High Pressure Reaction Of Unsaturated Metal Polycarbides

Carbon atoms will form carbon skeleton compounds with different dimensions due to different bonding modes.When carbon atoms are combined with metal elements,various metal carbides with unique properties will be formed.As a basic variable in thermodynamics,pressure can affect the microstructure,interatomic distance and electron spin state of materials.High pressure can induce the unsaturated carbon-carbon bonds to polymerize into carbon skeleton compounds with higher saturation.In this thesis,we mainly studied several different types of unsaturated metal polycarbides such as monosodium acetylide(NaC2H),magnesocene(Mg(C5H5)2)and metallofullerenes Gd@C82.The phase transition,polymerization pressure,reaction threshold(intermolecular atomic distance before polymerization),high pressure structure,electrical transport performance and polymerization products under high pressure are studied by in situ Raman spectroscopy,infrared spectroscopy,X-ray diffraction,and impedance spectroscopy,and ex situ gas chromatography-mass spectrometry.Pressure-induced polymerization(PIP)of metal acetylides is a novel method to synthesize metal-carbon framework and polycarbide materials with unique structures and properties.However,the polymerization pressure of disubstituted acetylide is mostly 25 GPa or higher,which limits its large-scale synthesis.Therefore,we chose monosodium acetylide with only one charge to study,and found that NaC2H undergoes a phase transition at 7 GPa and polymerizes at 14 GPa,which is the lowest pressure reported so far for carbide polymerization.At the reaction threshold,the nearest intermolecular C…C distance is about 2.9 A,which is almost the same as calcium carbide.Further research found that PIP process is mainly a free radical addition process.Our work discloses the threshold of the intermolecular distance for the PIP of acetylide and proposes reaction mechanism,which provided the basis for further study of high-pressure chemical reaction of acetylide.Magnesocene is a typical metal organic compound with sandwich structure.We have studied the PIP process of magnesocene and found it undergoes a phase change at 3 GPa.Then it turns into amorphization state and polymerized at about 15 GPa with the sp2 carbon tranferring into sp3 carbon.This polymerization pressure is also lower than common metal carbides such as CaC2 and Li2C2.The reversibility of polymerization is closely related to the reaction time.In situ impedance spectroscopy shows that the conductivity decreases first,then increases and finally decreases,which may be closely related to the phase transition and polymerization of magnesocene under high pressure.Gd@C82 is a typical endohedral metallofullerenes.The high-pressure study found that the polymerization pressure of Gd@C82 was about 13 GPa.The diffraction results showed that the carbon cage was destroyed under high pressure but the metal atoms remained in order up to 30 GPa.Impedance spectrum shows that the conductivity of Gd@C82 irreversibly increases by 103 times under external pressure.This shows that pressure can irreversibly change the conductivity of metallofullerenes.