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(Y 1- x Pr x )Ba 2 Cu 4 O 8 compounds with x =0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 were synthesized using a high-pressure oxygen technique. The solubility limit x s in this compound system was found to be 0.7. In this composition range, (Y 1- x Pr x )Ba 2 Cu 4 O 8 compounds have the YBa 2 Cu 4 O 8 (124) structure and the lattice parameters a , b and c increase with increasing Pr content x . The superconducting transition temperature T c decreases monotonically with increasing x , and the critical content x c , where superconductivity disappears, has been estimated to be 0.8. There is an obvious effect of Pr substitution on superconductivity, although the effect is smaller in the 124 system than that in the 123 system. X-ray diffraction results suggest that Pr is almost 3+ in this system. A model for the Pr substitution is suggested to make holes immobile to suppress superconductivity as the result of the hybridization of O 2p π with Pr 4f orbitals.

Pr 3d and 4d core levels have been studied for ${\mathrm{Y}}_{0.4}$${\mathrm{Pr}}_{0.6}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ and ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ using x-ray-photoelectron spectroscopy to elucidate the valence states. The features of Pr 3d and 4d are close to those for ${\mathrm{Pr}}_{2}$${\mathrm{O}}_{3}$ for both compounds, while they are different from those for ${\mathrm{Pr}}_{6}$${\mathrm{O}}_{11}$. It is concluded that praseodymium is almost trivalent with a 4${\mathit{f}}^{2}$ configuration.

The polarization dependence of x-ray absorption at the oxygen 1 s absorption edge in untwinned LuBa 2 Cu 3 O 7 and PrBa 2 Cu 3 O 7 single crystal specimens was measured. The hole states of oxygen in the CuO 2 plane in PrBa 2 Cu 3 O 7 locate at higher energy position than those of the oxygen in the CuO 2 in superconducting LuBa 2 Cu 3 O 7 , although the hole states of chain sites remain at the same energy position in both Pr123 and Lu123 samples. Although they are mainly of pσ character rather than of pπ and were attributed as an upper Hubbard band, a possibility was pointed out that a trace of pπ character hole states in the CuO 2 plane exists in Pr123. The formation of Zhang-Rice singlet is suppressed by pπ holes in the CuO 2 plane on account of the strong Coulomb interaction on an oxygen site.

The electronic structures and binding energies of a nitrogen atom adsorbed at three symmetry sites on the Ta(100) and (110) surfaces are obtained by means of the extended Huckel molecular orbital method. The Ta surfaces are represented by finite arrays of Ta atoms and the repulsive energy between nitrogen and Ta is calculated by using Anderson's approximation [A.B. Anderson and R. Hoffmann, J. Chem. Phys. 60 (1974) 4217]. The most stable site for nitrogen adsorption is found to be the five coordination number site for the (100) surface and the two coordination number site for the (110) surface. The calculated binding energies for the most stable sites are close to the value evaluated from the observed chemisorption heat. The difference in the binding energy between the adsorption sites on the (110) surface is fairly small as compared with the difference on the (100) surface. The charge of adsorbed nitrogen and the equilibrium bond distance between adatom and the nearest neighbor Ta atom do not vary sensitively with adsorption site: the charge is about −0.25¦e¦ and the bond distance is about 3.1 A.

The electronic structure of antiferromagnetic chromium (100) thin films with bulk interlayer distance and surface relaxation is calculated by the spin-polarised self-consistent-charge discrete variational X alpha method. The electronic bandstructures, the local density of states, atomic charges and spin polarisations of each of the atoms, including the spatial distributions and their dependence on the surface relaxation, are presented. The magnetic moments on the surface, second and third layers are 2.57, 0.96 and 0.64 mu B respectively. The surface spin polarisation decreases with surface contraction and increases with surface extension.

The electronic band structures of the paramagnetic and ferromagnetic phases of chromium spinels CdCr2S4 and CdCr2Se4 are calculated by the use of the extended Huckel method and assignments of the optical spectra are given.