The x-ray and neutron diffraction pattern of crystalline materials is usually dominated by the Bragg scattering contribution. It provides information about the content of the average crystalline unitcell. Moreover, frequently a less intense diffuse scattering intensity is also present. Analyzing it, differences from the average structure, e.g. short range order correlation between atoms, might be revealed. These two kinds of scattering types form the total scattering powder diffraction pattern.

Recently, modeling of the total scattering powder diffraction pattern of crystalline materials has become popular (see for instance [Keen 2015]). Several methods exist for this computation. One of them is the Reverse Monte Carlo (RMC [McGreevy 1988]) method, which does it on a diffraction data-driven way. During its application, the configuration containing atomic coordinates is varied by changing the positions of atoms in order that the total diffraction pattern calculated from configuration is gradually approaching the measured diffraction pattern. At the end of this procedure an atomic configuration is obtained that is consistent with the measured data. Then, investigations of correlations and short-range order can be performed using the atomic coordinates of the configuration(s). (In the form of popular science in Hungarian for the liquid phase see [Temleitner 2016]).

Among studying the structural disorder on liquids and amorphous materials, our group studies it on crystalline materials, using the method presented above (see, e.g. CBr₄ [Temleitner 2010]). The MSc student would join in the efforts of this last subject by studying the short range order on orthorhombic, monoclinic and plastic crystalline phases of hexachloroethane (C₂Cl₆) and hexagonal phase of phosphorous triiodide (PI₃). He/she would perform investigation based on the above presented method and molecular dynamics simulations, which is followed by the study of orientation correlations comparing them on the different crystalline and liquid phases. During the implementation it will be possible to do wider impulse transfer range neutron diffraction measurements at neutron centres. Major result(s) are expected to be published in international research journal(s).

References:

[Keen 2015] Keen, D. A., Goodwin, A. L.: "The crystallography of correlated disorder", Nature 521(2015), 303. DOI: 10.1038/nature14453.
[McGreevy 1988] McGreevy, R. L., Pusztai, L.: "Reverse Monte Carlo simulation: A new technique for the determination of disordered structures", Molecular Simulation 1(1988), 359, DOI: 10.1080/08927028808080958.
[Temleitner 2010] Temleitner, L., Pusztai, L., "Local order and orientational correlations in liquid and crystalline phases of carbon tetrabromide from neutron powder diffraction measurements", Physical Review B 81(2010) 134101. DOI: 10.1103/PhysRevB.81.134101.
[Temleitner 2016] Temleitner, L., Pusztai, L., Pothoczki, S.: "Tetraéderes molekuláris folyadékok. Egyszerű anyagok, fejfájást okozó problémák", Természet Világa 147(2016, augusztus) 346. http://epa.oszk.hu/02900/02926/00044/pdf/EPA02926_termeszet_vilaga_2016_08_346-349.pdf