KCoO 2 -type layered nitrides Ca 1- x Eu x TiN 2 : structural stability, electrical properties and Eu coordination chemistry.

Eu 2+ was used to substitute Ca in the orthorhombic KCoO 2 -type layered CaTiN 2 to form a Ca 1- x Eu x TiN 2 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 EuTiN 2 . This was in contrast with the full solid solution Ca 1- x Sr x TiN 2 , which realized a structural transition from orthorhombic to tetragonal at x = 0.5, even though Eu 2+ and Sr 2+ ions have similar sizes. The Eu substitution for Ca reduced the dielectric permittivity of CaTiN 2 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 EuTiN 2 . Compared with CaTiN 2 and SrTiN 2 , EuTiN 2 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- x Eu x TiN 2 solid solution showed a preference for the orthorhombic phase over the tetragonal phase for the hypothetical EuTiN 2 , revealing a different coordination chemistry of Eu 2+ -N to Eu 2+ -O through the comparison of the structural variations of ATiN 2 and ATiO 3 (A = Ca, Sr, Eu).