NEUTRON STUDY OF THE STRUCTURE AND CHEMICAL STABILITY OF (Y_1-xCa_x)(Ba_2-xLa_x)Cu₃O7-d. H. Rajagopal^*, A. Sequeira^*, R. Ganguly⁺, J. V. Yakhmi, (^*)Solid State Physics Division, (⁺)Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay - 400 085, India

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.