X-RAY CRYSTALLOGRAPHY
Definition:
X-ray crystallography (XRC) is the
experimental science determining the atomic and molecular structure of a
crystal, in which the crystalline structure causes a beam of incident X- Rays
to diffract into many specific directions..
PRINCIPLE:
The
underlying principle is that the crystalline atoms cause a beam of X-rays to
diffract into many specific directions. By measuring the angles and intensities
of these diffracted beams, a crystallographer can produce a 3D picture of the
density of electrons within the crystal.
INSTRUMENTATION OF X-RAY CRYSTALLOGRAPHY:
X-ray diffractometers consist of
three basic elements:
Ø X-ray
tube.
Ø A
sample holder.
Ø X-ray
detector.
WORKING:
•
The first step is the most
difficult which is to obtain an adequate crystal of the material under
study.
•
The crystal should be sufficiently
large, pure in composition and regular in structure.
•
The crystal is placed in an
intense (monochromatic) beam of X-ray producing the regular pattern of
reflections.
•
As the crystal is gradually
rotated, previous reflections disappear and new ones appear, the intensity of
every spot is recorded at every orientation of the crystal.
•
These data are now combined
computationally with complementary chemical information to produce and refine a
model of the arrangement of atoms within the crystal.
•
After the pattern is obtained the
data is then processed by a computer and the structure of the atom or molecule
is deduced and visualized.
APPLICATIONS OF X-RAY CRSTALLOGRAPHY:
Ø
New mineral identification,
crystal solution and refinement
Ø
Determination of unit cell,
bond-lengths, bond-angles and site-ordering
Ø
Characterization of cation-anion
coordination
Ø
Variations in crystal lattice with
chemistry
Ø
With specialized chambers,
structures of high pressure and/or temperature phases can be determined
ADVANTAGES
OF XRAY CRYSTALLOGRAPHY:
Ø
X-Ray crystallography provides a two-dimensional
view that gives an indication of the three-dimensional structure of a material
Ø
Relatively cheap and simple
Ø
Useful for large structures: Not limited by size
or atomic weight.
Ø
Can yield high atomic resolution.
DISADVANTAGES
OF XRAY CRYSTALLOGRAPHY:
Ø
The sample must be crystallizable
Ø
The types of sample that can be analyzed are
limited. In particular, membrane proteins and large molecules are difficult to
crystallize, due to their large molecular weight and relatively poor solubility
Ø
An organized single crystal must be obtained to
produce the desired diffraction
Ø
Non-dynamic method due to preparation of samples
and crystallization. Only a static three-dimensional analysis is produced.
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