Hello, A yearly allocation for the LCRC cluster has been requested with the following updated information: Submitter/PI: Michael Hu Project Name: SnI4 Division: XSD Project title: Crystalline and amorphous structures of SnI4 Associated funding: DOE and NSERC (Canada) Other Systems: Science: Although pressure-induced amorphization (PIA) has been observed in many solids, the true mechanism is elusive and remain a subject of debate and misinterpretation. Two scenarios have been proposed. In the first observation of this phenomenon in ice Ih , it was assumed the transition is akin to pseudomelting in which the crystalline solid crosses the extrapolated melting line by pressurization at low temperature. This suggests a possible thermodynamic connection between the amorphous state(s) to the liquid, and the prevalent but controversial “two-liquid” model for water is a consequence of this conjecture. Another proposal for PIA transition is that it is a mechanical process due to the instability of the crystal framework under compression and thus violates the Born stability conditions. Recent experiments have shown that in several materials that PIA believed to occur was found to bypass when compression was performed under quasi-hydrostatic conditions. Significantly, archetypal example of ice Ih and SnI4, the subject of our focus, are two solids that no crystalline form was yet to be found in lieu of the disordered structure and the nature of the amorphous phases is still an open question. SnI4 offers several advantages for the characterization of PIA transformation. It is a solid at room temperature and has a fairly high melting point making it comparatively easy to handle as compared to Ice Ih. The 119Sn nuclear resonant inelastic scattering spectra recently measured at sector ID-3, Advanced Phonon Source, Argonne National Laboratory, have revealed very interesting dynamic behavior on pressurization. The objective of the computational investigation is to elucidate the vibrational spectra and to reveal the mechanism of the PIA transformation. Project description: Our preliminary calculations have identified the PIA mechanism as mechanical instability, rather than pseudo-melting. A publication is in preparation. Next we plan to extend simulations to low temperatures, where experimental data were also taken. The aim is to clarify the source of spectrum broadening at room temperature and ambient pressure. Libration is the most likely reason. The objectives of this proposal are (i) to continue the study on the finite temperature effect on the vibration density of states of the Sn atoms in SnI4 and (ii) to start new calculation son the analogous SnBr4 in which experiments have been planned 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. The code Qbox will be used mainly for molecular dynamism calculations. Other codes such as SIESTA and VASP (one of the PI holds the license) will be used for static and properties 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. Industry partnership: Project URL: Current FY Hours Used: undetermined amount New FY Requested allocation: 499999 Q1: 124999 Q2: 125000 Q3: 125000 Q4: 125000 Justification: Storage requirements: Thank You, The LCRC Accounts System