Polar Ruddlesden-Popper Rare-Earth Aluminum Oxynitrides with Partial Anion Order

Anion order in heteroanionic compounds may induce electrical dipoles and the formation of noncentrosymmetric polar structures. The majority of perovskite oxynitrides are centrosymmetric, but some of them show high dielectric permittivities and ferroelectric response resulting from local anion order. Here, we report a study of polar, partially anion-ordered Nd2AlO3N and Sm2AlO3N n = 1 Ruddlesden-Popper oxynitrides, prepared by solid-state reaction in flowing N2 at 1500 °C. They crystallize in the noncentrosymmetric I4mm space group with parameters a = 3.703314(3) and c = 12.545382(14) Å for Nd2AlO3N and a = 3.674477(5) and c = 12.40414(3) Å for Sm2AlO3N. Neutron powder diffraction data of Nd2AlO3N indicate that oxygen occupies the equatorial sites 4b of the AlO5N octahedra, whereas both anions occupy two crystallographically independent 2a axial X2 and X3 sites, with different O/N populations (68/32 for X2 and 32/68 for X3) and a larger bond distance to the Al atom for the oxygen rich site. Dielectric permittivities of 260 and 150 are measured for Nd2AlO3N and Sm2AlO3N, respectively, and second-harmonic generation spectroscopy and scanning transmission electron microscopy furnish complementary evidence of the polar nature of both compounds. These results represent the first experimental demonstration of acentric functionalities in a Ruddlesden-Popper oxynitride.