Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Michael Hu Applicant's institution: ANL Applicant's division: XSD Project Name: SnI4 Project title: Crystalline and amorphous structures of SnI4 Associated funding: DOE and NSERC (Canada) Other Systems: Science: Molecular solid SnI4, when put under pressure, demonstrates rich structure transformations, and complex processes of polymerization, metallization, and amorphization. Adding to Mossbauer spectroscopic and x-ray diffraction studies, we recently conducted phonon spectrum measurements of SnI4 under pressure. In this proposal, we outline our research forward with material simulation using modern algorithms and supercomputers, which we believe is critical to interpret our experimental results and gain insights to material properties under extreme conditions. Project description: Tin iodide SnI4 is a van der Waals-bonded molecular solid with a face-center cubic structure (fcc) under ambient conditions. It is well established that under compression at room temperature to 15 GPa a reversible solid-state amorphization was found (S. Sugai, J. Phys. C 18, 799 (1985). using x-ray diffraction and Raman scattering. Later it was found that the amorphization was not originated directly from the cubic phase (phase-I) but through an intermediate crystalline metallic phase II stable above 7.2 GPa (N. Hamaya, K. Sato, K. Usui-Watanabe, K. Fuchizaki, Y. Fujii, and Y. Ohishi, Phys. Rev. Lett. 79, 4597 (1997). The precise structure of phase-II has not yet been clarified. The amorphous structure was found to crystallize to a fcc-like lattice at 61 GPa. Several theories have been proposed to account for the loss crystallinity in the diffraction pattern and, perhaps, the idea of pseudomelting (S. Sharma and S. Sikka, Prog. Mater. Sci. 40, (1996) is the most interesting. Recently, density of the vibrational states as a function of pressure were measured using nuclear inelastic x-ray scattering experiments to explore the mechanism of the amorphization. The theoretical study proposed below is aimed to elucidate all the experimental results obtained so as to uncover the mechanism for the amorphization process. We believe is critical to interpret our experimental results and gain insights to material properties under extreme conditions. The outline of the research is as follow. The stability of the lattice in the ambient structure will be examined using finite temperature self-consistent lattice dynamics and molecular dynamics calculations. The purpose is to investigate the role of (soft) phonons in the crystal -> crystal transition at ca. 7.2 GPa. Using evolution method, possible candidate structures will be predicted and their properties (i.e. structural and electrical) will be compared with experiment. Constant molecular dynamics calculations will then be performed on prospective phase II structures to examine the mechanism of amorphization. The final goal is to characterize the crystalline structure observed experimentally at 61 GPa. First-principles methods static and dynamic calculations based on density functional approximation will be performed. Specially, structural prediction using evolution algorithms (C. Glass and A. Oganov, J. Chem. Phys. 124, 244704 (2006); Y. Wang, J. Lv, L. Zhu and Y. Ma, Phys. Rev. B 82, 094116 (2010)) and constant pressure molecular dynamics calculations will be performed. The ambient fcc unit cell of SnI4 is consisted of 40 atoms. Any calculations using a supercell model will require exceeding 320 atoms. For this purpose, significant computational resource is required. The density functional codes to be used in this study are well maintained and proven highly-parallel public domain programs SIESTA and VASP. Both codes can be used for static and dynamic calculations. Structural search will be performed using our own implementation of the evolution algorithm (Y. Yao, J.S. Tse and K. Tanaka, Phys. Rev. B 77, 052103 (2008)). Structural search and molecular dynamics calculations are computationally (both on the speed of the CPU and memory capacity) very demanding. In the initial stage, an estimate of the computation requirement is about 400,000 CPU-hours on the IBM peta computer cluster. Project URL: Requested allocation: 400000 Q1: 100000 Q2: 100000 Q3: 50000 Q4: 150000 Justification: The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System