STRUCTURAL ASPECTS AND ANTIFERROMAGNETIC ORDERING IN THE 123 DERIVATIVE LNSR2CU2GA1-XFEXO7-DELTA (LN=Y, HO)

The magnetic ordering in the orthorhombic phases LnSr2Cu2 Ga1-xFexO7 ( Ln = Y, Ho; 0.03 less-than-or-equal-to x less-than-or-equal-to 0.95) h as been investigated by Fe-57 Mossbauer spectroscopy. Iron occupies th e Ga site (Cu(1) site) for small doping rates (x almost-equal-to 0.03) , but occupies the Cu(1) and Cu(2) sites for iron-rich compositions (0 .5 less-than-or-equal-to x less-than-or-equal-to 0.95). For x = 0.9, t he magnetic ordering takes place below T(N) almost-equal-to 420 K in b oth the Cu(1) and Cu(2) sites. The down concave curve of the temperatu re dependence of the hyperfine field in the Cu(1) site suggests a rath er weak coupling between the Cu(1) and Cu(2) sites. The principal EFG axis V(ZZ) is directed in both sites along the longest axis of the ort horhombic cell, i.e., perpendicular to the alternating sheets. The sig n of V(ZZ) is positive for the Cu(2) sites (DELTAE(Q) = 0.6 mm/sec) an d negative for the Cu(1) sites (DELTAE(Q) = -1.9 mm/sec). In the disto rted tetrahedral site, the EFG tensor is non-axial, however, its asymm etry is small (eta < 0.4). The magnetic hyperfine field is perpendicul ar to the V(ZZ) axis for both sites; i.e., H lies in the plane of the shortest axes. A structural analysis by electron microscopy reveals th e presence of a large number of defects in the (Fe, Ga)O chain orderin g as well as in the basic structure. The presence and the abundance of these defects is in agreement with the Mossbauer observations. (C) 19 94 Academic Press, Inc.