Diffraction Methods Flashcards
Diffraction
Photons interacting with electrons
Mass Spectrometry
Measures the mass-to-charge ratio of ions, reveals how atoms are linked or arranged
Spectroscopy
Used to investigate bonding and electronic transitions to characterise reactions
Lattice
An infinite array of points in an identical environment. Involves translational symmetry
Lattice Points
The identical points in a lattice
Motif
The element of a lattice that is associated with a lattice point, i.e. the part that repeats
Symmetry
An n-fold rotation axis of symmetry is aline of rotation about which 2π/n produces the identical position.
e.g. n=2, 2π/2=360°/2=180°
Symmetry (e.g. 2-fold, 3-fold, 6-fold)
An n-fold rotation axis of symmetry is a line of rotation about which 2π/n produces the identical position.
e.g. n=2, 2π/2=360°/2=180°
Note: No 5-fold symmetry possible
Centre of Symmetry
A point through which an inversion occurs
Mirror Plane
A plane such that positions on one side of the plane are mirror images of the other side
The Unit Cell
Smallest repeating unit, each corner contains 1/4 lattice point, 4 corners, (1/4*4)=1 lattice point
3 axial lengths: a,b,c
3 interaxial lengths,α,β,γ
The Primitive Unit Cell
A single lattice point unit cell, has coordinates (0,0,0)
P? I? F?
P=Primative (consists of one lattice point on each corner of the cube. Each atom at a lattice point is then shared equally between eight adjacent cubes, and the unit cell therefore contains in total one atom (1/8 × 8)).
I=Innenzentre/Body Centered (has one lattice point in the center of the unit cell in addition to the eight corner points. It has a net total of 2 lattice points per unit cell (1/8 × 8 + 1)).
F=face centered (has lattice points on the faces of the cube, that each gives exactly one half contribution, in addition to the corner lattice points, giving a total of 4 lattice points per unit cell (1/8 × 8 from the corners plus 1⁄2 × 6 from the faces)).
7 Crystal Systems
Triclinic: (a≠b≠c; α≠β≠γ≠90°) Monoclinic: a≠b≠c; α=γ=90° β≠90° Cubic: (a=b=c ; α=β=γ=90°) Orthorhombic: (a≠b≠c; α=β=γ=90°) Hexagonal: (a=b≠c; α=β=90° γ=120°) Trigonal ( a=b=c; α=β=γ≠90°) Tetragonal: (a=b≠c; α=β=γ=90°)
32 Point Groups
32 different ways in which rotation axes , mirror planes, centres of symmetry, and rotary inversion axes can be used to describe the 7 crystal systems
e.g. Point group: 4mm
Space Groups
230 Unique 3D structures (32 point groups + 14 Bravais lattices).
Designated by capital letter to identify lattice type: (Primative (P), Base (C), Body (I), Face (F))
Screw Axis
A line that is simultaneously the axis of rotation and the line along which translation of a body occurs
Close packed vs non-close packed contact
Close packed max contact = 6
Non-close packed max contact = 4
Hexagonal Close Packed (HCP)
Primitive
Cubic Close Packed (CCP)
Non-Primitive
5 items to describe metal structures
Crystal system, Lattice Type, Unit cell dimensions and angles, the atomic contents of the units cell, coordinates of atoms that comprise a lattice motif
Interstices
Gaps present in between even close packed molecules. Other molecules fill these gaps.
Can have tetrahedral coordination or octahedral coordination.
Tetrahedral = 1 pyramid, so 4
Octahedral = 2 pyramids attached by the base, so 8
Miller Indices (hkl)
A set of parallel, equally spaced lattice planes are defined by Miller Indices (hkl) with
reference to a particular unit cell. hkl are integers that make intercepts of length a/h,
b/k, c/l on the x, y, and z axes respectively.
d(khl)
The distance separating the planes (Interplanar spacing).
Determined by the dimensions of the unit cell and the associated Miller Indices
