Lanthanum substitution for barium in YBa 2Cu 3O 9−δ

Abstract

A detailed mapping is given for the existence range of the Y(Ba 1− y La y ) 2Cu 3O 9−δ solid solution phase with respect to y and δ. The findings are presented in the tetrahedral phase diagram of the Y(O) Ba(O) La(O) Cu(O) system. All samples were carefully prepared by citrate methods and gettering techniques giving high resolution in the degree of substitution y and oxygen content 9 − δ. The upper substitution limit of y = 0.36(2) can notably be exceeded if one at the same time allows substitution of Y by La, viz., by extending the phase region to include (Y 1− x La x )(Ba 1− y La y ) 2Cu 3O 9−δ. For Y(Ba 1− y La y ) 2Cu 3O 9−δ, the lower limit for the oxygen content 9 − δ increases strongly with y, from 6.00(3) for y = 0.00 to, say, 6.45(3) for y = 0.20. The upper limit is approximately given as 9 − δ = 6.95 + y O,T [0.00 < y < 0.36(2)], i.e., the maximum formal Cu valency remains constant. Hence, oxygen contents well above seven per formula can be achieved, and for such samples the crystal symmetry eventually turns tetragonal, as seen by X-ray and neutron diffraction. A three-dimensional representation of the degree of orthorhombic distortion together with the parameters y and 9 − δ is made in the range where orthorhombic symmetry is adopted. For fully oxygenated samples (saturation at 340°C; P O 2 = 100 kPa; Cu valence constant: 2.30(1) according to iodometry), the symmetry change occurs at y O,T = 0.140(5); 9 − δ = 7.10. At the somewhat lower oxygen contents between 6.9 and 7.0, the domain of the orthorhombic state extends, e.g, to y O,T = 0.160(5) for 9 − δ = 6.98. An interesting consequence of this is that oxygen rich samples from the intermediate composition interval 0.14 < y < 0.16 undergo the phase transition sequence tetragonal to orthorhombic to tetragonal upon thermal removal of oxygen.