“…This transition has been studied in a number of photoemission measurements, which show the disappearance of the DOS at the Fermi level as the transition is approached, with a corresponding shift in valence-band and core-level peaks (e.g. van Veenendaal, Sawatzky & Groen, 1994;van Veenendaal, Schlatmann, Sawatzky & Groen, 1993;Itti et al, 1991;Golden, Golden, Egdell & Flavell, 1991). The valence modulation caused by the addition of Y is accompanied by structural modifications, primarily contraction along the c axis of the structure, and expansion in the ab plane, which contains the Cu-O2 planes, with corresponding changes in Cu-O hybridization (Manthiram & Goodenough, 1988;Tamegai et al, 1989;Groen et al, 1990).…”