MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE
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16 Chapter 2<br />
a particular diffraction peak, or many crystallites randomly oriented can be<br />
used to get an orientation independent response.<br />
2.2.2.2.1 Powder X-ray diffraction<br />
In powder x-ray diffraction, the sample consists of many small crystallites<br />
which are assumed to be randomly oriented. This produces rings of diffracted<br />
x-rays, defined by the angle between the incident and diffracted beams, 2θ.<br />
The diffraction condition is determined by the atomic unit cell. In this way,<br />
the structure type of a material and unit cell size can easily be determined. If<br />
more than one type of material is present, the diffraction pattern will be a<br />
superposition of each of the components. If only phase identification is<br />
desired, this often provides enough information. Reference [40] gives a good<br />
example of how this is done.<br />
The intensities of each diffraction line is determined by the atomic<br />
constituents, and their placement in the unit cell. In principle, both of these<br />
can be determined from the intensities. In practice, this is difficult to achieve.<br />
A primary concern is texturing, or preferred orientation of the crystallites. If<br />
the crystallites are plate or needle like, they will tend to lay in the sample<br />
holder in a non-random orientation. This will cause a variation in the<br />
intensities. Also, individual diffraction peaks often overlap one another in a<br />
powder pattern. Since the determination of the structure depends on how<br />
much intensity is associated with each peak, having overlapping peaks<br />
complicates the solution process. It is often easiest to do structural analyses<br />
on single crystal samples.<br />
Powder diffraction sample holders are usually for flat, planar samples. If<br />
the incident angle of the incoming x-rays with respect to the sample plane is<br />
equal that of the diffracted beam being detected, the x-ray beam is partially<br />
focused (Bragg-Botano parafocusing) to give a narrower diffraction peak. For<br />
this reason, the sample plane is usually rotated (by an angle ω = θ) as the
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