KCoO₂-type layered nitrides Ca_1-EuTiN₂: structural stability, electrical properties and Eu coordination chemistry

Eu²⁺ was used to substitute Ca in the orthorhombic KCoO₂-type layered CaTiN₂ to form a Ca_1-EuTiN₂ solid solution, which showed a limited substitution around x = 0.35 with the tetragonality enhanced but the orthorhombic symmetry retained and inaccessibility for the end member EuTiN₂. This was in contrast with the full solid solution Ca_1-SrTiN₂, which realized a structural transition from orthorhombic to tetragonal at x = 0.5, even though Eu²⁺ and Sr²⁺ ions have similar sizes. The Eu substitution for Ca reduced the dielectric permittivity of CaTiN₂ owing to the reduced structural distortion arising from the enhanced tetragonality with the substitution. First-principle theoretical calculations on the total energies and formation energies considering the 4f electrons of Eu ions and the related magnetism were performed to understand the structural stability of the hypothetical EuTiN₂. Compared with CaTiN₂ and SrTiN₂, EuTiN₂ has much higher formation energies, making it inaccessible at high temperature. The evolutions of the experimentally observed and calculated lattice parameters of the Ca_1-EuTiN₂ solid solution showed a preference for the orthorhombic phase over the tetragonal phase for the hypothetical EuTiN₂, revealing a different coordination chemistry of Eu²⁺-N to Eu²⁺-O through the comparison of the structural variations of ATiN₂ and ATiO₃ (A = Ca, Sr, Eu).