Molten BaCN2 for the sintering and crystal growth of dielectric oxynitride perovskites Sr1−xBaxTaO2N (x = 0.04–0.23)

Abstract

Solid phase sintering of dielectric oxynitride perovskites above 1000 °C is accompanied by their decomposition. Post-ammonolysis is required to recover their stoichiometric nitrogen content and dielectric properties. In the present work, the oxynitride perovskite SrTaO₂N was sintered with a BaCN₂ flux at approximately 900 °C avoiding its thermal decomposition. The resulting solid product with a relative density of 68.9% showed relative dielectric constants in the range from 68 to 90 with loss values less than 0.11, without the post-ammonolysis. The interior of the solids contained rectangular Sr_1−xBa_xTaO₂N crystals for which 0.04 ≤ x ≤ 0.23 reflecting their euhedral form. These crystals were grown in molten BaCN₂ from 20 to 100 times larger than the original SrTaO₂N particles and had a maximum grain size of 3.7 μm. The Sr_1−xBa_xTaO₂N precipitated on the surfaces of the residual SrTaO₂N crystals that had partially dissolved in the BaCN₂ flux. A compositional gradient from barium-rich to strontium-rich was observed in a single crystal of the product on going from the exterior to the interior. This is the first-ever report of the preliminary liquid phase sintering and crystal growth of a dielectric oxynitride perovskite using a molten metal cyanamide.