Structural Distortion and Dielectric Permittivities of KCoO2-Type Layered Nitrides Ca1-xSrxTiN2.

Among the KCoO2-type phases, the orthorhombic layered nitride CaTiN2 is a newly reported high dielectric permittivity material (εr ∼ 1300-2500 within 104-106 Hz from 80 to 450 K) while the tetragonal SrTiN2 is reported to display an unintentional metallic conduction property. In this work, a Ca1-xSrxTiN2 solid solution was synthesized, in which the insulating SrTiN2 end member and some Sr-doped CaTiN2 samples were successfully obtained, and therefore, the dielectric properties of the Ca1-xSrxTiN2 solid solution were investigated. The Sr substitution for Ca drove an orthorhombic-to-tetragonal phase transformation in Ca1-xSrxTiN2, which reduced the dielectric permittivity significantly. The tetragonal SrTiN2 displays a much lower dielectric permittivity (εr ∼ 20-70 in 105-106 Hz and 10-300 K) than that of CaTiN2. The comparison on the dielectric permittivities and structures of CaTiN2 and SrTiN2 indicates that the structural distortion arising from the splitting of N planes between Ti layers within the TiN2 pyramidal layers could be a plausible structural origin of the high bulk dielectric permittivity of CaTiN2.