Niobate alkoxides

Lithium niobate is a ferroelectric material used as an optical switch. Preparation by the simple ceramic method leads to problems in obtaining the correct stoichiometry, and a mixture of phases often results. Several sol-gel preparations have been described, their advantage being the lower temperature required for the preparation and the greater homogeneity of the product. One such preparation starts with lithium ethoxide (LiOC2H5 (or LiOEt)) and niobium ethoxide Nb2(OEt)io. Each ethoxide was dissolved in absolute ethanol and the two solutions mixed. The addition of water leads to partial hydrolysis giving hydroxy-alkoxides, for example ... 

The hydroxy-alkoxides condense to form a polymeric gel with metal-oxygen-metal links. Lithium niobate is then formed when the gel is heated. Heating removes any remaining ethanol and any water formed during the condensation. The remaining ethyl groups are pyrolysed (i.e., oxidised to carbon dioxide and water) leaving the oxides. 

The sol-gel method of preparing lithium niobate used lithium and niobium alkoxides. Alkoxides are often used in CVD methods, but unfortunately for the preparation of lithium niobate, lithium alkoxides are much less volatile than niobium alkoxides and to get the two metals deposited together it is better to use compounds of similar volatility. One way around this problem is to use a more volatile compound of lithium. One reported synthesis uses a p diketonate of lithium in which lithium is coordinated to 2,2,6,6-tetramethylheptan-3,5-dione (Me3CCOCH2COCMe3) (Figure 3.11). 

For more than 30 years at Lomonosov Moscow State University, later at Karpov Institute of Physical Chemistry, and recently at the Swedish University of Agricultural Sciences, Uppsala (V. G. Kessler), the authors of this book have systematically studyiedthe synthesis and properties of homometallic alkoxides of Groups I-Vm of the Periodic Table, the bimetallic derivatives (alkox-oaluminates, -titanates, -zirconates, -niobates, -tantalates, -molybdates, -tung-... 

Some examples of application of metal alkoxides for synthesis of ferroe-lectrics such as BaTi03-based materials, complex niobates and tantalates, PZT andPLZT-materials, and high-temperature superconductors are discussed below. 10.1. Barium titanate and BaTiOj-based solid solutions... 

Application of metal alkoxides for the preparation of niobates and tanta-lates in the form of both powders and films has been first performed by Dand-liker [475]. In our study of hydrolysis of LiOEt andNb(OEt)5 solutions in anhydrous EtOH, it was demonstrated that although the freshly prepared powder is amorphous for X-ray, it already contains microcrystallites of LiNb03 [1778]. In addition to conventional techniques, the properties ofthe powder in the course of thermal treatment were studied by the second harmonic generation ofthe laser beam, which is a sensitive technique for detection ofnoncen-trosymmetric phase the first signal was registered at 350°C. 

All alkoxide routes appear to be limited to obtaining PNM [88a] and lithium or potassium niobate or tantalate [102a]. For the latter, the formulation of the mixed-metal alkoxide MM (OR)6, M = Nb, Ta, is in agreement with that of the material. Epitaxial thin films of LiNbOa on sapphire could be obtained [103]. The use of a mixed-metal MgNb2(OR)i2 species (R = Et [105], C2H40Me [104]) has been shown to promote the formation of the perovskite phase for PNM. 

Further to the synthesis of the OMS materials, researchers at Sandia National Laboratories reported the synthesis of a family of microporous niobates, general formula Na2Nb2 xM 06 x(0H)x.H20 (M = Ti, Zr x = 0.04-0.40), prepared from hydrothermal treatment of intimately mixed metal alkoxide precursors. The structure is made up of layers of edge-sharing octahedra interleaved with double ehains of edge-sharing niobate octahedra, in which additional, hydrated sodium eations reside. The niobate chains exhibit solid substitution. These SOMS (Sandia octahedral molecular sieves) demonstrate appreciable ion exchange eapacity and are selective for divalent cations. 

The Pechini method refers to an original process developed by Pechini (95) for the preparation of titanates and niobates for the capacitor industry. The method has since been applied to many complex oxide compositions (96,97). Metal ions from starting materials such as carbonates, nitrates, and alkoxides are complexed in an aqueous solution with a-carboxylic acids such as citric acid. When heated with a polyhydroxy alcohol, such as ethylene glycol, polyesterification occurs, and on removal of the excess liquid, a transparent resin is formed. The resin is then heated to decompose the organic constituents, ground, and calcined to produce the powder. The typical steps in the method are illustrated in Figure 2.34 for the preparation of SrTiOs powders (98). 

De G., Chatterjee A., Ganguli D. Zirconia fibres from zirconium n-propoxide-acetyleacetone-water-isopropanol system. J. Mater. Sd. Lett. 1990 9 845-846 Eichorst D.J., Payne D.A. Evolution of molecular structure in alkoxide-derived lithium niobate. 

Hashimoto T., Kamiya K., Nasu H. Strengthening of sol-gel-derived Si02 glass fibers by incorporating colloidal silica particles. J. Non-Cryst. Solids 1992 143 31-39 Hirano S., Hayashi T., Nosaki K., Kato K. Preparation of stoichiometric Cryst. lithium niobate fibers by sol-gel processing with metal alkoxides. J. Am. Ceram. Soc. 1989 72 707-709 Horikiri S., Tsuji K., Abe Y., Fukiu A., Ichiki E. US Patent 4,101,615 (1978)... 

Yogo et al. have reported a UV patterning process for alkoxy-derived lithium niobate film (Yogo et al, 1995). Benzoylacetone was selected as a ligand on an alkoxide precursor for the sensitive absorption of UV light. Lithium ethoxide and niobium ethoxide were used as starting materials. Patterned lithium niobate films with preferred c-axis orientation were crystallized on sapphire substrates by a heat treatment at 550 C. 

---