Study On The Preparation Of Perovskite Oxynitrides Powder Based On A Novel Solid-state Nitridation

Perovskite oxynitrides have received considerable attentions because of their promising optical,dielectric,and photocatalytic properties.Generally,oxynitrides were obtained by the ammonolysis of the oxide precursors,which were prepared by either solid state reaction or soft chemical route.However,these routes were tedious and usually required a long period to obtain the product with the single phase.Moreover,the toxic and corrosive nature of NH₃inhibited its systematic research and scalable production for commercialization.Therefore,in the present work,we attempted to explore the feasibility of direct synthesis of perovskite oxynitrides ABO₂N?A=Sr,Ba,Ca;B=Ta,Nb?powder using carbonate?ACO₃,A=Sr,Ba,Ca?and nitrides?BN,B=Ta,Nb?as raw materials in flowing N₂ atmosphere at different temperatures,and the phase evolution and reaction mechanism were systematically investigated.The X-ray diffraction,Rietveld refinement,and nitrogen and oxygen content analysis were employed to analyze the chemical composition of the products.The reaction behavior of the raw material mixture was investigated by thermogravimetry-differential thermal analysis combined with mass spectrometry.Scanning electron microscope,transmission electron microscope,and laser particle size analysis were used to analyze the morphology and particle size of the products.The result showed that almost single-phase orange SrTaO₂N powder was able to be directly synthesized from the mixture of SrCO₃/TaN in flowing N₂ atmosphere at 1200°C for 5h.Phase analysis showed that the TaN?Aladdin?reacted with CO₂,which was from the decomposition of SrCO₃,to produce TaON,and then TaON reacted with SrO to produce SrTaO₂N.Also,the Sr₂TaO₃N phase was detected as the intermediate product during the reaction process.During the reaction,TaN was expected to act as a template and SrTaO₂N initially was formed between the two-phase interface of the reactants.Therefore,the fine SrTaO₂N powder with uniform size was synthesized by this route.Analysis showed that SrTaO₂N powder median diameter(d₅₀)was 6.63?m,and the surface area volume average diameter(d_3,2)was 3.47?m.After the success of direct synthesis of SrTaO₂N powders,the attempts were then to explore the feasibility of direct synthesis of perovskite oxynitrides BaTaO₂N and CaTaO₂N powder using the mixture of BaCO₃/TaN and the mixture of CaCO₃/TaN as raw materials in flowing N₂ atmosphere,respectively.The result showed that almost single-phase brown-red BaTaO₂N powder could be directly synthesized from the mixture of BaCO₃/TaN in flowing N₂ atmosphere at 1200°C for 5h.The reaction process was similar to that of SrCO₃/TaN,and the reaction activity was higher than that of the SrCO₃/TaN.During the reaction,the TaN was anticipated to act as a template and the BaTaO₂N phase initially was formed between the two-phase interface of the reactants.Fine BaTaO₂N powder with uniform size was then synthesized by this route.BaTaO₂N powder median diameter(d₅₀)was 10.21?m,and the surface area volume average diameter(d_3,2)was 5.53?m.Due to the mismatch between decomposition temperature and reaction temperature,only trace amounts of CaTaO₂N phase were synthesized from CaCO₃/TaN in flowing N₂ atmosphere.Lastly,the synthesis of perovskite oxynitrides ANbO₂N?A=Sr,Ba,Ca?powder was conducted using the mixture of ACO₃/NbN?A=Sr,Ba,Ca?as raw materials in flowing N₂atmosphere.The result showed that almost single-phase brownish black SrNbO₂N powder and brownish black BaNbO₂N were obtained by the direct synthesis from the mixture of SrCO₃/NbN and BaCO₃/NbN in flowing N₂ atmosphere at 1300°C for 5h,respectively.Unlike Ta system,niobium oxynitride-based compound does not exist?such as NbON?.The reaction mechanism of SrCO₃/NbN was mainly that NbN reacted with CO₂ and SrO,which were generated from the decomposition of SrCO₃ to form the SrNbO₂N by one-step reaction.There was no intermediate phase observed during the reaction process.The phase formation and reaction mechanism of BaNbO₂N from BaCO₃/NbN was similar to that of SrNbO₂N.During the reaction,NbN was anticipated to act as a template,and phases of SrNbO₂N and Ba NbO₂N were gradually formed from the inside to the outside on the template.However,due to the mismatch between decomposition temperature and reaction temperature,single phase CaNbO₂N was not able to be directly synthesized from the mixture of CaCO₃/NbN at any temperature in flowing N₂ atmosphere.Furthermore,analyses showed that the as-prepared perovskite oxynitrides possessed nitrogen deficiency.The raw materials of SrCO₃,Ba CO₃,CaCO₃,TaN,and NbN used in the experiment was all commercially available,and the reaction process was carried out in an ammonia-free environment.Therefore,this synthesis process employed in this study could have a potential to be applied for a large-scale production of perovskite oxynitrides.