Ab Initio Investigation Of Phase Transitions Of Solid Iodine And Ammonia Under High Pressures

Typical small molecular systems,including diatomic molecular crystals composed of single element,or hydrogen bond dominated crystals(including water and ammonia et al.),as a result of their abroad distribution in our universe,our lives are greatly affected by them,so their properties under extreme conditions become important topics of high pressure physical scientists.They are constitutes of some celestial bodies,so some properties of these celestial bodies are decided by them. Meanwhile,for people's recognizing and understanding the evolvement of structures and properties of these systems is very important in theoretical aspect.In this article,we made research on the pressure induced molecular dissociation effect of solid iodine and the phase transition and dynamical properties of ammonia under high temperatures and high pressures.As to the previous reported results,we educed some accepted properties through our analysis and comparisons of them,and then we make a careful analysis and comment.In combination of basic physical theory,including group theory,X-ray diffraction theory,and so on,we put forward our main point of view.We think that there is another phase (phaseâ… ') during the dissociation process of solid iodine.Then we made a series of geometry optimization calculations of phaseâ… and phaseâ… ',the vibrational frequencies at gamma point,X-ray diffraction patterns of these structures are also calculated.Then we compare them with experiments,which further prove our assumptions.Based on analysis of Raman spectra,our main result is that new vibrational modes should be assigned to a new phase(a transition phase),we thought that this new phase coexist with phaseâ… at wide range of pressure.This transition process carried on gradually.Through our X-ray diffraction experiments, our results show that the amount of phaseâ… ' increase with pressure distinctly. Our result is an important supplement of molecular dissociation mechanism theory of iodine,states where van der waals bonds and covalent bonds coexist to states pure covalent-bonded solids,people discover that new phases exist,so the structures of them become critical points for people to understand the molecular dissociation process.Our research shows that,during the transition process,solid iodine will translate to a new structure which is more close to phaseâ… ,which belong to molecular crystal,but with some new characters.Molecules boned more close to each other,not only weak van der waals-bonded,but to covalent bonded.Not only this,new changes in molecules,bond length of some molecules is longer than other molecules,ended with a abrupt change of bond lengths,which transit to phaseâ…¤(incommensurate phase). It is not a normal period structure,but modulated from phaseâ…¡,that is to say,this structure is more close to phaseâ…¡.On the contrary,our phase can be modulated from phaseâ… .A common feature of these two new phases (phaseâ… ' and phaseâ…¤) is that more distances between atoms exist in this system.They are just two transition states of two normal structures.We believe that the appearance of this phenomenon is in connection with metastable states.The possible feature of metastable states is that the potential barrier to them is much lower than to the most stable structure from the initial structure.Aside from the main conclusions above,we calculated the electronic structures of them,our results show that the energy band of phaseâ… began to overlap at around 16 GPa(process of metallization),which is close to the experimental results,so the methods we used in our calculations are reliable.We used ab initio molecular dynamics in our research on the high temperature and high pressure phase transition of ammonia,this is main attributed to the fact that the behavior of protons are very important during this process.The kind of molecular dynamics method we use can describe the structure and motion character(especially on how these atoms bonded together).The significance of our research on these structures is that ammonia is an important constitutes of some Giant Planets,whose properties can be greatly affected by the properties of ammonia on them.Recent theoretical research showed that,ionic phases of ammonia may be formed under those extreme conditions.So the dynamical property of these ionic crystals is very important,which may be helpful in people's research of some phenomena.Based on our research,a new phase which is dynamical stable was put forward.And we make some dynamical analysis of these ionic crystals and found that P4₃ phase is the only protons-conducted phase. There are 16 molecules in one unit cell,under the effect of temperature and based on the results of molecular dynamics and through static optimization,we found that the attraction of hydrogen atoms from amides is weaker than the attraction in ammoniums,which is demonstrating that there are at least two potential barriers for protons,protons with lower potential barrier can be easily excited.Our molecular dynamics results show that protons in ammonium never leave their original positions(120 GPa and 600 K),only jump sometimes and never diffuse through this system,which is a very important character,which is showing that protons in ammonium is not electronic carriers,not only this,amides form a three dimensional net,the protons on them move frequently,which is main attributed to the weak bondage from nitrogen atoms,this configuration make the hydrogen atoms jump from on amide to another. Some times they didn't jump back but diffuse in this system,so we deduced that there will be an invariable electronic current and becomes a protons-conducted conductor.As to the Pma2 phase,the protons with the lowest potential barrier are not on amides,but on ammoniums.There are different potential barriers for protons on ammoniums.Some N-H bonds are shorter than others.Protons with lowest potential barrier will jump to amides nearby.But this new configuration is not stable,so they will jump back to their original positions.There is no protons diffusion in this phase, which is different from that of P4₃ phase.No electronic carrier for Pma2 phase at low temperatures,which is more like an insulator than P4₃ phase. There is no obvious protons diffusion in the region where P2₁/m is stable, which is just stable at low temperatures and not stable at high temperatures(like 320 GPa and 1000 K).We should mention that this structure is not stable at high temperatures,but as a chemical group,our molecular dynamics results show that ammonium is stable,even though the equilibrium state is not reached.Nitrogen atoms are more likely to attract four hydrogen atoms,so we predict that another ionic phase will be formed under this condition.Our recently results show that,solid ammonia will form partially protons disordered phase will be formed under high temperature.Owing to the fact that a lot of disordered phases exist in these systems,especially under high temperatures,so a important topic of future research will be on these disordered phase,which is hinting that molecular dynamics is essential.In above research,we make a brand new interpretation of Raman spectra and X-ray diffraction patterns of solid iodine at high temperatures. We make carefully analysis on the pressure induced phase transition and explain that different bond lengths coexist in this system is logical.We also made research on the structures and properties of ammonia under high temperatures and high pressures and put forward a brand new structure(P4₃ phase).This new phase is protons-conducted conductor under proper temperatures and pressures,while other two structures is not, which is mainly attributed to their different configuration.