Spherical ultrafine (average 30-50nm) codeposited Al
2O
3-TiO
2 powders were prepared by the oxidation of vapor mixture of AlCl
3 and TiCl
4 in a combustion flame. γ-Al
2O
3 and anatase and rutile TiO
2 were detected in the as-prepared powders, but β-Al
2TiO
5, which was one of the major phases in the Al
2O
3-TiO
2 powders prepared from AlBr
3 and TiCl
4 by r.f. plasma or in a tubular reactor, was scarcely observed. The simultaneous deposition of Al
2O
3 and TiO
2, leading to the formation of β-Al
2TiO
5, became difficult due to the adoption of less reactive AlCl
3. It appeared that the anatase TiO
2 particles were not accompanied by Al
2O
3, while the rutile TiO
2 crystallites dispersed in the Al
2O
3 contained noticeable amounts of Al
2O
3 metastably. The solution of Al
2O
3 into TiO
2 appeared to promote the phase change of TiO
2 from anatase to rutile. The calcination experiment was performed to investigate the stability of phases in the codeposited powders. The TiO
2 appeared to accelerate the formation of α-Al
2O
3 due to the solution of TiO
2 into γ-(or δ-)Al
2O
3. An unknown phase was detected in the powders calcined at 1200°C, whose X-ray diffraction pattern was similar to that of Na
2Ti
2Ti
6O
16. The new phase was considered to be a bronze-type compound containing H
+ and Al
3+, possibly expressed by H
2-X(Al
YTi
1-Y)
2Ti
6O
16-Z.
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