Properties Of VIB Group Transition-Metal Nitrides Prepared At High Temperature And High Pressure

Transition metal?TM?and light element?LE?compounds have been penetrating into all aspects of industrial production due to their excellent phy-chemical properties,such as catalysis,oxidation resistance,corrosion resistance,some specific electrical and magnetic properties and etc.Their hardness,especially the mechanism of hardness,have always been the focus of researchers.After conscientious analyzing the relationship between the basic materials'properties?elastic constant,chemical bond strength,Debye temperature and etc.?and hardness,two main common characteristics on the atomic scale of high hardness materials were concluded that high valence electron number and high light element content.And researchers established many hardness models.However,the mentioned hardness models could not explain all experimental phenomenon very well.Implicit in the differences between experimental and theoretical hardness is the unconsidered hardness factors.In TMLE compounds,researches about the influences from TM,in particular how TM d electron states affect hardness,are very lacking.Thus,in-depth studies and discussion of the influence of TM electronic behavior on hardness is of great significance.It will be not only conducive to the design and development of new high-hardness multifunctional materials,but also to the deeper understanding the mechanism of hardness and the improvement of hardness model.Transition metal nitrides are one of the important class of TM compounds,which have been widely used in the industrial field.However,due to their thermodynamic instability at high temperature,it is still very difficult to prepare high-quality nitrides by conventional methods.For example,the ammonia method with metal powders requires a long high-temperature reaction time,and the obtained nitrides usually contain a large amount of nitrogen vacancies.In contrast,the high-pressure could effectively inhibit the dissipation of nitrogen atoms and improve nitrides'thermodynamic stability.It is more advantageous to synthesize high-quality nitride under high pressure.Most importantly,there should be suitable precursors and chemical reaction path under high pressure for synthesis of nitrides.In this dissertation,the VIB transition metal nitrides are the main research subjects.Firstly,high-quality transition metal nitrides were successfully synthesized by new metathesis reactions between NaN₃ and TM chloride or oxides.Then combining with theoretical calculation,DAC high-pressure technique and many other auxiliary experimental techniques,the relationship between hardness and electronic structures,in particular hardness and the electronic states of d electrons was uncovered.The innovative results are listed as below:1.By new metathesis reactions under high pressure,the high-quality CrN?F m-3m?,MoN?P 6₃mc?and W₂N₃?P 6₃/mmc?were successfully synthesized.Then the WN?F m-3m?was obtained by a decomposition reaction under extremely high temperature with the precursor of W₂N₃.The preferred crystallographic orientation of CrN and MoN is<111>and c axis,respectively.There are two kind of layer morphologies of W₂N₃,the thickness of micro-size layer less than 70 nm while the thickness of nano-size layer less than 20 nm.2.With the experimental results and theoretical simulation,we uncovered the hardness mechanism of the hardest TM mononitride--?-MoN.By analyzing all kind of neighboring atom pairs of MoN with Mulliken Population,it indicates that Mo-N is the only bonding atom pair.So Mo-N bonds is the source of the high hardness of MoN.Moreover,strong coupling between Mo d,p electrons and N p electrons dramatically enhances Mo-N bond strength.3.To study the impacts of connection between TM atoms?mainly d,p electron coupling?on hardness,we studied the differences of Vickers hardness and electronic structures between CrN and WN,which is of the same structure,lattice parameter and valence electron number.It is found that there is a strong local electronic instability occupied near Fermi surface at Cr-Cr,while strong coupling between d electrons and p electrons exist at deep energy level at W-W.It is the main cause of the hardness difference between CrN and WN.It indicates that the TM-TM connection is not merely the source of metallicity.The specific electronic states at TM-TM could also seriously affect TM-TM stability,even lead to lattice distortion.This result is very helpful to reveal the abnormal increase or decrease of the TMLE compounds'hardness during their evolution.4.Compressed experiments of well crystalline CrN show a similar bulk modulus from F m-3m phase to P nma phase and clarify the controversy of the CrN phase transition point.The X-ray photoemission spectra dependence on temperature indicate no remarkable electron transferring from localized electrons at Cr-N to itinerated electrons at Cr-Cr.No change of p-d hybridization across the phase transition takes place.It clarifies the controversy about whether bulk modulus softening across phase transition.We firstly measure the variation of monocrystalline CrN electrical resistance with the function of temperature under different pressure.It is confirmed that at each pressure point,CrN is both metallic of two phases and its Néel temperature shift to room temperature with increase of pressure.It is also clarified that under ambient pressure,CrN is a metal to metal phase transition,but not a non-metal to non-metal phase transition or a metal to non-metal phase transition with decreasing temperature.