Structures of solids Flashcards
What happens to al inorganic substances if cooled sufficiently
- Form a solid phase
- The majority are crystalline solids
- The atoms are molecules pack together in regular repeating units
What are the four main solid types
- Ionic solid e.g. NaCl
- Metal e.g. Fe
- Covalent network e.g. Si
- Molecular solid e.g. H2O
Briefly describe an ionic solid
- Strong ionic (electrostatic interactions) between oppositely charged ions
- High melting point
- Conduct electricity in solution and the liquid phase but not normally as a solid
Briefly describe a metal
- Bonding via delocalised electrons
- Wide range of melting points
- Conduct electricity when a potential difference is applied
Briefly describe a covalent network
- Strong covalent bonds
2. Very high melting points
Briefly describe a molecular solid
- Strong intramolecular forces
- Weak intermolecular interactions
- Low melting point
Give four examples of where solid materials are important for research and technology
- Heterogeneous catalysts- e.g Fe, Pt, CeO2 (>80% Industrial chemicals)
- Semiconductors- e.g. Si, GaAs
- Ionic conductors in batteries e.g. LiCoO2
- Solar cells e.g Si, CdTe, perovskite, CH3NH3PbI3
What are the two ways of close packing
- Cubic close packing (ccp) or (fcc)- face centred cubic
2. Hexagonal close packing (hcp)
What is close packing
- The most efficient way of packing atoms so the empty space is minimised
Describe hexagonal close packing
- First start with a single layer of atoms
- Second layer placed above spaces in the bottom layer
- Third layer directly above first layer
- ABABAB
What is the coordination number for hexagonal close packing and give 4 examples
- 12
2. Mg, Zn, Ti, Co
Describe cubic close packing/ face centred cubic
- Same as hcp but third layer is not directly above previous layer
- Layer sequence= ABCABC
What is the coordination number for cubic close packing and give 4 examples
- 12
2. Cu, Ag, Al, Ni
What is the coordination number in a solid state structure
- Number of nearest neighbours an atom has
Why do hcp and ccp/fcc have a coordination number of 12
- There are 6 nearest neighbours in the same row
2. And 3 in row above and below
Why is ccp also known as fcc
- There are atoms on each face of the cube
2. As well as the 8 vertices
Describe the body centred cubic structure bcc
- Not as common as hcp or ccp
- Has lattice site at centre of a cube
- Other lattice sites at the corner
What is the coordination number for body centred cubic and give 3 examples
- 8
2. Fe, Na, K
What are the gaps between atoms called
Interstitial sites
What are the two types of interstitial site
- Octahedral sites
2. Tetrahedral sites
What is a tetrahedral hole
- Formed by a planar triangle of atoms capped by a single atom
- So it is surrounded by 4 atoms
What is an octahedral hole
- Lies between two oppositely directed planar triangles of atoms
- Surrounded by 6 atoms
How many octahedral and tetrahedral sites do you get per atom of a close-packed structure
- One octahedral site
2. Two tetrahedral sites
What is a unit cell
- Smallest repeating units that shows the fully symmetry of the solid structure
- Repeated in 3D gives infinite solid lattice
How many atoms are in one unit cell of the ccp
- Looks like 14- one on each of 8 vertices and one on each of the 6 faces
- But all of the atoms are shared with other unit cells, so don’t contribute fully to each cell
- Total number is 4
What is the total number of atoms contributing to the unit cell of a vertex atom
- 8 atoms in the vertices
- Each atom contributes 1/8 to each unit cell
- Total number of atoms contributing to the unit cell= 8*1/8= 1
What is the total number of atoms contributing to the unit cell of a face atom
- 6 atoms at the faces
- Each atoms contributes 1/2 to each unit cell
- 6*1/2 =3
How can you find the volume of atoms in a unit cell
- Multiply the number of atoms e.g. 4 by 4/3pir^3
What is a cell projection diagram
- 2D representation of the unit cell- view from above
- The x- and y- coordinates are shown on the projection as normal
- But the z-coordinate is given as a number between 0-1 for each atom
What do the z-coordinates represent
- 0 is the bottom of the unit cell
2. 1 is the top of the unit cell
How many different crystal systems are there
- There are 7 different geometric shapes of unit cells so 7 different crystal systems
What are the 7 different crystal systems
- Cubic
- Tetragonal
- Orthorhombic
- Monoclinic
- Triclinic
- Hexagonal
- Trigonal/ rhombohedral
What are the parameters that define a unit cell
- Cell lengths (a,b,c)
2. Cell angles ( alpha, beta, gamma)
What are the 4 lattice sites and where are the lattice points for each
- P= primitive= lattice points on cell corners only
- I= body-centered = lattice points on cell corners + centre of cell
- F= face-centered= lattice points on cell corners + centre of each face
- C= Base-centred= lattice points on cell corners + each face of one pair of opposite faces
