Hiroshi Mizoguchi

The electronic structures of the antifluorite‐type compound Mg₂Si is described in which a sublattice of short cation–cation contacts creates a very low conduction band minimum. Since Mg₂Si shows n‐type conductivity without intentional carrier doping, the present result indicates that the cage defined by the cations plays critical roles in carrier transport similar to those of inorganic electrides, such as 12 CaO⋅7 Al₂O₃:e⁻ and Ca₂N. A distinct difference in the location of conduction band minimum between Mg₂Si and the isostructural phase Na₂S is explained in terms of factors such as the differing interaction strengths of the Si/S 3s orbitals with the cation levels, with the more core‐like character of the S 3s leading to a relatively low conduction band energy at the Γ point. Based on these results and previous research on electrides, approaches can be devised to control the energy levels of cation sublattices in semiconductors.

In view of the strong electron-donating nature of H^– and extensive vacancy formation in metals by hydrogen insertion, a series of LnH_2+x (Ln = La, Ce, or Y) compounds with fluorite-type structures were verified to be the first hydride-based electride, where itinerant electrons populating the cage are surrounded by H^– anions. The electron transfer into the cage probably originates from Ln–cage covalent interaction. To the best of our knowledge, anion-rich electrides are extremely rare, and a key requirement for their formation is that the cage site is not occupied by lone pair electrons of the adjacent ions. In the case of LnH₂, the cage site is surrounded by eight H^– anions with isotopic electronic character caused by the lack of mixing of H p-orbital character. Notably, Ru-loaded LnH_2+x electride powders synthesized by hydrogen embrittlement (Ln = La or Ce) were found to work as efficient catalysts for ammonia synthesis at ambient pressure, without showing serious signs of hydrogen poisoning. There are several possible origins of the observed high catalytic activity in the hydride promotors: the small work function of LnH_2+x derived from the covalent interaction between Ln cation and the H^– σ donor, and the formation of Ln nitride during catalytic reaction.

Using a high pressure technique and the strong donating nature of H⁻, a new series of tetragonal La₂Fe₂Se₂O₃-type layered mixed-anion arsenides, Ln₂M₂As₂H_x, was synthesized (Ln=La or Sm, M=Ti, V, Cr, or Mn; x≈3). In these compounds, an unusual M₂H square net, which has anti CuO₂ square net structures accompanying two As³⁻ ions, is sandwiched by (LaH)₂ fluorite layers. Notably, strong metal–metal bonding with a distance of 2.80 Å was confirmed in La₂Ti₂As₂H_2.3, which has metallic properties. In fact, these compounds are situated near the boundary between salt-like ionic hydrides and transition-metal hydrides with metallic characters.

We found that La₂Sb with a layered structure composed of alternate stacking of La square nets and LaSb layers exhibits bulk superconductivity with a critical temperature of 5.3 K. This suggests that the presence of the square net with strong La–La metal bonding is essential for the emergence of superconductivity.

Electronic structure in the conduction band of CaTiO₃ was examined by the bremsstrahlung isochromat spectroscopy (BIS), X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS). Two broad bands were observed at 2 and 7eV above the Fermi energy in the BIS spectrum. The features of the XAS spectra for Ca L_{2, 3}, Ti L_{2, 3}, and O K edges were almost the same as those of the EELS spectra, and the XAS spectrum for each element was in reasonable agreement with its partial density of states estimated by an energy band calculation. A trial spectrum was synthesized by the sum of the XAS spectra, and it was found that the feature of the synthesized spectrum is similar to that of the BIS spectrum. From the comparison between XAS and BIS spectra, the two broad bands at 2 and 7eV in the BIS spectrum were assigned to Ti 3d bands and Ca 4s bands, respectively.

The new thallium mixed valence compound Tl₂Nb₂O_6+x (0.1 < x < 1.1) with a defect pyrochlore structure was synthesized by inserting excess oxygen under an oxygen atmosphere up to 2 MPa. A distinct red shift with ca. 0.25 eV of absorption edge was observed on going from x = 0.1 to x = 1.1, and a new and weak absorption band was observed for the samples with large x, at 0.6 eV for x = 1.1 and at 0.8 eV for x = 0.9. The induced band was tentatively ascribed to be due to intervalence charge transfer between Tl⁺ and Tl³⁺. As x increased, full width at half maximum of Tl4f_7/2 increased from 2.4 to 3.0 eV in X-ray photoemission spectra. ²⁰⁵Tl NMR spectra showed the unusual behavior that the absorption due to Tl⁺ vanished as x increased to ca. 1. Electrical conducting properties of these compounds were semiconductive, independent of x. The optical and electrical properties demonstrated the presence of the mixed valence states of Tl in these oxides.