects provide colour.

Also transition metals can adopt different oxidation states giving a mechanism for maintaining charge balance when defects are created

34
Q

Give three examples of non stoichiometric salts

A

FeO
KCl
FeO

35
Q

What happens to non-stoichiometry when temperature increases

A

The entropy term T delta S increases with increasing temperature therefore an increase in temperature increases the value of x.

36
Q

How is a solid classified as non-stoichiometric?

A

1) chemically significant composition range (measured under chemical analysis)
2) unit cell size varies linearly (vegards law - deviations occur at high defect concentrations)

37
Q

What does the composition of a non-stoichiometric solid depend on?

A

Partial pressure of O2 in equilibrium with solid
The chemical potential of O2 must be equal to chemical potential of the solid
When enthalpy change is small, there’s a significant composition range

38
Q

How does the size of the unit cell allow determination of non-stoichiometric solids?

A

As composition of the cation decreases, the size of the unit cell increases - interaction between defects results in non-linear behaviour

39
Q

In a non-stoichiometric solid, how does the interplay of enthalpy and entropy change dictate the structure?

A

Entropy favours high concentration of defects and random distribution of defects

Enthalpy favours small concentration of defects and an ordered distribution of defects

40
Q

What are point defects?

A

Non-interacting defects, usually metallic compounds that have d orbital overlaps

41
Q

Explain the structure of SrFeO2.5 and explain the change in structure upon hearing

A

Vacancies order to form brownmillerite with vacancies located in alternative layers leaving layers of tetrahedrally and octahedrally coordinated Fe.

Upon heating, a transition occurs at approximately 850 degrees Celsius where:

  • Oxygen vacancies become randomly distributed
  • a deficient oxygen perovskite is formed
  • Increase influence of entropy as temp increases
42
Q

Give two methods that allow variations of oxidation states of transition metals

A

1) formation of very high concentrations of oxide ion vacancies (reduction with hydrides) e.g. CaH2
2) reduction of Fe oxidation stage while maintaining low vacancy content e.g. PVDF and -CH2CF2- 400 degrees c

43
Q

Within perovskites the cations occupying A can be….

A

Different oxidation states of the same cation

44
Q

How are structures ordered?

A

Ions with different sizes and/or charges will tend to prefer order to minimise strain and electrostatic repulsion

45
Q

Explain the large perovskites structure

A

A unit twice as large as the conventional perovskites that is face centre with systematic absences such as Sr2FeMoO6

46
Q

Explain the Ruddlesden-Popper structure

A

Perovskites type units separated by rock salt type layers

A(n+1) B O(3n+1)

E.g. Sr Ln 0.5 Sb 0.5 O4

47
Q

On an electronic spectroscopy spectra, what gives rise to unsymmetrical peaks

A

eg–> t2g

48
Q

When analysing spectra, what should be commented on?

A

Number of bands
Energies of bands
Intensities of bands
Width of bands

49
Q

Why does crystal field stabilisation energy fail?

A

It ignore electron electron repulsion

50
Q

How does the repulsion energy of xz and z^2 compare to xz and z^2?

A

Yz and z^2 have less repulsion than xz and z^2

51
Q

When labelling orbitals in CFSE what does T and E stand for

A 
T = 3 triplet 
E = 2 doublet
52
Q

What does the orgel diagram show

A

The relationship between the d1, d4, d6 and d9 showing the ground state. This is identified where all of the lines cross over.

For these four configurations you will observe a single band with energy equivalent to delta.

53
Q

When you see a g subscript, what does this relate to?

A

Octahedral complexes

54
Q

Eg to eg transition has which symmetry and which state?

A

3A2g

Single find symmetry

55
Q

Eg to t2g transition has which symmetry and which state?

A

3 fold symmetry therefore 3T2g

56
Q

T2g to t2g transition has which symmetry and which state?

A

3 fold degenerate low energy

3T1g

57
Q

When does la Porte selection rule apply?

A

Only to octrahedral

58
Q

What three factors determine the Band widths

A

Vibrations
Spin orbit coupling
Reduced symmetry

59
Q

If vibrations are the same and the two vibrationsl lines run
(A) parallel
(B) antiparralel

What would you expect

A

(A) essentially the same energy and therefore a sharp peak

(B) large energy differences therefore big energy variation and broad peak

60
Q

How does spin orbit coupling affect the broadness of s peak

A

Typically a small effect but causes splitting of T terms

61
Q

How does Jahn Teller affect the peak broadness

A

Removes the degeneracy of the eg level resulting in splitting of the 2Eg term which causes the peak to become more broad and asymmetric

62
Q

On a tanabe sugano diagram, what is the difference between the left and the right of the diagram

A

Left is high spin

Right is low spin

63
Q

What is MLCT favoured by

A

Metals with low oxidation state and ligand with empty low energy orbitals (usually pi*)

64
Q

What is LMCT favoured by

A

Usually m in high oxidation states and ligand with low ionisation energy

Inorganic (1 decks)

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