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Viva > XRD values > Flashcards

XRD values Flashcards

1
Q

Formula

A

Simple - the molecular formula of the unit cell

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2
Q

Dcalc/ g cm-3

A

calculated density of unit cell
mass/v

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3
Q

Mu/mm-1

A

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

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4
Q

Formula Weight

A

combined molecular weight of everything in unit cell

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5
Q

Size/mm3

A

size of crystal - 0.1-0.5 mm

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6
Q

T/K

A

temperature of crystal - acheived by liquid N2

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7
Q

Crystal System

A

“sest of point groups”
crystal system of the unit cell
there are 7

eg triclininc, monoclinic etc)

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8
Q

Space group

A

-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.

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9
Q

a/b/c

A

size and shape of unit cell
length

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10
Q

alpha/beta/gamma

A

size and shape of unit cell
angles

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11
Q

V/ A3

A

volume of the unit cell

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12
Q

Z’

A

the number of molecules in the asymmetric unit

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13
Q

Wavelength/ A

A

Wavelength of X-ray radiation used given.
The wavelength refers to the radiation used
to measure intensities (i.e. mean K rather t

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14
Q

Radiation type

A

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
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15
Q

theta min

A

lowest angle reflections - high intensity

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16
Q

theta max

A

highest angle - reflections - low intensity copper source, better

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17
Q

Measured reflections

A

Total number of reflections measured

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18
Q

Indepentant reflections

A

Number of independent reflections measured (not repeated, not
related by symmetry)

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19
Q

Refl’s I≥2 sigma(I)

A

Intensity/error - signal to noise ration - big is better more signal to noise

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20
Q

Rint

A
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21
Q

Parameters

A

parameters (coordinates, displacement parameters, scale factor) in the model.

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22
Q

Restraints

A

constraints - softer
restraints - stricture
RIGU
SAME
DELU
ISOR
even HFIX is a restrainy

23
Q

Largest Peak

A

feature of the difference map, and are useful for deciding how to further improve the model

24
Q

Deepest Hole

A

feature of the difference map, and are useful for deciding how to further improve the model

25
Q

GooF

A

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

26
Q

wR2 (all data)

A

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.

27
Q

wR2

A
28
Q

R1 (all data)

A

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.

29
Q

R1

A
30
Q

HOw many unique space groups are there?

A

230
they are frequently specified by means of space group symbols that are not unique to a particular space group

31
Q

How many crystal systems are there and name them

A

7 crystal systemes
triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic

32
Q

Define centrosymmetric

A

In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements.

33
Q

Define enantiomorphic point groups

A

The point groups that possess no improper rotations are called enantiomorphic. Enantiomorphic molecules have right-handed and left-handed forms,

34
Q
A

The 7 crystal systems consist of 32 crystal classes (corresponding to the 32 crystallographic point groups)

35
Q

What is a Laue Class

A

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.

36
Q

Define triclinic

A

a ≠ b ≠ c; α ≠ β ≠ γ
no symmetry other than inversion or identity
C1 or Ci

37
Q

Define monoclinic

A

a ≠ b ≠ c; α = γ = 90°; β ≠ 90°
one 2-fold axis of rotation of 1 mirror plane

38
Q

Define Orthorhombic

A

a ≠ b ≠ c; α = β = γ = 90°

39
Q

Tetragonal

A

a = b ≠ c; α = β = γ = 90°
requires 1 four-fold axis of rotation

40
Q

Trigonal

A

a = b = c; α = β = γ ≠ 90°

41
Q

Hexagonal

A

a = b ≠ c; α = β = 90°; γ = 120°
requires 1 six-fold axis of rotation (60°).

42
Q

Cubic

A

a = b = c; α = β = γ = 90°
4 three-fold axes of rotation (120°) along all 4 body diagonals.

43
Q

where does alpha lie in respect to a

A

orthogonal at the bottom

44
Q

The two Triclinic space groups are ?

A

P-1 enantiomorphic
P1 centrosymmetric

45
Q

Monoclinic examples

A

P2 P21
C2
P, PC
P21M P21x
anything with a 2/

46
Q

Where do the 230 space groups arise from

A

there are 7 space groups
14 bravais lattices
32 crystal classes

47
Q

Why do all space groups start with a letter

A

Space group symbols all begin with the lattice type as the first character of the symbol

48
Q

What is a lattice type and what are the 4 examples

A

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

49
Q

wtf is HKL

A
50
Q

braggs law

A
51
Q

Asymmetric unit

A

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

52
Q

Z

A

value in crystallography can be referred to as the number of molecules (or formula units) present in the unit cell.

53
Q

how xrd actually works

A

Viva (5 decks)

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