  DTA measurements with mixtures of aluminum oxide and lutetium oxide around
the 1:1 perovskite composition were performed up to 1970 deg. C. A peak with
onset 1901 deg C was due to the melting of the eutectic Lu4Al2O9 (monoclinic
phase) and LuAlO3 (perovskite). Neither peritectic melting of the perovskite
nor its decomposition in the solid phase could be resolved experimentally. The
maximum of the eutectic peak size is near x=0.44, on the Lu-rich side of the
perovskite, which is consistent with the conclusion that LuAlO3 melts
peritectically at ca. 1907 deg. C as proposed by Wu, Pelton, J. Alloys Compd.
179 (1992) 259. Thermodynamic equilibrium calculations reveal, that under
strongly reducing conditions (oxygen partial pressure <10^{-13} bar)
aluminum(III) oxide can be reduced to suboxides or even Al metal. It is shown
that under such conditions a new phase field with liquid Al can appear.
