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Mg4Nb2O6

Mg4Nb2O6

Figure 56. (a) Powder XRD patterns of the calcined MgNb2O6 powders prepared from a dual source after calcination at different temperatures and times. (b) TEM micrograph of the calcined MgNb2O6 powder, which shows the coexistence of a parasitic Mg4Nb2O9 phase. Reprinted with permission from [580], S. Ananta et al., J. Eur. Ceram. Soc. 19, 355 (1992). © 1992, Elsevier Science.

MgNb2(OEt)12(EtOH)2 (Fig. 57) containing Mg and Nb in a single molecule and appropriate ratio [278]. The Mg-Nb ethoxide has been successfully used to deposit stoichiometric films of MgNb2O6 [264]. The evaporation of MgNb2(OEt)12(EtOH)2 occurs in the range of 200-320 °C, which is comparable to the evaporation temperature of Pb(thd)2 (thd = 2,2,6,6-tetramethylheptane-3,5-dionate), showing that the volatilities of the two precursor systems (Mg-Nb and Pb) are well matched for liquid injection applications. This approach is of help, especially in the case of systems based on three or four different elements for which the precursor systems have not been investigated. The Mg-Nb bimetallic alkoxide is stable only at low deposition temperatures (<450 °C); at higher temperatures the coordinating EtOH ligands dissociate to leave an unsatu-rated four-coordinate Mg center that is highly susceptible to attack by oxygen. This leads to interaction between the lig-ands (liberated from the Pb(thd)2 precursor) to form Mg-thd species (e.g., Mg(thd)2 or EtOMg(thd)), which have a high thermal stability and lead to Mg-deficient films [264]. The reactivity of pure Mg-Nb alkoxide may be controlled by introducing chelating acetate ligands to obtain a modified acetato-alkoxide precursor, MgNb2(^-OAc)2(OPr')10 (Fig. 58) [323]. Similarly, Ba(Mg1/3Nb2/3)O 3 ceramics could

Figure 57. TGA data for (a) Mg[Nb(OEt)6]2-2EtOH and (b) Pb(thd)2. Reprinted with permission from [278], A. C. Jones et al., J. Mafer. Chem. 11, 544 (2001). © 2001, Royal Society of Chemistry.

Figure 55. Molecular structure of [{PPh3}2AgIn(SCOPh)4]. Reprinted with permission from [237], T. D. Devarajet al., Chem. Commun. 2304 (2001). © 2001, Royal Society of Chemistry.

Figure 57. TGA data for (a) Mg[Nb(OEt)6]2-2EtOH and (b) Pb(thd)2. Reprinted with permission from [278], A. C. Jones et al., J. Mafer. Chem. 11, 544 (2001). © 2001, Royal Society of Chemistry.

Figure 58. (a) Molecular structure of Mg{Nb(OEt)6}2(2EtOH). Reprinted with permission from [264], H. O. Davies et al., Adv. Mater. Opt. Electron. 10, 177 (2000). © 2000, John Wiley & Sons. (b) Molecular structure of MgNb2(^-OAc)2(O'Pr)10. Reprinted with permission from [323], L. G. Hubert-Pfalzgraf et al., Polyhedron 13, 1181 (1994). © 1994, Elsevier Science.

Figure 58. (a) Molecular structure of Mg{Nb(OEt)6}2(2EtOH). Reprinted with permission from [264], H. O. Davies et al., Adv. Mater. Opt. Electron. 10, 177 (2000). © 2000, John Wiley & Sons. (b) Molecular structure of MgNb2(^-OAc)2(O'Pr)10. Reprinted with permission from [323], L. G. Hubert-Pfalzgraf et al., Polyhedron 13, 1181 (1994). © 1994, Elsevier Science.

be prepared by a sol-gel method involving the reaction of the heterometallic MgTa2(OEt)12 with a barium precursor [582].

A SSP is not always a guarantee of control over stoi-chiometry, and heterometallic precursors may become a disadvantage when they disproportionate during the gas-phase transport, as observed for SrTa2(OR)12 compounds, because of the large disparity in the vapor pressure of the parent alkoxides [277]. Nevertheless, the replacement of simple (monoanionic) alkoxide ligands by chelating alkoxides (donor functionalized) offers the possibility of increasing the strength of the Sr-(O)R-Ta bridge. Jones et al. have used dimethyl aminoethanol (dmae) and bis(dimethylamino) iso-propanol (bis-dmap) to obtain novel Sr-Ta heteroleptic alkoxides, SrTa2(OEt)10(R)2 (R = dmae, bis(dmap)). It can be discerned from the solid-state structure of the dmap derivative (Fig. 59) that in contrast to SrTa2(OPri)12(PriOH)2, the Sr atom in SrTa2(OEt)10(bis-dmap)2 more highly coordinated (C. N. = 8) (Fig. 59). The use of SrTa2(OEt)10(dmae) in MOCVD has produced crystalline films of SrTa2O6 [277].

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