Preparation and Optical Properties of Polycrystalline Aluminum Germanate
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PREPARATION AND OPTICAL PROPERTIES OF POLYCRYSTALLINE ALUMINUM GERMANATE S. Prochazka, G. A. Slack INTRODUCTION Aluminum germanate, the germanium analog of mullite, 3 AlO203 2 GeO 2, was first prepared by Gelsdorf, Muller-Hesse and Schweite (Ref. 1), who demonstrated that both partial and complete substitution of SiO2 by GeO 2 in mullite was possible. They also determined the lattice constants of the solid solutions. The following studies reported some physical properties of aluminum germanate such as density (Ref. 2) optical constants (Ref. 2) crystal structure (Ref. 3) and I. R. absorption (Ref. 4). Phase equilibria in the system AlO 3 -GeO^ were investigated by Miller et al. (Ref. 5) and Perez-y-Jorba (Ref. 6). The ratter author found several compounds of which germanium mullite was the most stable and showed a relatively wide compositional range. Miller et al. observed only 3 Al0. . 2 GeO2 with no evidence of solid solutions. The compound was reported to melt incongruently at 1530 0 C. Recently Yamaguchi et al. (Ref. 7) prepared aluminum germanates of a wide range of stoichiometry from alkyl oxides and found, in addition to germanium mullite, the compound AlO -2GeO which was stable between 1190 and 1310 C. The authors also report in~rared absorption spectra. Germanium mullite has recently attracted interest as an electromagnetic window material because it is expected to possess transmittance further into the infrared than mullite. So far, however, except for i.r. powder data by Muller-Hesse (4) and Yamaguchi (7),no information on optical absorption has been published. The objective of the present work was to prepare polycrystalline 3 A13 03 . 2 GeO samples sufficiently dense to make possible infrared transmission measuremenis on macroscopic specimens. The preparation of aluminum germanate in polycrystalline form is complicated by the volatillity of GeO that precludes the use of high temperatures necessary for its synthesis from oxides in open systems. In addition GeO 2 reduces easily to GeO or Ge at elevated temperatures, thus making the use of hot-pressing impractical. These constraints make the synthesis via amorphous coprecipitated precursors a very attractive approach. PRECURSOR PREPARATION Hydrolysis of alkyloxides, a procedure used previously for aluminum silicate compositions (7, 8, 9) and many other compounds, was the selected route. Amorphous GeO 2 is soluble in water and, therefore, a nonaqueous medium was preferred for the hydrolysis. The use of alcohols, however, resulted in the formation of insoluble complexes at room temperature; cyclohexane was finally found to be a convenient solvent. In the preparation procedure germanium ethoxide (Thiokol Co.,Danvers, Mass.) and redestilled aluminum isopropoxide (Chattem Chem. Inc., Chattanooga, Tenn.) were dissolved separately in cyclohexane. The oxide concentration was determined gravimetrically, and the solutions were mixed to obtain the required stoichiometry. The mixture was diluted with the solvent to contain about 5% oxides and were hydrolyzed at room t
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