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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 effect of Pr on the superconductivity in (Pr x Y 1−x )Ba 2 Cu 4 O 8 were studied and compared with that in (Pr x Y 1−x )Ba 2 Cu 3 O 7 . Transition temperatures of the (Pr x Y 1−x )Ba 2 Cu 4 O 8 compounds decrease with increasing Pr content x, although the rate of decrease is slower than that of (Pr x Y 1−x )Ba 2 Cu 3 O 7 . Thus the Pr has a weaker effect to the superconductivity suppression in (Pr x Y 1−x )Ba 2 Cu 4 O 8 than in (Pr x Y 1−x )Ba 2 Cu 3 O 7 . From the analysis of X-ray absorption spectra(XAS) near O K-edge in (Pr x Y 1−x )Ba 2 Cu 4 O 8 and in (Pr x Y 1−x )Ba 2 Cu 3 O 7 , the effect of Pr substitution on the O 2p electronic state is found to be weaker for (Pr x Y 1−x )Ba 2 Cu 4 O 8 , which is parallel to the effect on superconductivity. These results were well explained with the hole localization model based on the hybridization of Pr 4f and O 2p orbitals.

The compounds of (Pr x Y 1−x )Ba 2 Cu 4 O 8 (x=0, 0.1 and 0.6) were prepared in the oxygen atmosphere of 33 atm using a electrical furnace, where the materials were annealed at 930°C for 48 h. Magnetic and electrical measurements were performed: Transition temperatures for compounds of x=0, 0.1 and 0.6 were 80 K, 75 K and 44 K, respectively. The critical content X c , at which superconductivity disappears, was estimated to be 0.9. Effective moment and the Pauli paramagnetic component of Pr 0.6 Y 0.4 Ba 2 Cu 4 O 8 were 3.05±0.15 and (1.3±0.2) x 10 -6 cm 3 /g, respectively.

The electronic states of a tetrahedral cluster of a Fe ion and four S ions are calculated in order to obtain the electronic structure around a Fe ion in I-III-VI 2 compounds. The LCAO-MO scheme is used with account of configuration interactions (CI). The ground state is found to have 6 A 1 symmetry and two 6 T 2 states, to which allowed transitions are expected, are obtained in an energy region below the fundamental absorption edge, in agreement with experiments. It is remarked that the ground 6 A 1 state arises by no means from the pure atomic d 5 configuration: mixing of charge transferred configurations is quite large. The energies of quartet states ( 4 T 1 , 4 T 2 ) are lower than those of the corresponding sextet states ( 6 T 1 , 6 T 2 ) in harmony with the observed optical spectra. Account of CI is essential to the findings.

The electronic structure and spin-polarisation of five-layer vanadium (100) thin films have been calculated using the self-consistent-charge spin-polarised discrete variational X alpha method. The calculations were made for the three thin-film spacings between the surface and the inner adjacent layers at the bulk value, at a 10% contracted value and at a 10% expanded value. The bandstructures the layer density of states and the charge and spin densities are presented. It is shown that the spin polarisation appears on the surface layer and that the magnetic moment is more than 0.2 mu B for each atom on the surface. The surface relaxation affects the distribution of the spin polarisation on the inner layers as well as on the surface layer.

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 band structure of a ferromagnetic semiconducting spinel Hg${\mathrm{Cr}}_{2}$${\mathrm{Se}}_{4}$ is self-consistently calculated by using the discrete variational $X\ensuremath{\alpha}$ method. The overall features of the band structure are quite similar to those of Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$ and Cd${\mathrm{Cr}}_{2}$${\mathrm{Se}}_{4}$ which have been calculated in the first paper of this series [T. Oguchi, T. Kambara, and K.I. Gondaira, Phys. Rev. B 22, 872 (1980)]. The fundamental energy gap is 1.8 eV, somewhat narrower than those of the Cd compounds. The narrowest band gap appears between the $d\ensuremath{\epsilon}$ and $d\ensuremath{\gamma}$ bands, and its width is 0.4 eV, much smaller than those of the Cd compounds. The top of the valence bands has the ${\ensuremath{\Sigma}}_{4}$ symmetry and the bottom of the conduction bands ${\ensuremath{\Gamma}}_{1}$. The spin polarization of the $3d$ orbitals of Cr is about 3.5, of which 0.5 comes from the $3d$ components mixed in the valence bands, while the spin polarization of the $4p$ orbitals of Se has the opposite sign, the magnitude of which is about 0.3.

The effect of antiferromagnetic ordering on the optical absorption edge in alpha -MnS is studied experimentally and theoretically. The absorption edge (3.33 eV) is conclusively determined from the dispersion of the interference fringe in a thin platelet crystal. The edge shows a blue shift of 0.22 eV on decreasing the temperature from room temperature to 4.2K. The electronic structures of MnS in both paramagnetic and antiferromagnetic phases are calculated by using the self-consistent-charge extended Huckel method in order to clarify the nature of the transition and the mechanism of the edge shift. The observed blue shift can be reasonably explained by a model which attributes the edge to a transition corresponding to the magnetic exciton constructed with a hole in the 3d orbitals and an electron in the 4p orbitals of Mn ions.

