FORMATION OF NEW COMPOUNDS LN(4)AL(2)O(9) IN THE LN(2)O(3).AL2O3 SYSTEM (LN=LA, PR, TB)

The new compounds La4Al2O9, Pr4Al2O9, and Tb4Al2O9 were identified in the system 2Ln(2)O(3) . Al2O3. X-ray powder diffraction showed that th ey mere 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) (Lp = La, Ce, Pr, Nd, Sm, Ey, Gd). La4Al2O9:P2(i)/c, a = 7.97 Angstrom, b = 11.09 Angstrom, c = 11.41 Angstrom, beta = 108.8 degrees, Z = 4; Pr 4Al2O9: P2(1)/c, a = 7.80 Angstrom, b = 11.03 Angstrom, c = 11.31 Angs trom, beta = 109.6, Z = 4; and Tb4Al2O9:P2(1)/c, a = 7.40 Angstrom, b = 10.58 Angstrom, c = 11.16 Angstrom, beta = 107.2 degrees, Z = 4. The lanthanum and praseodymium compounds mere metastable phases in the Ln (4)Al(2)O(9) systems and transformed into LaAlO3 and La2O3, and PrAlO3 and PrO1.83 upon prolonged heating at 1200 degrees C, respectively. L n(4)Al(2)O(9) (Ln = La, Pr) were synthesized by precipitation of Al(OH )(4)(-) with Ln(3+)(aq) and subsequent heating at 900 degrees C for 10 h. Tb4Al2O9 required calcination at 1200 degrees C for 10 h. The form ation of La4Al2O9 and Pr4Al2O9 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 synthe sis route. Particularly, for Nd4Al2O9 this represented a significantly improved method.