Structures of Ionic solids Flashcards
1
Q
What are ionic solids
A
- Class of solid that can be considered as an assembly of oppositely-charged ions
- Many are based on ccp or hcp arrays of ions
- With the other ion occupying the octahedral or tetrahedral holes
2
Q
What are the different ionic structures
A
- Rock-salt structure (NaCl)
- Caesium chloride (CsCl)
- Fluorite structure (CaF2)
- Nickel arsenide structure (NiAs)
- Wurtzite structure (ZnS)
- Rutile (TiO2)
- Perovskite structure (CaTiO3)
- Zinc Blende/ sphalerite (ZnS)
3
Q
Describe the Rock-salt structure
A
- Based on ccp of large Cl- ions
- All octahedral holes occupied by Na+ ions
- Coordination number Na+=Cl-=6
- Octahedral coordination- geometries
- e.g.
- Alkali halides (NaBr, KCl not CsX),
- AgCl, AgBr (photographic film)
- MgO, NiO (catalyst)
4
Q
Describe the Caesium chloride structure
A
- Starts from primitive cubic packing not close packing
- Does not have octahedral or tetrahedral sites
- All interstitial sites are equivalent and lie at the centre of the cube so have cubic geometry
- Primitive cubic lattice of Cl- ions with Cs+ ions in the cubic interstitial sites
- Coordination number = 8
- CsBr and TiCl
5
Q
Describe the fluorite structure CaF2`
A
- Based on ccp of Ca2+
- F- in tetrahedral holes
- Coordination Number Ca2+ = 8
- Coordination Number F- = 4
- E.g.
- CeO2 (catalytic converter)
- UO2 (Nuclear fuel)
- ZrO2 (Fuel cells)
6
Q
Describe the Nickel Arsenide (NiAs)
A
- Based on hcp arrangement of arsenic atoms
- Nickel atoms in octahedral holes
- Coordination Number Ni=As=6
- Geometry Ni= octahedral
- Geometry As= trigonal prismatic
- hcp analogue of ccp-based NaCl structure with metal atoms occupying all of the octahedral sites
- E.g. CoSe and NiTe
7
Q
Describe Wurtzite (ZnS)
A
- Polymorph of ZnS
- Based on hcp of S2- ions
- 1/2 tetrahedral holes occupied by Zn2+ ions
- Coordination number= Zn = S = 4
5 Tetrahedral geometries - E.g. ZnO, Bn
8
Q
Describe Zinc Blende/ sphalerite (ZnS)
A
- Polymorph of ZnS
- Based on ccp of S2- ions
- With Zn2+ occupying 1/2 of tetrahedral holes
- Coordination number= Zn= S=4
- Tetrahedral geometries
- E.g. Si, GaAs, Diamond
9
Q
Describe rutile (TiO2)
A
- Most common polymorph of TiO2- Pigment in white paint + suncream
- Not cubic- one cell lengths longer than the other
- Ti 4+ = 6 coordinate- distorted octahedral geometry as two TiO longer than other 4
- O2 2- = 3 coordinate- trigonal planar geometry
10
Q
Describe perovskite (CaTiO3)
A
- Common structure and mostly oxides (some halides) with general formula AMO3
- A= 2+ , M=4+ or A=3+ ,M=3+
- Based n ccp of Ca2+ and O2- ions with Ti4+ in octahedral holes
- Unit cell= Ca2+ at centre, Ti at 8 corners, O2- at 12 edge sites
- Coordination number for Ca2+ = 12
- Coordination number for Ti4+ = O2-= 6
- E.g. LaMnO3 (magnetic), LaCoO3 (catalyst), MgSiO3 (common mineral), (CH3NH3)PbI3 (Solar cells)=MAPI
11
Q
How can the distance between the centres of two ions in an ionic solid be measured accurately
A
- X-ray crystallography
2. Sum of anionic + cationic radii = r+ + r-
12
Q
What are the general trends in ionic radii
A
- Increase down a group with the increase in principle quantum number
- For ions of the same electronic configuration (isoelectronic ions), the greater the nuclear charge, the smaller the ion
13
Q
What makes a structure more stable
A
- In an ionic solid, the ions are assumed to be in contact with their nearest neighbours that have the opposite charge
- Most stable structures maximises contact between cations and anions
- Structure adopted has the maximum coordination number for the specific ions
14
Q
What is the radius ratio
A
- Radius ratio= r+/r-
- Gives an indication of the likely coordination number
- The higher the ratio, the greater the coordination number
15
Q
What geometry do you get with coordination number 8 and 12
A
- 8- cubic
2. 12- cuboctahedral
