Modeling and Measurements of the XRD Patterns of Extended Solids under High-Pressure

Evolutionary structural search simulations were conducted to identify stable P-N and H-N network-type structures. Density functional theory-based calculations of relevant high-pressure systems formed the basis for our approach. High-density covalently bonded structures were created in silico using variable and fixed concentration methods. Rank sorting of the most promising (viable) systems is initially based on thermodynamic stability assessments and direct comparison with our measured experimental X-ray diffraction (XRD) patterns. The high pressure stability of predicted systems was estimated from convex-hull plots. XRD line profiles were calculated using a virtual diffraction algorithm that computes kinematic diffraction intensities in three-dimensional reciprocal space. A dimensional reduction from 3-D to 1-D was made to yield two-theta line profiles. Direct computation of structure factors enables implementation of distinct atomic scattering factors for each atomic constituent, thus allowing (with confidence) calculation of diffraction patterns for multicomponent systems. Computed XRD patterns were constricted (optimized) using experimental results and commensurately calculated, pressure and structure dependent enthalpy values.