THE LIGAND COORDINATIONS AND VALENCIES OF THE PRIMARY IRON SITES IN YBA2CU3-XFEXOY

The now voluminous and often conflicting literature concerning the number and identification of the locations, valencies and spin quantum states adopted by iron when doped into the high-Tc superconductor YBa2Cu3Oy is analyzed. Although it now seems clear that the details depend on sample thermal history as well as stoichiometry, a considerable amount of experimental evidence pertaining to the problem appears to be at least qualitatively well established. By looking at the totality of this evidence a number of conclusions have been reached. It is clear that iron in YBa2Cu3-xFexOy always replaces copper in some general sense and can do so both in the unique Cu(2) site as well as in several, but not all, of the n-coordinated (2{slanted equal to or less-than}n{slanted equal to or less-than}6) ligand geometries which can be envisaged in an oxygen disordered Cu(1)-plane. The allowed ligand coordinations are identified and, by correlating them with their Mössbauer signatures, their population trends as functions of x and y are established. Associated evidence relating to the oxidation state and spin quantum number of iron at each topologically different location is assembled and analyzed. The preponderance of evidence points to the presence of a mixture of valencies and of both high and intermediate spin states. © 1990.