Synthesis And High-pressure Studies Of Alkaline Earth Metal Diazenides

In recent years,new materials and new energy have been explored and developed to achieve the rapid and sustainable development of the society.Metal diazenides have significant application prospects owing to their energetic characters and metal conductive.However,the research on the metal diazenides is still in its infancy in China.There are still some problems to be studied,such as the synthesis of metal diazenides,high-pressure phase transitions,and conductive mechanism etc.In this paper,the alkaline earth diazenides have been synthesized by decomposing corresponding alkaline earth azides under high-pressure and high-temperature conductions,based on the studies of alkaline earth azides.The high-pressure studies of the synthesized of the alkaline earth diazenides have been explored by the in-situ synchrotron X-ray diffraction.The experimental studies support for the systematic exploration of the high-pressure nature of alkaline earth diazenides,and provide the experimental support and new ideas for the synthesized of conductive energetic materials.First,Sr?N₃?₂ was chosen as precursors.The symmetrical diamond anvil cells combined with laser heating technology were chosen to provide high-pressure and high-temperature conductions.We present the in-situ synchrotron X-ray diffraction of the synthesized samples.The results reveal the synthesized samples contained initial reactants Sr?N₃?₂.We improved the experimental method.An Walker-type module was chosen to provide high-pressure and high-temperature conductions.The results reveal that we successful synthesized SrN₂.Then,Ca?N₃?₂ was chosen as precursors.The symmetrical diamond anvil cells combined with laser heating technology were chosen to provide high-pressure and high-temperature conductions.We present the in-situ synchrotron X-ray diffraction of the synthesized samples.The results reveal the synthesized CaN₂ contained initial reactants Ca?N₃?₂.In this work,we present the in-situ synchrotron X-ray diffraction of the synthesized SrN₂ up to 43.2 GPa by a symmetrical diamond anvil cell.The results reveal a pressure-induced phase transition of tetragonal to orthorhombic at 12.0 GPa.We temporally ascribe the phase transition of SrN₂ to the rotation of diazenide units and the rearrangement of Sr ions.SrN₂ shows anisotropic compressibility due to the orientation of the diazenide anions.As pressure is released to 0.6 GPa,the tetragonal structure recovered from the high-pressure orthorhombic structure,demonstrating a reversible phase transition of SrN₂.The bulk modulus of SrN₂ fitted by Birch-Murnaghan equation is 128.8?12.7?GPa,which is intermediate between Sr?N₃?₂ and PtN₂.And the bulk modulus of PtN₂ is the largest.However,the distance of N-N is decreasing in Sr?N₃?₂,SrN₂,PtN₂.To obtain deeper insight into the nature of chemical bonding between metal atoms and N atoms,we performed crystal orbital overlap population?COHP?analysis and integrated crystal orbital overlap population?ICOHP?values.The figures of COHP reveal metal atoms combine N atoms with covalent bond.The values of ICOHP are in decreasing order of PtN₂,SrN₂,Sr?N₃?₂.Thus,the larger bulk modulus of SrN₂ is attributed to the stronger covalent strength between Sr and N atoms in SrN₂.