Lanthanum pyrochlores and the effect of yttrium addition in the systems La 2−xY xZr 2O 7 and La 2−xY xHf 2O 7

Abstract

The crystal structures of the compounds La 2− x Y x Zr 2O 7 and La 2− x Y x Hf 2O 7 with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La 0.8Y 1.2Zr 2O 7 and La 0.4Y 1.6Hf 2O 7, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48 f ( x, 3 8 , 3 8 ) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases.