Synthesis in solid state chemistry : use of sol-gel and hydrothermal techniques for the preparation of molybdates and silicophosphates

Abstract

Several non-classical methods of synthesis were used for the preparation of transition metal molybdates and sodium zirconium silicophosphates. In addition to synthesizing new compounds in each group, the results were used to better explain the formation, stoichiometry, and structure of these materials. A series of hydrated manganese molybdates formed by reacting aqueous solutions of manganese sulfate and sodium molybdate followed by refluxing. Three of these products were determined to be MnMoO4*H2O with different structural characteristics. Hydrothermal treatment of these solutions led to NaMn2(OH)(H20)(MoO4)2 or the high pressure form of MnMoO4 depending on reaction time. Reactions involving Ni produced the isomorphous form of the sodium manganese molybdate phase. Similar reactions involving Cu led to the formation of Cu3(MoO4)2(OH)2 and NaCu(OH)MoO4. A different approach to hydrothermal synthesis involved the reaction of CuO and MoO3 in H2O which formed C4Mo6O20. The silicophosphates that were studied belonged to the NASICON (Na Super Ionic Conductor) system with the general formula Na[1+x]Zr2Si[x]P[3-x]O12. Stoichiometric and nonstoichiometric compositions in this series were prepared by the sol-gel technique. Attempts to prepare Na3Zr2Si2PO12 resulted in the formation of a single phase. However, heating the product above 1100 °C showed a slight deficiency of ZrO2. In addition, a segregation was observed between the Si and P atoms in the two possible crystallographic positions. Synthesis of Na3.3Zr1.65Si1.9P1.1O11.5 led to a single phase. Structural studies ruled out the placement of any Na+ ions in the Zr4+ sites as had been observed for the product of hydrothermal reactions. These structural differences between the sol-gel products and those of other techniques may be explained in terms of different phase transformations which occur during the heat treatment...