Hydrothermal Synthesis And Properties Of Transition Metal Fluoride And Oxide Compounds

Fluoride compounds are usually prepared by tranditional high temperature solid-state methods which require calcination of fluoride raw materials at temperature in excess of 1000℃with frequent grindings. Aothough these methods are adequate for the synthesis of X-ray pure specimens, it is often difficult to control the total oxygen content in the mixed valence systems because of the high temperature used in products of the formation of undesirable phases. Particle size reduction by milling can introduce chemical impurities into the ceramic product, which may severely influence the final properties of fluoride compounds.Soft chemical methods, such as the use of sol-gel precursors or molten salts as reaction media, have been adopted for the synthesis of oxide, but these methods involve often complex operating procedures. Hydrothermal technique has been widely applied to the synthesis of metastable phases and microporous solids. This method provides an attractive alternative to the synthesis fo complex oxides and fluorides, since it is carried out under relatively mild conditions (autogeneous pressure and～240℃) with controllable particle size distribution of the product. Hydrothermal method, involving heating metal salts, fluorides iin a liquid elevated temperature and pressure, offers an alternative synthesis route for the multicomponent ceramic oxides prepared using the traditional, high-temperature methods of solid-state chemistry: the use fo a solvent would permit rapid mixing fo several chemical elements, leading to homogeneous products, and also offers the potential for control of crystal growth leading to particles of desired morphologies, something rather difficult to achieve when using the high temperatures that must be employed in solid-state reactions. In particular, the hydrothermal method has great scope for the preparation of multinary fluoride phases, i.e. containing two ro more metals, where the rapid mixing fo the constituent elements would provide a great synthetic advantage, as well as potentially leading to the discovery of new materials. In this study, we focus on the hydrothermal crystal growth of fluoride compounds. In the system that has been prepared by traditional methods, a more soft synthesis route is explored and the results are compared with those obtained by other synthesis methods for the purpose to establish the relationship between the synthesis method and properties of final crystalline products.The complex fluorides K₂NaVF₆, (NH₄)₂VF6 and (NH₄)₂NaVOF₅ with elpasolite-type structure have been synthesized under mild hydrothermal conditions. The details of hydrothermal synthesis conditions deeply discussed. We know that V2O3 is good source of V. F anions is not only a reaction material but also act as a mineralizer in the process. We found that the reducing agent K2S2O3 and (NH₄)₂S2O3 played an ed an important role for the compounds of K₂NaVF₆ and (NH₄)₂NaVF6 respectively, and also they can bring A site cations for the target products. From the experiment we concluded that the reaction condition of compound K₂NaVF₆ as follow: the molar ratio of raw material is V2O3 :NaF :(NH₄)₂S2O3 :NH4HF2 :HF=0.5:1:2:0.1,the temperature is 180℃, the reaction time is 3 days. The synthesis reaction conditions of compound (NH₄)₂NaVF6 as follows: raw materials molar ratio of V₂O₃ :NaF :(NH₄)₂S₂O₃ :NH₄HF₂ :HF=0.5:1:1:7:0.1, reaction temperature is 180℃, the reaction time is 3 days.The chiolite-type compound of K₅V₃F₁₄ was prepared for the first time from hydrothermal system, and deeply discussed the molar ratio of the reaction materials, the reaction conditions: the reaction temperature is 180℃, reaction time is 3 days, during the synthesis processes, the reducing agent of K2S₂O₃ was the critical role because of the raw material of V₂O₃ can be easily oxide by oxygen.The hexafluoride vanadium(III) K₃VF₆, Na₃VF₆ and (NH₄)₃VF₆ with cryolite-type were prepared for the first time under the mild hydrothermal synthesis. Deeply discussed the molar ratio of the reaction materials, reaction temperature and reaction time that influence the products. Thermal stability of of (NH₄)₃VF₆ and (NH₄)₃FeF₆ were studies by TG-DTA in air. They decomposed in two steps at the first step and losses the compound of NH₄F, and the second step they loss NH₄F.The perovskite-type oxide compounds of AgMO₃(M = Nb, Ta), BaTaO₃ and the pyrochlore-type compound of Cd₂Ta₂O₇ were prepared for the first time under the hydrothermal synthesis. Deeply discussed the reaction temperature, molar ratio of the reaction materials and reaction time that influence the products. The photocatalytic properties of the two compounds of AgNbO₃ and AgTaO₃ were investigated, the results shown that they have good photocatalytic properties for photodegradation of dyes.