Effects Of Ion Substitution On The Structure And Electrical Properties Of KCoO₂-type Layered Nitride

The development of information industry puts forward high requirements for the materials of electronic components.High dielectric permittivity materials are considered to be the key factor to determine the storage density and device size of capacitors.At present,the research on high dielectric permittivity materials mainly focuses on oxides.The high atomic polarizability of nitrogen relative to oxygen makes（oxy）nitride compounds have great potential in developing new dielectric materials.Among them,KCo O₂-type layered metal nitride CaTiN₂ exhibits a high bulk dielectric constant of～1500 at 10⁶Hz at room temperature.However,the mechanism of its high dielectric permittivity has not been reasonably explained at present,and further research is needed.In this paper,CaTiN₂ and Sr Ti N₂ with the same structure type are used as parent phase compounds,and partially or completely ion substituted compounds are synthesized by high-temperature solid-state method.The effects of ion substitution on the structure and dielectric properties of CaTiN₂ and Sr Ti N₂ are studied,and the mechanism of high dielectric permittivity of layered metal nitrides is further studied.1.Ca_1-xEu_xTi N₂ and Ca_1-xBa_xTi N₂ solid solutions were synthesized by doping the A-site ions of CaTiN₂.TGA-DSC and SEM showed that the designed target solid solution was obtained in the experiment,and the structure refinement showed that the two solid solutions were still KCo O₂-type layer metal nitride structure.With the increase of doping content,the structure of the two solid solutions shows a trend of transformation from orthogonal phase to tetragonal phase.Among them,Ca_1-xBa_xTi N₂solid solution is completely transformed into tetragonal phase,and the unit cell parameters continue to increase with the increase of doping content,but still remain in tetragonal phase structure.2.After partial substitution of Ti ions of CaTiN₂ and Sr Ti N₂,no solid solution can be formed,but after complete substitution of Ti by Mn,Hf and Zr,Ca₃Mn N₃,Sr Hf N₂ and Sr Zr N₂ are obtained.The synthesized compounds do not maintain the structure type of parent phase compounds.The coordination polyhedron of the B-site ion of Ca₃Mn N₃ becomes a Mn N₃ planar triangle.The ion coordination polyhedron in Sr Hf N₂ and Sr Zr N₂ changed into 6-Coordinated Zr/Hf N₆ polyhedron.The neutron diffraction data structure analysis showed that the polyhedron had no obvious structural distortion.Theoretical calculations show that both Sr Hf N₂ and Sr Zr N₂ tend to be formed in the lower energyα-Na Fe O₂-type structure crystal.3.The Ca_1-xEu_xTi N₂ solid solution has the same trend as the previously reported Ca_1-xSr_xTi N₂ solid solution in the change of electrical properties and structure with doping amount.Doping makes the crystal structure change from orthorhombic phase to tetragonal phase,and reduces the structural distortion of Ti N₅ polyhedron.The dielectric permittivity of Sr Hf N₂ and Sr Zr N₂ at 10⁶ Hz and 5-280 K is～290-650,and the conductivity is～10^-3S/cm.The electrical property impedance test shows that the two compounds contain the internal barrier layer capacitance effect（IBLC）,so they have an intrinsic dielectric permittivity.The high dielectric constant source of CaTiN₂provides a meaningful reference.Theoretical calculations show that both Sr Hf N₂ and Sr Zr N₂ are semiconductors with a band gap of～1.0 e V and have high bulk dielectric permittivity of 280 and 470,respectively.The density functional perturbation theory calculation of the Born effective charge tensors together with the mode eigenvectors indicate that the large dielectric permittivities are attributed to the ionic dielectric tensor contributions arising from the individual infrared active phonon modes.The high ionic polarizability and low vibration frequency in the xy plane of the lattice play a major role in the high dielectric constant.