Spontaneous polarization study in A 3+ B 4+ (O 2 N) 7− and A 2+ B 5+ (O 2 N) 7− perovskite-type oxynitrides: a first principles investigation

Synthesized oxynitride perovskites materials exhibited very interesting properties, in particular ferroelectricity, which allow their uses in various applications and qualify them as very promising candidates for multifunctional materials. This study highlights the use of first-principle calculations based on density functional theory to carry out investigations of spontaneous polarizations into two groups of perovskite-type oxynitrides. The first group is A 3+ B 4+ (O 2 N) 7− where A 3+ = Y, Sc, La, Lu and B 4+ = Si, Ge, Sn while the second one is A 2+ B 5+ (O 2 N) 7− where A 2+ = Mg, Ca, Sr, Ba and B 5+ = V, Nb, Ta. Therefore, we have computed the structural parameters, formation energy, tolerance factor, octahedral factor, bond ionicity, spontaneous polarization, and band gaps. As a result, we have deduced (1) that all studied oxynitride perovskites are distorted and can be synthesized, and are all insulators or semiconductors, some of which have narrow energy gaps. (2) Some perovskites exhibit high spontaneous polarization values that appear to be stimulated by the large displacements of B-cations away from their centrosymmetric positions, by the lower B-cation ionic radius, and the higher A-cation ionic radius of the tetragonal ABO 2 N perovskite structure. (3) Spontaneous polarization in oxynitride perovskites increases for higher electronegativity of B-cations and lower interatomic distance between the B-cation and the N-anion. It also noted that spontaneous polarization was more pronounced in the valence group A 3+ B 4+ (O 2 N) 7− than that of A 2+ B 5+ (O 2 N) 7− .