High Pressure Single-Crystal X-ray Diffraction Study Of Polyhalides And Solid Bromine

Pressure,as a fundamental thermodynamic variable,can dramatically change the properties of materials.The discovery of new materials and new chemical processes are essential for exploration of fundamental scientific issues and innovations in industrial applications.Examples include the recent development of novel high-pressure compounds with unusual stoichiometry and new chemical reactions that have attracted attention because they demonstrate novel types of chemical bonding and number of interesting properties.Despite the tremendous progress in high-pressure studies during the past decades,we are still at the beginning of the development of the fundamental concepts for chemistry under pressure.The behavior of halides and halogens under pressure provides an insight into high-pressure fundamental chemistry.Problems to be addressed are,for example,new chemical reactions enabled under high-pressure,unconventional stoichiometric compounds synthesized featuring unprecedented bonding and properties,and how simple molecular systems respond to extreme conditions.In this thesis,we present high-pressure and high-temperature studies on the alkali halide systems and solid bromine using single-crystal synchrotron X-ray diffraction measurements in diamond anvil cells.Combined with our own ab initio calculations,the unexpected chemical reactivity of alkali halides under high-pressure,unconventional sstoichiometric compounds in alkali halide systems,and the phase transitions during the pressure-induced molecular dissociation of solid bromine were studied.Detailed outlines and conclusions are as follows:(1)Unexpected reactivity of the alkali halides at high pressures.Chemistry of pure sodium and potassium chlorides under high-pressure is considered so far to be extremely simple-they are chemically inert,do not react with pure metals,oxides,silicates,etc.and therefore widely used as a solid pressure-transmitting medium in the diamond anvil cells and multi-anvil experiments.Here we demonstrate the unexpected reactivity of the halides(NaCl and KCl)with metals(Y,Dy,and Re)and iron oxide(FeO)in a laser-heated diamond anvil cell,thus providing a synthetic route for halogen-containing binary and ternary compounds.So far unknown chlorides,Y2Cl and DyCl,and chloride carbides,Y2ClC and Dy2ClC,were synthesized at～40 GPa and～2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction.Also,FeCl2 with the HP-PdF2type structure,previously reported at 108 GPa,was synthesized at～160 GPa and～2100 K.The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.(2)Unraveling the bonding complexity of polyhalogen anions.The field of polyhalogen chemistry,specifically polyhalogen anions(polyhalides),is rapidly evolving.The bonding concepts in polyhalogen anions is of great interest because of its central importance in chemistry.However,their experimental database is very limited to have a general concept of the chemical bonding,formation laws,and physical properties.Here,we present the synthesis of three sodium halides with unpredicted chemical compositions and structures(tP10-Na2Cl3,hP18-Na4Cl5,and hP18-Na4Br5),a series of isostructural cubic cP8AX3 halides(NaCl3,KCl3,NaBr3,and KBr3),and a trigonal potassium chloride(hP24-KC13).The high-pressure syntheses were realized at 41 to 80 GPa in diamond anvil cells laser-heated at about 2000 K.Single-crystal synchrotron X-ray diffraction provided the first accurate structural data for the symmetric trichloride Cl3-anion in hP24-KCl3 and revealed the existence of two different types of infinite linear polyhalogen chains,[Cl]∞n-and[Br]∞n-,in the structures of cP8-AX3 compounds and in hP18-Na4Cl5 and hP18-Na4Brs.In Na4Cl5 and Na4Br5 we found unusually short,likely pressure-stabilized,contacts between sodium cations.Ab initio calculations support the analysis of structures,bonding,and properties of the studied halogenides.(3)Polytypism in solid bromine at high pressures.Although the behavior of simple molecular systems under extreme conditions is of particular interest as a fundamental science,accurate structural information on bromine at high pressures is still missing.Here we present single-crystal synchrotron X-ray diffraction measurements in diamond anvil cells that clearly reveal the structural evolution of solid bromine under high-pressure(45(3)-105(3)GPa)and high-temperature(up to 2600 K).We found evidence of bromine’s molecular dissociation above～80 GPa.The molecular allotrope oC8(Br-I)transforms to the phases with incommensurately modulated structure Fmmm(00γ)s00(Br-Ⅱ;γ=0.493(3))and,depending on pressure,to several incommensurate polytypes Br-Ⅲγ similar to Br-Ⅱ,but with different discrete values of the y components(ranging in the interval of～0.18 to～0.3).The diversity of the incommensurate bromine polytypes and their coexistence at the same pressure,as well as indications that similar behaviour may be characteristic for other halogens,suggest that incommensurate polytypism is common for systems undergoing transformations from molecular(consistent of strongly bonded units)to none-molecular crystal structures.