Structural Aspects from Diffraction Measurements and Computer Simulations

2 Structural Aspects from Diffraction Measurements and Computer Simulations 

Consider an ion L placed at the origin of coordinates with other ions of either sign surrounding it. The number of particles of these species (subscript j) in a spherical shell of thickness dr at a distance r from the center of the ion at die origin is  

The choice of the upper limit of the integration, /, is somewhat arbitrary commonly it represents the distance at which 4 rr [g0(r)-l]/ bu]k reaches its minimum after the first peak. An alternative is to assume syimnetty of the peak, to use the peak distance to represent and to take the coordinatiOTi number as twice the integral (3.2) up to this point. The second peak in the 4 rr [gu(r)-l]/7buik curve represents the distance of the ion firom its next-nearest neighbors, and a third peak may also be observed, representing more distant correlations. 

The primary information of the diffraction measurements is the intensity of the scattered beam I q) as a function of the angle p to which the radiation is scattered, where q = (4jtM)sin( /2) and X is the wavelength of radiation employed, whether x-rays or neutrons. The space variable q has the dimension of a reciprocal length, its units usually being The structure factor S q) is obtained from the intensities of 

A model of the molten salt obtained by computer simulations is nowadays generally used to refine the structiwe, avoiding errors arising from incomplete knowledge of S(q) at very small and very large angles, q(r — 0) and q r oo) required for the integration (3.3). 

---