Combustion synthesis and bulk thermal expansion studies of Ba and Sr thorates

Nanocrystalline BaThO3 and SrThO3 powders have been prepared at a relativel y low calcination temperature by a gel combustion technique using citric ac id as a fuel and the nitrate as an oxidizer. These powders resulted in high ly dense (sintered density > 90% of th.d.) pellets on sintering at 1923 K. The orthorhombic lattice parameters of BaThO3 were found to be as a = 0.635 (1), b = 0.6387(9) and c = 0.8995(9) nm whereas SrThO3 was monoclinic with a = 0.6319(4), b = 0.3240(1), c = 0.4928(3) nm and beta = 117.38(5)degrees It was observed that the average linear thermal expansion coefficients, <(a lpha)over bar>, for BaThO3, SrThO3 and ThO2 are 11.09 x 10(-6), 10.49 x 10( -6) and 9.10 x 10(-6) K-1, respectively, which ale about 21.9% and 15.3%, r espectively, higher than that of pure ThO2 between room temperature and 112 3 K. (C) 2000 Elsevier Science B.V. All rights reserved.