The electronic band structures of ferromagnetic Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$ and Cd${\mathrm{Cr}}_{2}$${\mathrm{Se}}_{4}$ are self-consistently calculated by using the discrete variational $X\ensuremath{\alpha}$ method. The general features of the band structures are quite similar between sulfide and selenide; each structure consists of relatively narrow valence bands, fairly wide conduction bands, and very narrow $d$ bands. The $3d\ensuremath{\epsilon}$ and $3d\ensuremath{\gamma}$ bands for up spin lie in the energy region near the top of the valence bands and around the bottom of the lowest conduction band, respectively, and both $d$ bands for down spin fall in the conduction bands. The maximum point of the valence bands has ${\ensuremath{\Sigma}}_{4}$ symmetry for both compounds, and the minimum point of the conduction band has ${\ensuremath{\Lambda}}_{1}$ for sulfide and ${\ensuremath{\Gamma}}_{1}$ for selenide. The fundamental energy gap at the $\ensuremath{\Gamma}$ point is 2.6 eV for sulfide and 2.3 eV for selenide. The spin polarization of the $3d$ orbitals of Cr is about 3.5, in which 0.5 comes from the $3d\ensuremath{\gamma}$ components mixed with the valence bands, while the spin polarization of the outermost $p$ orbitals of chalcogen ion has the opposite sign, the magnitude of which is about 0.3.

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 structure of a ferromagnetic chromium spinel CuCr2S4 is calculated self-consistently by using the discrete variational X alpha method. The main features of the band structure are: relatively narrow valence bands constructed mainly from the S 3p orbitals lie in the lower energy region; very narrow bands having mainly d character range over the intermediate energy region and a few d bands with only majority spin cross the Fermi level, which corresponds to the p-type metallic conduction observed; true conduction bands composed of Cu and Cr 4s, 4p orbitals lie in the higher energy region. The valency of each ion obtained from the self-consistent calculation is Cu1.3+(Cr1.55+)2(S1.11-)4 and the atomic spin-polarisation is 3.2, -0.32 and -0.27 for Cr, Cu and S, respectively.

Several transition-metal compounds change their volume abruptly under high pressure through their electronic spin-state transition. The pressure-induced spin-state transitions in transition-metal compounds are studied on the basis of the ligand-field theory by using the model in which the coupling of transition-metal ions with crystal distortions is taken into account. The various types of transitions occur due to the crystal distortion induced by pressure, that is, the continuous and discontinuous high-spin (HS) to low-spin (LS) transitions with increasing pressure in the weak-crystal-field compounds, the continuous and discontinuous LS to HS transitions and the multiple LS to HS to LS to HS transition in the strong-crystal-field compounds. The pressure against volume curve shows various features of changes due to the crystal distortion associated with the spin-state transitions. The observed pressure against volume curve of haematite Fe2O3 is well explained by the discontinuous HS to LS transition of Fe3+ ions.

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.

The electronic band structure of chalcopyrite CuFe${\mathrm{S}}_{2}$ in the antiferromagnetic phase is calculated by spin-polarized self-consistent-charge discrete-variational $X\ensuremath{\alpha}$ method. The valence bands consist of many rather narrow bands constructed from S $3p$, Cu $3d$, and majority-spin Fe $3d$ orbitals. The upper valence bands are constituted mainly of Cu $3d$ and Fe $3d$ orbitals. The top of the valence band is at the $X$ point. The conduction bands composed mainly of $4s$ and $4p$ orbitals of Cu and Fe have the bottom at the $\ensuremath{\Gamma}$ point and the direct gap is 3.1 eV. The narrow bands composed mainly of minority-spin Fe $3d$ orbitals are in the so-called fundamental gap. The direct minimum gap of these $d$ bands is 0.7 eV at the $X$ point. These gap energies are in good agreement with the observed optical spectra. The minimum gap of the minority-spin $d$ bands is indirect and about 0.3 eV. The magnetic moment of an Fe ion is $3.88{\ensuremath{\mu}}_{B}$, which agrees with the observed value.

The spin-polarized electronic structures of the two 6-layer (100) slabs, Ni 1 Al 5 and Ni 2 Al 4 are determined by using the discrete-variational X α method. The results show that the magnetic moment of the Ni layers in contact with Al is noticeable reduced but not completely suppressed.

The electronic band structures of ferromagnetic semiconductors CdCr2S4, CdCr2Se4, and HgCr2Se4 are calculated self-consistently by the DV-Xα method. The general features of the band structures are quite similar for three substances except for the relative positions of the 3d bands: Each structure consists of relatively narrow valence bands, fairly wide conduction bands, and very narrow d bands. The d ε and dγ bands for up-spin lie in the energy region near the top of the valence bands and around the bottom of the conduction bands, respectively, and both d bands for down-spin fall in the conduction bands. The fundamental energy gap at the Γ point is 2.6, 2.3 and 1.8 eV for CdCr2S4, CdCr2Se4, and HgCr2Se4, respectively. The spin polarization of the Cr-3d orbitals is about 3.5, while the spin polarization of the valence p orbitals of a chalcogen ion has the opposite sign.

Electronic band structures of CuGaS2 and CuAlS2 are selfconsistently calculated by using the discrete variational Xα method and the results are compared with the optical data. The contents of the valence and core bands are reasonably coincident with the XPS mesurements. The top of the valence bands is an admixture state of Cu3d and S3 p orbitals. The crystal-field splitting of the uppermost valence band is about 0.2 eV. The fundamental energy gaps are direct transitions from \varGamma4 to \varGamma1.