XRD values Flashcards
Formula
Simple - the molecular formula of the unit cell
Dcalc/ g cm-3
calculated density of unit cell
mass/v
Mu/mm-1
The absorption coefficient
When X-rays with intensity I0 penetrate a uniform substance,
the intensity I after transmission through distance x is given by.
I = IO e^-xmu
mu = linear absorbtion coefficient
-increases with more atomic number, increases with wavelength
-dependant on wavelength and density of substance
Formula Weight
combined molecular weight of everything in unit cell
Size/mm3
size of crystal - 0.1-0.5 mm
T/K
temperature of crystal - acheived by liquid N2
Crystal System
“sest of point groups”
crystal system of the unit cell
there are 7
eg triclininc, monoclinic etc)
Space group
-A space group is the symmetry group of the unit cell.
=230 of them
-combination of the different symmetry operations results in a total of 230 different space groups describing all possible crystal symmetries.
a/b/c
size and shape of unit cell
length
alpha/beta/gamma
size and shape of unit cell
angles
V/ A3
volume of the unit cell
Z’
the number of molecules in the asymmetric unit
Wavelength/ A
Wavelength of X-ray radiation used given.
The wavelength refers to the radiation used
to measure intensities (i.e. mean K rather t
Radiation type
For Copper the characteristic
wavelengths (λ) are:
* Cu Kα1 = 1.540Å
* Cu Kα2 = 1.544Å
* Cu Kb = 1.392Å
- For Molybdenum they are:
- Mo Kα1 = 0.70932Å
- Mo Kα2 = 0.71354Å
- Mo Kb = 0.63225Å
- We use MoKα (avg.) radiation
- (λ) = 0.71073Å
- Or CuKα (avg.)
- (λ) = 1.54178Å
theta min
lowest angle reflections - high intensity
theta max
highest angle - reflections - low intensity copper source, better
Measured reflections
Total number of reflections measured
Indepentant reflections
Number of independent reflections measured (not repeated, not
related by symmetry)
Refl’s I≥2 sigma(I)
Intensity/error - signal to noise ration - big is better more signal to noise
Rint
Parameters
parameters (coordinates, displacement parameters, scale factor) in the model.
Restraints
constraints - softer
restraints - stricture
RIGU
SAME
DELU
ISOR
even HFIX is a restrainy
Largest Peak
feature of the difference map, and are useful for deciding how to further improve the model
Deepest Hole
feature of the difference map, and are useful for deciding how to further improve the model
GooF
GooF is another value which describes the
quality of our model
In contrast to the R-factor, which also depends on the signal-tonoise ratio, S is relatively independent from the noise.
S should be around 1.
S > 1: bad model or bad data/parameter ratio
S < 1: model is better than the data: problems with the absorption
correction, space group problems
formula number of independant reflec and parameters
wR2 (all data)
The wR2 and R1 numbers are known as R-values, and measure how well the model agrees with the experimental data. As the model improves, these numbers decrease.
wR2
R1 (all data)
The wR2 and R1 numbers are known as R-values, and measure how well the model agrees with the experimental data. As the model improves, these numbers decrease.
R1
HOw many unique space groups are there?
230
they are frequently specified by means of space group symbols that are not unique to a particular space group
How many crystal systems are there and name them
7 crystal systemes
triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic
Define centrosymmetric
In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements.
Define enantiomorphic point groups
The point groups that possess no improper rotations are called enantiomorphic. Enantiomorphic molecules have right-handed and left-handed forms,
The 7 crystal systems consist of 32 crystal classes (corresponding to the 32 crystallographic point groups)
What is a Laue Class
The Laue classes are eleven geometric crystal classes containing centrosymmetric crystallographic types of point groups and their subgroups. When absorption is negligible and Friedel’s law applies, it is impossible to distinguish by diffraction between a centrosymmetric point group and one of its non-centrosymmetric subgroups; only point groups belonging to different Laue classes can then be distinguished.
Define triclinic
a ≠ b ≠ c; α ≠ β ≠ γ
no symmetry other than inversion or identity
C1 or Ci
Define monoclinic
a ≠ b ≠ c; α = γ = 90°; β ≠ 90°
one 2-fold axis of rotation of 1 mirror plane
Define Orthorhombic
a ≠ b ≠ c; α = β = γ = 90°
Tetragonal
a = b ≠ c; α = β = γ = 90°
requires 1 four-fold axis of rotation
Trigonal
a = b = c; α = β = γ ≠ 90°
Hexagonal
a = b ≠ c; α = β = 90°; γ = 120°
requires 1 six-fold axis of rotation (60°).
Cubic
a = b = c; α = β = γ = 90°
4 three-fold axes of rotation (120°) along all 4 body diagonals.
where does alpha lie in respect to a
orthogonal at the bottom
The two Triclinic space groups are ?
P-1 enantiomorphic
P1 centrosymmetric
Monoclinic examples
P2 P21
C2
P, PC
P21M P21x
anything with a 2/
Where do the 230 space groups arise from
there are 7 space groups
14 bravais lattices
32 crystal classes
Why do all space groups start with a letter
Space group symbols all begin with the lattice type as the first character of the symbol
What is a lattice type and what are the 4 examples
Types of Lattices
Lattices are either:
1. Primitive (or Simple): one lattice point
per unit cell.
2. Non-primitive, (or Multiple) e.g. double,
triple, etc.: more than one lattice point
per unit cell
Simple (P)
Body-Centred (I)
Face-Centred(F)
Base-centred(C)
Rhombohedral (R)
http://www.chm.bris.ac.uk/webprojects2003/cook/latticetypes.htm
wtf is HKL
braggs law
Asymmetric unit
An asymmetric unit in unit cells is the smallest portion of a crystal structure, which can be duplicated and moved by crystallographic symmetry operations so that the entire unit cell may be created in order to produce the entire crystal
Z
value in crystallography can be referred to as the number of molecules (or formula units) present in the unit cell.
how xrd actually works
