solid state chemistry Flashcards
(18 cards)
What is a solid?
A solid is a state of matter, in which the constituent particles (atoms/ions/molecules) are arranged so that their shape and volume are relatively stable.
What is a molecular solid?
Solid structures made up of molecules. The bonds within the molecules are covalent = strong. The intermolecular bonds are much weaker. This results in relatively soft materials that display low melting points, as only the weak intermolecular interactions need to be broken to change state.
Examples of molecular solids include; iodine and sulfur.
What is a covalent network solid?
Covalent network solids are formed of atoms that are all bonded by covalent bonds to form a continuous, infinite network. They can be formed by both elements (e.g. carbon, C, with all allotropes; graphene, graphite, fullerene, diamond) and compounds (e.g. silicon dioxide, SiO2, with allotropes like quartz). * These solids show high melting points and boiling points, and are hard and brittle, as strong covalent bonds must be broken to change state.
What is a ionic solid?
They formed of cations and anions. They are held together by strong electrostatic interactions between the opposingly charged ions. Elements that are more readily ionised (e.g. have lower ionisation energies) are more likely to form ionic compounds. Ionic solids have high melting and boiling points and are often hard and brittle, due to the strong
ionic bonds holding them together. They are also conductive when molten or dissolved in aqueous solution.
What is a unit cell and how by what parameters can they be described?
The smallest repeating unit of the crystal is called the unit cell. The unit cell is described by 6 parameters: it’s 3 vector lengths: a, b and c and, the 3 angles between them: α, β, and γ, and all equal 90.
What are the most common close packing arrangements?
and what are their repeat layer stacking arrangement
Ccp and hcp.Cubic close packing (ccp) has the repeat layer stacking arrangement ABCABC…(rock salt) Hexagonal close packing (hcp) has the repeat layer stacking arrangement ABABAB…
Describe fcc, bcc and primitive.
Face-centred cubic (fcc)
- Atoms on all vertices
- plus atoms in centre of
all faces
Body-centred cubic (bcc)
- Atoms on all vertices
- plus one atom in centre
of cell
- CN = 8 (cubic geometry
Primitive cubic
- Atoms on all vertices only
- CN = 6 (octahedral geometry)
Draw a hexagonal unit cell.
look it up.
Describe octahedral (intestitial) sites.
Lie between a triangle of atoms in the row above, and a triangle of atoms in the row below. Number of octahedral sites in a close packed structure = N (N = number of atoms in host).
Describe tetrahedral (intestitial) sites.
Lie between a triangle of atoms in one row (e.g. above), and a single atom in another (e.g. below). Number of tetrahedral sites in a close packed structure = 2N.
What is packing efficiency and its equation and steps for the calculation?
The packing efficiency is the percentage of the structure that is filled by the atoms/ions.
packing efficiency = (volume of atoms in unit cell)/(total volume of unit cell) × 100%
steps
1. Calculate the total volume of the atoms in the unit cell.
a) Determine how many atoms there are in the unit cell and how much each one contributes.
b) Find the total number of atoms in one unit cell.
c) Find the volume of one atom and therefore the total volume of all atoms in the unit cell.
2. Calculate the total volume of the unit cell.
a) The cubic unit cell has all lengths equal: 𝑎 = 𝑏 = 𝑐. So the volume of the cell is: 𝑉 = 𝑎3. use pythagoras the cell length 𝑎?
b) Find the total volume of the unit cell.
3. Calculating the packing efficiency of the unit cell.
How do you find the density of crystals?
3 steps
density = mass/volume
1. Calculating the volume of the unit cell.
a) 𝑎^2 + 𝑎^2 = (4𝑟)^2
2𝑎^2 = 16𝑟^2
𝒂 = 𝟖^1/2 𝒓
b) calculate the lattice parameter, 𝑎.
c) calculate the volume of the cell in the unit cell v = a3
2. Find the total mass of the unit cell by multiplying number of atoms by amu.
3. calculate density.
What is the radius ratio?
What is the limiting radius ratio?
What is the radius ratio rule?
radius ratio = 𝑟+/𝑟−
The limiting radius ratio is the smallest possible value of the radius ratio that still allows the cations and anions to be in contact.
The limiting radius ratio is defined for a particular geometry around the cation. Using these ranges, it is possible to predict a compound’s structure from the radius ratio of its ions: this is the radius ratio rule.
Why does the radius ratio rule not always work?
It relies on the ionic model, which is based on some over-simplifications:
Ions in crystals are not really hard spheres.
We do not know the exact radii of individual ions: values taken from experiment are determined in cation-anion pairs and are dependent on the compound studied.
Deviations from the ionic model: we know that not all compounds are not 100 % ionic. There will be a varying degree of covalency depending on the electronegativity difference between the two atom types and this will cause
a discrepancy between the real and predicted structure types.
What is a point defect and an extended defect?
A point defect is a defect that occurs at a single site in the crystal structure.
An extended defect is a defect that occurs in 1D, 2D or 3D through the crystal (i.e. along a line, plane or throughout the crystal bulk).
What are intrinsic and extrinsic defect?
An intrinsic defect is a defect that occurs in the pure substance.
An extrinsic defect is a defect that arises due to the presence of impurities.
What is a Schottky defect?
A Schottky defect is point defect where atoms / ions are missing (vacant) from their expected position in an otherwise ideal structure. The overall charge balance of the structure is not affected, because Schottky defects occur stoichiometrically.
What is a Frenkel defect?
A Frenkel defect is a point defect where an atom / ion is displaced into an interstitial site, leaving a vacancy at its original position. The charge balance and stoichiometry of the solid remains unchanged. A Frenkel defect can involve the displacement of either the cation, the anion, or even both. Frenkel defects more commonly occur in structures with lower coordination numbers, so that atoms in the interstitial sites are more easily accommodated.
