The bond ionicity in biborides MB2 (M = Mg, Ti, V, Cr, Mn, Zr, Hf, Ta, Al and Y) has been studied by using the complex chemical bond theory based on a generalization of the Phillips-Van Vechten-Levine scheme. The ionicity of M-B bonds decreases in the following order: Mg, Al, Mn, Y, Cr, Zr, Hf, Nb, Ta, V and Ti. The Mg-B bond in MgB2 has the largest value of ionicity of 96.8% among these examined diborides. Our calculations support the recent results of band structure calculations that Mg atoms are fully ionized so that they can donate valence electrons to the system. The observed superconductivity loss in the solid solution Mg1-xAlxB2 with x>0.1 can be understood in terms of the ionicity and the number of valence electrons.

The crystal structure and magnetic properties of new intermetallic compounds have been investigated by x-ray powder diffraction and magnetic measurement. The lattice parameters of with the rhombohedral -type structure are a = 8.4971 - 8.5006 Å, c = 12.3370 - 12.3424 Å. V atoms show a strong preference for occupying 6c crystallographic positions. The compounds are ferromagnetic. The Curie temperature decreases from 926 to 827 K and the saturation magnetic moment decreases from 22.5 to with x = 0.8 - 1.2. The compound exhibits favourable uniaxial anisotropy and the easy magnetization direction is parallel to the c axis. The anisotropy field is 19 kOe at 1.5 K for the compound. The reasons that does not exist and has uniaxial anisotropy are discussed.

In this study, as a first attempt, we choose Al and transition metals (TMs), Fe, as codoping atoms to synthesize (Al, TM)-codoped SiC. X-ray diffraction and Raman analysis showed that a series of single-phase codoped 4H-SiC samples were obtained and no trace of any other impurity phases, such as Fe3Si and polytypes of other types of SiC, was found. Measurement of magnetic properties showed that codoping by Al and Fe elements changed the original glassy ferromagnetism (FM) features in Al-doped SiC and induced a robust room temperature FM order that gradually dominated (Al, Fe)-codoped 4H-SiC with increase in Fe content. The contribution of Al elements to the magnetic properties of (Al, Fe)-codoped 4H-SiC can be neglected and the magnetic origin should be ascribed to be induced by Fe doping. The only major role of Al is to stabilize the codoped crystal structure as 4H- single phase. As the first investigation on (Al, TM)-codoped SiC, this study opens a new pathway to obtain TM-doped SiC-based diluted magnetic semiconductors with a high T c via the codoping strategy.

The room-temperature phase diagram of the Gd-Fe-Nb ternary alloy system in the Fe-rich corner has been investigated by means of x-ray powder diffraction and thermomagnetic scans. In the system there exist (x = 0-1.0) `2:17' (-type structure), `1:12' (-type structure) and a new compound (x = 0.9-1.3, Curie temperature ). The homogeneous ranges of the three above mentioned compounds are determined by a precise lattice parameter method. Further annealing the new compounds at for 10 days results in no trace of decomposition, implying that may be a room-temperature phase.

A series of La 2 CuO 4+δ samples chemically oxidized at 50 • C with different oxidation times using NaClO as the oxidizing agent have been prepared. All the oxidized samples are single-phase bulk superconductors with an onset transition temperature at 42-45 K measured by dc magnetic susceptibility. X-ray powder diffraction indicates that all the oxidized samples are orthorhombic, and show an enhanced orthorhombic distortion with the oxidation time, compared with that of the starting material. The excess oxygen contents of the oxidized samples determined by iodometric titration are large and increase with the oxidation time. The fit to the experimental data reveals that the excess oxygen content versus the oxidation time obeys the law of the second-order exponential decay. No obvious changes in the grains of the samples before and after oxidation have been observed by scanning electron microscopy.