Formation mechanism and growth of MNbO₃, M=K, Na by in situ X‐ray diffraction

Abstract

Hydrothermal synthesis is a well-established method to produce complex oxides, and is a potential interesting approach to synthesize stoichiometric lead-free piezoelectric K 0.5 Na 0.5 NbO 3. Due to challenges in obtaining the desired stoichiometry of this material, more knowledge is needed on how the end members, KNbO 3 and NaNbO 3 , are nucleating and growing. Here we report on the formation mechanisms and growth during hydrothermal synthesis of KNbO 3 and NaNbO 3 by in situ synchrotron powder X-ray diffraction. We show that tetragonal KNbO 3 crystallites form from

Defect-Accommodating Intermediates Yield Selective Low-Temperature Synthesis of YMnO3 Polymorphs

“…For example, it is known that hexagonal rare earth manganites RMnO 3+δ synthesized under high oxygen pressure can accommodate excess oxygen via interstitials due to their more open structure. 60 Hence the constructed phase diagram provides a route to understanding thermodynamic stability when approached from a synthesis route in which cation vacancies exist or are likely to form, such as from an intermediate structure containing Mn(IV). We anticipate that the inclusion of anharmonic effects could change the calculated Gibbs free energies of the YMnO 3 phases considerably at high temperature.…”

Chemical stability of KNbO3, NaNbO3, and K0.5Na0.5NbO3 in aqueous medium

“…Contrary, no intermediate phase precipitate appears until | 1077 the final KN phase meaning that potassium has higher solubility than sodium. 53 From a thermodynamic point of view, the correlation may be bridged with formation enthalpies (ΔH f OX ) of the species. It was addressed that having more exothermic formation enthalpy of KN (À207.12 AE 1.74 kJ mol À1 ) than NN (À141.63 AE 2.27 kJ mol À1 ) states higher tolerance factor.…”

“…49 The report introduced a correlation between ionic field strength (Z/r 2 ) ratio and corrosion rate of glass powder in deionized water by Feng,50 where Z is the valence of ion and r is the ionic radius based on the coordination number. 53 From a thermodynamic point of view, the correlation may be bridged with formation enthalpies (ΔH f OX ) of the species. The principle has been conveyed to explain preferential hydrolysis of BZT-BCT 38 and BST 39 systems.…”

Perovskite Oxides Prepared by Hydrothermal and Solvothermal Synthesis: A Review of Crystallisation, Chemistry, and Compositions

“…[99] More recently Skjaervø et al studied the kinetics of the hydrothermal growth of NaNbO3 and KNbO3 using time-resolved in situ X-ray diffraction with high intensity synchrotron X-rays. [100] This work showed how KNbO3 crystallises directly from dissolved Nb2O5 precursor at 250°C-300°C and 250 bar, whereas NaNbO3 forms via crystalline intermediate phases at 225°C-325°C and 250 bar. [100a] .…”