Hydrothermal Syntheses And Characterization Of Niobate

Lead zirconate titanate （PZT）-based piezoelectric material is a kind of importantmultifunctional material, which are widely used. However, recently lead has beenexpelled from many commercial applications and materials owing to concernsregarding its toxicity. For the benefit of the environment, PZT-based piezoelectric willbe gradually replaced by the lead-free piezoelectric materials. The niobate is one ofthe most promising ferroelectric materials for both fundamental and applied research,as this material exhibits high ferroelectric polarization at room temperature. Hence itis one of the numerons hot topics in recent years. we successfully synthesiszed theAgNbO₃and K_1-xBa_xNbO₃（x=0,0.2,0.5） nanoparticlers by mild hydrothermalconditions.1. The phase-pure AgNbO₃has been prepared through the hydrothermal reactionsof NH₄HF2, Ag₂O, and Nb2O5. The one-step and easy handling character ofhydrothermal synthesis desides the low temperatures used for reaction makes thisprocedure a convenient method for the synthesis of AgNbO₃based compound. Thepure AgNbO₃sample was also prepared using the solid state reaction reported in theliterature.2. The continuous phase transitions are believed to occur in AgNbO₃prepared bythe hydrothermal synthesis, which have been confirmed by the powder X-raydiffraction patterns measured in two different temperatures873K and723K. Thecrystal has cubic symmetry above813K. The crystal structure transformssubsequently into the tetragonal phase below813K due to tilted octahedral. Thisphase is stable in the temperature range of718-813K. Both transition temperaturesare clearly different from that of AgNbO₃prepared by the solid state reaction.Hydrothermal synthesis of AgNbO₃is a compound with a high stucture order. M1, M2,and M₃phases or by the proposed monoclinic phase were no monitored by in situX-ray diffraction patterns for the title compound under the atmospheric pressureabove room temperature which were observed in image of the temperature depedndences of dielectric permittivity. They are typical for silver niobate and exhibitcharacteristic anomalies at549K,461K and340K, respectively.340K and549Kcorrespond to M₁—M₂and M₂—M₃phase transitions, respectively. The phasetransition apperaring at a relatively high temperature of630K is suggested to bedriven by the antiferroelectric displacement of B-sit atons Nb in a AgNbO₃perovskitestructure. Thebroad relaxation mode observed around540K is due to the dynamics ofantipralled Nb displacements, and further cooling should transform theantiferroelectric ordering of Nb atoms into the ferroelectric ordering at342K, leadingto the apperarance of weak ferroelectricity at room temperature. The Ag and Nb ionsexhibit only one chemical state characteristic for the bulk of sample. In contrary, theO1s spectra are not observed as one component singlet. A main peak and weak peakare attributed to lattic oxygen and adsorbed oxygen, respectively. The core level linesof AgNbO₃are broadened in comparison to those of AgNbO₃by solid state reaction.We evaluate AgNbO₃powder photocatalytic activity toward the photodegradation ofMB at room temperature as a test reaction.3. We prepare a hydrothermal synthesis of a series of potassium barium niobiumnanoparticlers without precursor whose barium content is varied controllably. Wepresent the synthesis of K_1-xBa_xNbO₃nanoparticlers with x=0,0.2,0.5. Thecomposition of the niobate depends on the mineralizer KOH, tempertaure andreaction time. High resolution spectra of the Nb3d states in the K1-xBaxNbO3（x=0,0.2,0.5） are determined. As it is clearly seen Nb ions exhibit only one chemical statecharacteristic for KNbO₃. In contrary, the Nb ions of K_0.8Ba_0.2NbO3and K_0.5Nb_0.5O₃are two chemical states characteristic. The substitution of Ba ions for K decreasessaturation polaritation of this system.