Using Ewald's method, we calculated the ion-site potentials and Madelung energy of Y,Ba2Cu90, (orthorhombic, Pmmm, No. 47). We considered the effects of copper-ion and oxygen-ion valences. Among seven suggested copper-oxygen ion-charge contigurations, only two give a low electrostatic bonding energy to agree with oxygen vacancies on 0 ( 1) sites. Only one configuration gives a realistic bulk modulus. We perturbed this configuration by equalizing the Cu ( 1) -Cu (2) valences and introducing a hole into the CuOz plane at the oxygen sites. This configuration also agrees well with observation.
Several authors reported ionic-model calculations of lattice properties of the new high-T, metal-oxide superconductors.
Iguchi and Yonezawa [ 1 ] calculated ion-site Madelung potentials, which determined the most stable Cu3+ site and showed the remarkable stabilization of O*-in Y,Ba2Cu30, compared with Y,BazCu30s. Wright et al. [ 21 focused on forces in YIBazCuXO, and found that mechanical stability imposes constraints on coppercharge distribution. Kress et al. [ 31 predicted phonon-dispersion and phonon-density-of-states curves and concluded that interatomic interactions in Y,Ba2Cu307 resemble those in perovskites and other oxides. Further, they concluded that the ionic polarizability suggests strong electron-phonon interactions. Cohen et al. [4] considered lattice dynamics in La2Cu04 and Y ,Ba2Cu30, and reached similar conclusions: important ionic contributions and large electron-phonon matrix-element enhancements. Also, they failed to find evidence for Cu3+. Kondo et al. [ 5 ] used Madelung-energy calculations to find the hole-carrier distribution among lattice sites. They found that the holes enter the Cu02 sheets. Torrance and Metzger [ 6 ] focused on the Cu-0 Madelungsite-potential difference to separate semiconductors and superconductors for a wide class of copper oxides. Feiner and de Leeuw [ 71 considered the Madelung energy to determine hole distribution between the Cu02 planes and other structural subunits.