The new compounds La 4 Al 2 O 9 , Pr 4 Al 2 O 9 , and Tb 4 Al 2 O 9 were identified in the system 2Ln 2 O 3 . Al 2 O 3 . X‐ray powder diffraction showed that they were isomorphous with the previously characterized Ln 4 Al 2 O 9 (Ln = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Y) and Ln 4 Ga 2 O 9 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd). La 4 Al 2 O 9 : P2 1 /c, a = 7.97 Å, b = 11.09 Å, c = 11.41 Å, β= 108.8°, Z = 4; Pr 4 Al 2 O 9 : P2 1 /c, a = 7.80 Å, b = 11.03 Å, c = 11.31 Å, β= 109.6, Z = 4; and Tb 4 Al 2 O 9 : P2 1 c, a = 7.40 Å, b = 10.58 Å, c = 11.16 Å, β= 107.2°, Z = 4. The lanthanum and praseodymium compounds were metastable phases in the Ln 4 Al 2 O 9 systems and transformed into LaAlO 3 and La 2 O 3 , and PrAlO 3 and PrO 1.83 upon prolonged heating at 1200°C, respectively. Ln 4 Al 2 0 9 (Ln = La, Pr) were synthesized by precipitation of A1(OH) 4 − with Ln 3+ ( a q) and subsequent heating at 900°C for 10 h. Tb 4 AI 2 O 9 required calcination at 1200°C for 10 h. The formation of La 4 Al 2 O 9 and Pr 4 Al 2 O 9 was followed by TGA‐DSC, which was also used to assess their thermal stability. Several other members of the above family of compounds were similarly synthesized by the new synthesis route. Particularly, for Nd 4 Al 2 O 9 this represented a significantly improved method.
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Dohrup et al. (1996) studied this question.