Ionic Interaction Models for MX2 Glass-Forming Materials

In a neutron diffraction experiment on a liquid or glassy MX2 system, the coherent scattered intensity measured with respect to the magnitude of the scattering vector k can be represented by the total structure factor [44] 

The full set of Sap (k) functions for an MX2 system can be extracted from the measured diffraction patterns by applying the NDIS method, provided that isotopes are available with a sufficiently large neutron scattering length contrast [44, 46, 47], The total structure factor can also be expressed in terms of the Bhatia-Thornton [48] number-number, concentration-concentration and number-concentration partial structure factors denoted by Sccik) and nc) ), respectively. These partial 

If bu = bx the incident neutrons in a diffraction experiment cannot distinguish between the different scattering nuclei and the measured total structure factor gives Sxxik) directly (see (1.4)). The corresponding Fourier transform nn(c) therefore 

In practice, a diffractometer can only access a finite k-space range with a maximum cutoff value kmax- Provided that sufficiently small k-values can be accessed, a reciprocal-space function such as F(k) will therefore be truncated by a modification function given by M(k) = 1 for k kmax and M(k) = 0 for k kmax- lu consequence, the real-space information corresponding to F k) is obtained by the Fourier transform relation 

The desired r-space information is therefore convoluted with the Fourier fiansform of M(k), the effect of which becomes negligible if fcmax is sufficiently large that F(k) is featureless at higher k-values. To give smoother r-space functions, other expressions for M(k) are used such as the Lorch [49] modification function where 

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Ionic Interaction Models for MX2 Glass-Forming Materials... 

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