The structures of the precursor Hf(OnBu)4 and its modification in solution:: EXAFS-investigation in combination with XANES- and IR-spectroscopy

The solution structure of hafnium-n-butoxide Hf(O"Bu)(4), an important precursor for the sol-gel preparation of hafnium oxide containing materials, was studied in solution in pure toluene, and in presence of acetylacetone, iso-propanol and tetrahydrofuran by EXAFS-, XANES- and IR-spectroscopy. The combination of these three methods and the additional measurements and discussions of well known solid state references allow us to identify structural motives and to gain a detailed picture of the structures formed in solution without any a priori knowledge about these structures. The dimeric structure of crystalline Hf(O"Bu)(4) is not preserved in solution and a trimeric cyclic structure is formed. While the addition of (HOPr)-Pr-i and THF does not induce any change of the formed complexes, one equivalent of Hacac reduces the degree of aggregation by the partial formation of dimers. In the presence of two and three equivalents of Hacac, solely monomeric species are formed. The obtained structural parameters are used to establish for the first time an empirical correlation of a spectral feature in the XANES region with the average oxygen coordination number.