Structures And Properties Of Typical Main Group Hydrides And Borides

In 1935,E.Winge and H.B.Huntington predicted that solid hydrogen would transform into metallic phase at high pressure.This metallic phase is also one kind of room temperature superconductor.Since the prediction was proposed,much attention has been paid to metallic hydrogen.But the pressure for metallization of hydrogen is too high to achieve based on current technological level.So people try to find other ways to obtain metallic hydrogen at lower pressure condition.In 2004,N.W.Ashcroff proposed that heavier atom introduced in the lattice of hydrogen will provide a precompression effect on the hydrogen atom,and this precompression effect could lower the pressure of metallization.Hydrogen-rich compounds are proposed as potential materials with possibly high temperature superconductivity at lower pressures,as the hydrogen in these structures may be considered as “chemically precompressed”.In recent years,high-pressure studies on hydrogen-rich compounds have set off a new wave of upsurge.The most prominent one is sulfur hydride with high temperature superconductivity under high pressure.H2 S decomposes into H3 S and sulfur under high pressure.The Tc of H3 S with the Im-3m structure tops out at 204 K at 200 GPa.Similar to H2 S,other hydrides such as PH3 and HBr also show the high temperature superconductivity under high pressure.So our work is amied at the behavior of main group hydrides such as PH3 and HBr under high pressure.And The hexaborides MB6 of the alkaline earth elements,such as Ca B6,YB6,Ce B6 and Ba B6 have long been the hot topics of the theoretical and experimental studies.Among these hexaborides,The divalent alkaline-earth hexaborides are renowned for having properties of high melting point and hardness,low density and coefficient of thermal expansion,and excellent chemical stability.As a kind of ferromagentic semiconductor with high Curie temperature,Ca B6 is used in Sspin-electron device.But for the behavior of Ca B6 and Ba B6 under high pressure,the information have not been abundant up to now.So our work is amied at the behavior of Ca B6 and Ba B6 under high pressure.The investigation of them under high pressure is beneficial to understand the phase transiton mechanism of alkaline-earth hexaborides and maybe provide new ideas about preparing new metarials.In this work,in order to ascertain the behavior of PH3 and HBr under high pressure at room temperature,we have studied them by means of in situ high pressure Raman spectroscopy and sychrontron XRD spectra.Our detailed experimental study allowed us to identify the changes of both crystal structure and stoichiometry of compressed PH3 and HBr by the effect of pressure.Futher more,we have reported a joint of synchrotron XRD and Raman spectra study of Ca B6 and Ba B6 to explore the crystal structures and vibrational properties.The obtained results are as follows:1.As for PH3,PH3 decomposes under high pressure.The products are P2H4 and hydrogen.And PH3 or P2H4 could decompose into hydrogen and element phosphorus under high pressure.The experimental results indicate that the generated element phosphorus belong to white phosphorus.With pressure downloading to ambient pressure,the PH3 did not occur again.P2H4 was acquired by releasing pressure from 27 GPa to ambient pressure.Maybe P2H4 and other phosphorus hydrides could explain the high temperature superconductivity of PH3 reported in experiment.2.As for HBr,the behavior of HBr under high pressure has been investigated experimentally.By applying pressure,HBr decomposes into Br2,H2 Br or other bromine hydrides at 18.7 GPa.With pressure increasing,Br2 molecular transforms to incommensurate structure,then to monatomic bromine.Incommensurate structure of bromine was observed for the first time in the synchrotron radiation X-ray measurement.With pressure downloading,Br2 formed again,but no Raman signal of HBr was observed.It demonstrates that the monatomic bromine can recovered with pressure downloading.3.We have successfully studied the structural stability and vibrational characteristics of Ca B6 by means of in situ high pressure Raman spectra and synchrotron X-ray diffraction.With pressure increasing up to 40 GPa,no phase transitions occurred and the lattice constant decreased linearly with pressure.Upon compression,all the vibration modes have smooth monotonic pressure dependence.Our experimental results reveal that the covalent network of Ca B6 is difficult to break only under pressure conditions.In order to promote the phase transition occurs,it must be supplied with an alternative pathway to break the energy barrier.4.We have successfully studied the structural stability and compression behavior of Ba B6 by means of in situ high pressure synchrotron X-ray diffraction and first-principles calculations.The experimental results reveal that it is very difficult to break the covalent network under only high pressure conditions,Ba B6 remains stable up to 49.3 GPa.Our theoretical calculations propose that Ba B6 is still a semiconductor at 50 GPa.Three candidate dynamic stable structures(Cmmm,Cmcm and I4/mmm)might replace the typical cage structure at 78 GPa,97 GPa and 105 GPa.,respectively.Attempting to accomplish a phase transition,it must be supplied with other pathway to break the energy barrier.