KCoO₂-type layered nitrides Ca_1-xEu_xTiN₂: structural stability, electrical properties and Eu coordination chemistry

Eu2+ was used to substitute Ca in the orthorhombic KCoO2-type layered CaTiN2 to form a Ca1-xEuxTiN2 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 Ca1-xSrxTiN2, 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 Ca1-xEuxTiN2 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 ATiN(2) and ATiO(3) (A = Ca, Sr, Eu).