Low-Temperature Synthesis of Lanthanum Monoaluminate (LaAlO₃)

    * Ercan Taspinar and A. Cüneyt TAS, "Low-Temperature Chemical Synthesis of Lanthanum Monoaluminate," Journal of The American Ceramic Society, Vol. 80, 133-141 (1997).     (------> download pdf:  laalo3.pdf)

    * Ercan Taspinar and A. Cüneyt TAS, "Low-Temperature Chemical Synthesis of Lanthanum Aluminate," 2nd International Ceramics Congress, Istanbul, Turkey, Turkish Ceramic Society, October 24-28, 1994.
 

One of the promising candidates for ferroelectric substrate materials, LaAlO₃, was successfully synthesized by two separate chemical powder preparation techniques: (a) homogeneous precipitation from aqueous solutions containing urea in the presence of nitrate salts, and (b) self propagating combustion synthesis from aqueous solutions containing the respective nitrate salts of La and Al, and urea. The sub-micron, spherical-like particles of the precursors were completely converted to pure LaAlO₃ at 850° C in the homogeneous precipitation route, and the same conversion temperature was observed to be 750°C, which becomes the lowest temperature ever reported for the powder synthesis of a pure lanthanide monoaluminate phase. The materials were characterized by powder X-ray diffraction, simultaneous TG/DTA, scanning electron microscopy, and energy dispersive X-ray spectroscopy.

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              Crystal Structure of LaAlO₃
(Rhombohedral, R-3c, a=5.3619 and c=13.1045 Å (Hex. Parameters))

 

Structural refinements by Rietveld analysis showed that LaAlO₃ was iso-structural with BaTbO₃ and had the space group R-3c in contrast to the previously assumed space group of R-3m for this phase. The significant structural differences between the two spa ce groups could be summarized as follows; while the R-3m structure would place Al ions at -3m sites, La ions at 3m sites, and oxygen ions at m sites, the R-3c structure would have Al ions at -3 sites, La ions at 32 sites, and oxygen ions at 2 sites. The a tomic positions in the structure of LaAlO₃ were refined and presented for the first time with respect to the present space group. The atom positions in the hexagonal settings of the LaAlO₃ unit cell are 2La^{3+ :} 0.0 (0.25), 0.0 (0.25), 0.25 (0.25), 2Al³⁺ : 0.333 (0.0), 0.667 (0.0), 0.667 (0.0), and 6O^2- : 0.167 (-0.295), 0.333 (0.769), 0.583 (0.25). The positions in the rhombohedral settings are given in parentheses for the x, y, and z fractions.