QUADRUPOLE INTERACTIONS AND VIBRATIONAL ANISOTROPY OF TETRAHEDRAL FE(III) IN THE 123-DERIVATIVE LNSR2CU2GA1-XFEXO7 (LN=Y, HO)

The layered copper oxides LnSr2Cu2Ga1-xFexO7 (Ln = Y, Ho) have been sy nthesized for 0 less-than-or-equal-to x less-than-or-equal-to 1. An or thorhombic structure (S.G. Ima2, a almost-equal-to 22.9 angstrom, b al most-equal-to 5.5 angstrom, c almost-equal-to 5.4 angstrom) forms in t he oxides prepared in Ar atmosphere for x rising up to 0.95. The trans ition from orthorhombic to tetragonal ''123'' structure (S.G. P4/mmm, a123 almost-equal-to 3.84 angstrom, c123 almost-equal-to 11.4 angstrom ) in the range of 0.6 < x < 0.95 is induced by heating in an oxygen fl ow. The Mossbauer study reveals that iron exhibits the predominant tet rahedral coordination in the orthorhombic phase, but the pyramidal coo rdinations in the oxygenated tetragonal LnSr2Cu2FeO7.5. The characteri stic Mossbauer quadrupole doublet associated with the tetrahedral ''Ga '' site turned out to be asymmetric suggesting an anisotropic thermal vibration of iron in this site. The anisotropy of recoil-free fraction is consistent with a large distortion of the tetrahedral site, where the EFG is parallel to the a direction of the unit cell. The negative sign of V(zz) and the large value of quadrupole splitting (\DELTAE(Q)\ almost-equal-to 1.93 mm/sec) suggest a similarity between the geometr y of FeO4 tetrahedra in both oxides LnSr2Cu2Ga1-xFexO7 and LnBa2Cu3-xF exO-y. (C) 1994 Academic Press, Inc.