NEUTRON STUDY OF THE STRUCTURE AND CHEMICAL STABILITY OF
(Y1-xCax)(Ba2-xLax)Cu3O
It has been shown recently that appropriate amounts ( x [[circleplus]] 0.4 ) of cation substitutions in the title compound make it remarkably corrosion resistant and retain its Tc above 80K. An orthorhombic to tetragonal transition also occurs at x = 0.4. We have made a comparative structural study of three samples ( x = 0.0 , 0.2 and 0.4 ) with Tc values of 93K, 80K and 79K respectively. Influence of cation substitution on the structural parameters as deduced by neutron Rietveld analysis is reflected in Table below:
Sample x = 0.0 x = 0.2 x = 0.4
Sp.Group Pmmm Pmmm P4/mmm
a (Å) 3.8190(9) 3.8192(8) 3.8480(6)
b (Å) 3.8826(9) 3.8773(8) 3.8480(6)
c (Å) 11.6892(35) 11.6590(29) 11.6575(35)
O-content 6.97 6.95 7.00
Cu(2) Valence 2.17 2.18 2.18
Cu(1)-O(4) (Å) 1.895(7) 1.885(6) 1.855(8)
Cu(2)-O(4) (Å) 2.245(9) 2.271(7) 2.299(9)
]O(2)-Cu(2)-O(2) 163.0deg. 165.1deg. 167.4deg.
Ba-O (avg) (Å) 2.862 2.856 2.841
La3+ is observed to substitute at Ba2+ site and Ca2+. at Y3+ site. As x goes up, the bridging oxygen O(4) moves away from Cu(2) towards Cu(1), thus decreasing the puckering of CuO2 sheets. This could be the cause of improved chemical stability.