The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.

The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties

Development of a non-noble-metal hydrogen-producing catalyst is essential to the development of solar water-splitting devices. Improving both the activity and the stability of the catalyst remains a key challenge. In this Communication, we describe a two-step reaction for preparing three-dimensional electrodes composed of CoSe2 nanoparticles grown on carbon fiber paper. The electrode exhibits excellent catalytic activity for a hydrogen evolution reaction in an acidic electrolyte (100 mA/cm2 at an overpotential of ∼180 mV). Stability tests though long-term potential cycles and extended electrolysis confirm the exceptional durability of the catalyst. This development offers an attractive catalyst material for large-scale water-splitting technology.

CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction

A general strategy is described for inducing magnetostriction in ferromagnetic martensitic materials. An analysis of domain redistribution caused by a magnetic field is given, and certain relations...

Magnetostriction of martensite

The magnetic and magnetostrictive properties of Fe/sub 1-x/Ga/sub x/ and Fe/sub 1-x-y/Ga/sub x/Al/sub y/ (a+y<0.3) single crystals were measured under compressive stresses up to 120 MPa and in magnetic fields up to 1 kOe. Values of /spl lambda//sub 100/ exceeding 200 ppm were observed in samples of Fe/sub 83/Ga/sub 17/ from 180 K to room temperature. Only a small monotonic decrease in magnetostriction with temperature was found. On the other hand, /spl lambda//sub 111/ maintained small negative values close to those of b.c.c. Fe. In all cases, no higher order magnetostrictive terms were detectable. With 15 to 20 atomic percent replacement of Fe by Ga, the magnetizations remain high (M, /spl cong/1.8 T) and the magnetic anisotropies reduced (K/sub 1//spl cong/10/sup 4/ J/m/sup 3/). Values of the 'stiff' (high magnetic field) Young's modulus, Bulk modulus, Poison's ratio, and elastic constant C/sub 11/-C/sub 12/ for Fe/sub 85/Ga/sub 15/ at room temperature are, respectively, 77 GPa, 111 GPa, 0.38, and 56 GPa.

Magnetostrictive properties of body-centered cubic Fe-Ga and Fe-Ga-Al alloys

Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. λ100 exhibits two peaks as a function of Ga content. At room temperature, λ100 reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, λ100 drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling |b1| of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c11−c12) which approaches zero near 27% Ga. λ111 is significantly smaller in magnitude than λ100 over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1027&context=ameslab_conf

Extraordinary magnetoelasticity and lattice softening in bcc Fe-Ga alloys

The magnetic properties of dichalcogenide compounds, CoS 2 , CoSe 2 , NiS 2 , NiSe 2 and the system Co(S x Se 1- x ) 2 , were studied by means of magnetic measurement, neutron diffraction and NMR. The exchange interaction J s w of the ferromagnetic CoS 2 ( T c =124°K, θ p =220°K, p eff =1.76µ B , n =0.84µ B and H i =52 kOe) was estimated to be 24 cm -1 per Copair from the analysis of spin wave theory. While, CoSe 2 ( T N =90°K, θ p =-160°K and p eff =1.72µ B ) showed an antiferromagnetic spin ordering with MnSe 2 type. NiS 2 (θ p =-740°K and p eff =2.48µ B ) had no magnetic ordering even at 4.2°K. NiSe 2 exhibited weak constant paramagnetism with metallic conductivity. In Co(S x Se 1- x ) 2 , (0≤ x ≤1), the magnetic properties and the magnetic phase diagram were determined, and this system showed to have “low spin state” with 1µ B per Co. Some discussions were given from the theoretical stand points.

Magnetic Properties of Cobalt and Nickel Dichalcogenide Compounds with Pyrite Structure

The lattice parameters of the α-phase (disordered bcc), the metastable α''-phase (Fe 3 Al type) and the stable β-phase (Cu 3 Au type) in the Fe-Ga system containing 15∼30% Ga were determined and a phase diagram is proposed according to X-ray and magnetic measurements. The magnetic properties of these phases were investigated between 4.2 K and 1100 K. The magnetic moment per Fe atom was found to be 2.05±0.06µ B and 2.39±0.06µ B for the ordered bcc and the ordered fcc Fe 3 Ga respectively. Mossbauer measurements on the α'' and β phases were carried out and the internal fields were estimated to be 274 kOe for the fcc Fe 3 Ga and 230 and 360 kOe for the A and D sites respectively of the bcc Fe 3 Al-type Fe 3 Ga. Large quadrupole interactions were found in the fcc Fe 3 Ga and its spin direction is suggested to be along . Some discussions are given in the last section.

Magnetic Properties and Mössabauer Investigations of Fe-Ga Alloys

Further investigations, succeeding to the works of J. Phys. Soc. Japan 26 (1969) 631 and 29 (1970) 323, on the magnetic properties of Co(S x Se 1- x ) 2 in the whole concentration ranges (0≤ x ≤1) were performed by means of high magnetic field, electrical resistivity and low temperature X-ray diffraction measurements. The first kind magnetic transition was found in the ferromagnetic phase except CoS 2 . The metamagnetic behaviors were observed in the ferromagnetic (0.89< x <1.0) as well as paramagnetic ( x <0.89) phases. The saturation moment per Co atom was obtained to be 0.855 µ B for CoS 2 and it decreased gradually with increasing Se content. The electrical resistivity in low temperatures showed a T 2 -dependence. It was concluded that the itinerant electron model base on the spin fluctuation theory is more adequate than models based on a localized spin scheme.

Further Investigations on Magnetic Properties of Co(S x Se 1− x ) 2 , (0≤ x ≤1)

Proceeding the previous paper (J. Phys. Soc. Jpn. 46 (1979) 1474) the high field magnetization process of Co(S x Se 1- x ) 2 system for 0.70 ≤ x ≤1.00 has been investigated by means of pulse fields up to 500 kOe at fixed temperatures. The metamagnetic behaviors were observed and the phase diagram between the transition field and temperature was given. The saturation moment seemed to disappear at x ≃0.6.

Magnetization of Co(S x Se 1− x ) 2 under High Magnetic Field Up to 500 kOe

A new fct phase of disordered Fe–Pd Invar alloys was found at low temperatures. The low temperature phase diagram was obtained from X-ray diffraction experiments (4.2 K to 300 K). The fct phase ranges from 28% to 33.5% Pd at 0 K and the temperature dependence of lattice constants was determined. A decrease of the X-ray Debye temperature associated with a lattice softening was observed for 34% Pd in the low temperature region.

Kengo Adachi

Papers

Magnetic Properties of Cobalt and Nickel Dichalcogenide Compounds with Pyrite Structure

Magnetic Properties and Mössabauer Investigations of Fe-Ga Alloys

Further Investigations on Magnetic Properties of Co(S x Se 1− x ) 2 , (0≤ x ≤1)

Magnetization of Co(S x Se 1− x ) 2 under High Magnetic Field Up to 500 kOe

A New Low Temperature Phase (fct) of Fe–Pd Invar