The alloy CuPt is one of the few that order into a L1{sub 1} crystal structure, i.e. planes of copper and planes of planes of planes of platinum perpendicular to the < 111 > direction. For disordered CuPt, the calculated Warren-Cowley short-range order parameter indicates an instability to concentration fluctuations with a wave-vector of ({1/2}, {1/2}, {1/2}), consistent with L1{sub 1} ordering. We show that this rare tendency is due to this ordering vector arising from the large joint density of states associated with L point and X point van-Hove singularities which lie near the Fermi energy.

Oxygen doped La{sub 2}CuO{sub 4+{delta}}, due to the unusually large mobility of the oxygen ions at high temperatures, is the only cuprate which has macroscopic phase separation of doped holes. We discuss our studies of La{sub 2}CuO{sub 4+{delta}}, including phase separation, distributed local structure whose onset coincides with-phase separation and the observation that in the presence of doped holes two distinct copper sites copper sites are generated, an observation which contrasts with results of diffraction studies. The superconducting transition temperature Tc in this material changes by over 10% in response to altered cooling history. Below a temperature T{sub f}, the situation becomes similar to other cuprates since oxygen is no longer sufficiently mobile to allow further macroscopic phase separation. In this regime the magnetic behavior of metallic La{sub 2}CuO{sub 4+{delta}} is conventional in context of cuprates. However {sup 139}La NMR spectroscopy has shown the local structure of the La-O layer to be very sensitive to the presence of doped holes and to be strongly temperature dependent in this same regime. This sensitivity is evident in the CuO{sub 2} planes where, in the presence of doped holes a second, distinct copper site is present. This provides an opportunity to explore the …