ZnTaO2N: Stabilized High-Temperature LiNbO3-type Structure

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO 2 N that crystallizes in a centrosymmetric ( R 3̅ c ) high-temperature LiNbO 3 -type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric ( R 3 c ) LiNbO 3 -type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12 c site instead of 6 a, implying an order–disorder mechanism of the phase transition. It is found that the closed d-shell of Zn 2+, as well as the high-valent Ta 5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO 2 N coordination. Interestingly, only 3% Zn substitution for MnTaO 2 N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.