Shigeharu Naka

Charge disproportionation in CaFeO3 studied with the Mössbauer effect

Abstract The Mossbauer effect spectrum of stoichiometric CaFeO 3 at 4 K consisting of two magnetic hyperfine patterns with nearly the same intensities is explained assuming a charge disproportionation 2Fe 4+ →Fe 3+ + Fe 5+ . The disproportionation is supposed to set in at or in the vicinity of the Neel temperature. The single magnetic hyperfine pattern for SrFeO 3 at 4 K, on the other hand, indicates a rapid electron exchange between Fe 3+ and Fe 5+ ions, for the center shift and the hyperfine field coincide approximately with the average values of the corresponding parameters for CaFeO 3 .

Preparation and characterization of stoichiometric CaFeO3

Abstract Stoichiometric CaFeO 3 was prepared and was characterized by crystallographic, magnetic and electrical measurements. A slight tetragonal distortion from the ideal cubic perovskite structure was found. The tetramolecular unit cell has a = 5.325 (3) A and c = 7.579 (5) A. The susceptibility showed a maximum at about 115 K and the temperature dependence of electrical resistivity changed from metallic to semiconductive in the vicinity of the magnetic transition temperature. These indicate a phase transition from the metallic-paramagnetic (the high-temperature phase) to the semiconductive-antiferromagnetic phase. The Mossbauer spectra indicated that a charge disproportionation 2Fe 4+ → Fe 3+ + Fe 5+ associates with the transition.

Determination of cation distribution in spinels by X-ray diffraction method

Abstract A method for the determination of the cation distribution in spinels by X-ray powder diffraction technique was modified so as to take into consideration the effective temperature factor for integrated diffraction intensities of the sample. The optimum values of the oxygen parameter u and degree of inversion x for the spinel sample were determined as the ones gave the best linearity of the relation between log ( I obs I calc ) and sin 2 θ λ 2 . Eight diffraction lines were used, five of which were attributed to the cations in tetrahedral site and other three to all the cations in the spinel. The modified method was applied to three samples GeCo2O4, CoAl2O4 and NiAl2O4.

High temperature and pressure preparation and properties of iron carbides Fe7C3 and Fe3C

The formation regions of Fe7C3 and Fe3C were determined at high temperature and high pressure in the iron-graphite system. Fe7C3 formed at relatively higher pressures and Fe3C at lower pressures. Both Fe7C3 and Fe3C were isolated from coexisting excess carbon powders by a magnetic method. Fe7C3 had a Curie point of 250° C and a saturation magnetization of 120 emu g−1 at room temperature and Fe3C had those of 210° C and 125 emu g−1. Fe7C3 decomposed to Fe3C and carbon at 600° C, but to α-Fe and carbon at 700° C at atmospheric pressure, and Fe3C to α-Fe and carbon at 700° C. The substitution of other metals (Cr, Mo and W) for iron in these carbides leads to changes in the thermal stabilities and the magnetic properties.

On the formation of solid solution in Co3−xMnxO4 system

The formation of solid solution in the Co3−xMnxO4 system in atmospheres of oxygen, air and argon was examined at a constant temperature of 1000 °C. In oxygen, a small amount of the NaCl-type compound was found to co-exist with the cubic spinel in the composition rangex≤0.1. A single phase of the cubic spinel was found in the range 0.1 to 1.3 and the tetragonal spinel above 1.9. In the rangex=1.3 to 1.9 where the cubic and tetragonal spinels co-exist they both have very broadened diffraction line profiles. In air, the identified phases and the changes in their lattice constants with composition were very similar to those in oxygen, except that the NaCl-type compound and the cubic spinel co-existed over a larger range. In argon, the cubic spinel was not observed over any of the composition range and the NaCI-type compound and the tetragonal spinel co-existed in the wide range of 1.1 to 2.3. The experimental results are discussed with regard to the cation distribution in the spinel and also to the relative stability of Co3+- and Mn3+-ions under the low partial pressure of oxygen.

Formation process of tungsten borides by solid state reaction between tungsten and amorphous boron

Various kinds of tungsten borides were synthesized by solid state reaction between tungsten and amorphous boron powders. The mixed powders with various compositions (B/W = 0.4 to 13.0) were treated at 800 to 1550° C for 0 to 120 min in a stream of argon. Four kinds of boride phases such as W2B, WB, W2B5 and WB4 were formed, although the boride phase having the composition of the highest boride, WB12, did not appear. The formation of W2B was initiated approximately at 1000° C in excess of tungsten content. On the other hand, in the excess boron content, the formation of WB, W2B5 and WB4 was initiated approximately at 800, 950 and 1200° C, respectively. The maximum formation amount and crystallinity of WB and W2B5 was found in nearly 10 at % excess boron content in their own stoichiometric compositions. The only crystalline phase of WB4 was prepared with a large excess boron content. However, the formation behaviour of WB4 showed that WB4 is metastable above 1400° C. The stability of WB4 phase could be increased by the presence of excess boron.

A preparation and polymorphic relations of sodium iron oxide (NaFeO2)

Abstract The formation of α-NaFeO 2 and β-NaFeO 2 below 700°C was found to depend strongly on the structure of starting iron oxide; α-Fe 2 O 3 with hcp of oxygen yielded β-NaFeO 2 with hcp and γ-Fe 2 O 3 with ccp of oxygen yielded α-NaFeO 2 with ccp by a reaction either with Na 2 CO 3 or Na 2 O 2 . The doping of Na-ions into the γ-Fe 2 O 3 structure and its rapid change to α-NaFeO 2 were observed. The transformation from the metastable β-NaFeO 2 to the stable α-form was very much sluggish in the solid state, but it was accelerated by the presence of the liquid Na 2 O 2 .

Formation of non-graphitizable isotropic spherulitic carbon from poly-divinylbenzene by pressure pyrolysis

Isotropic spherulites of carbon stable at 2000° C were synthesized by the pressure pyrolysis of divinylbenzene polymer sealed in a capsule. The morphology of the synthesized carbon was pressure and temperature dependent being influenced by the state of polymerization of the starting polymer. Using a polymer prepared at atmospheric pressure and 150° C without catalyst, isolated spherulitic carbon was formed at 700° C and pressures of 1000 to 1250 kg cm−2. These spherulitic carbons were optically isotropic, hard and non-graphitizable after heat treatment at 2000° C. Such carbons originate in the co-existence of higher and lower molecular weight products of pressure pyrolysis and the survival cross-linkages in the original polymer.

Hydrothermal synthesis and properties of Na0.70MnO2.25 layer crystal

Abstract The single crystals of Na 0.70 MnO 2.25 were hydrothermally synthesized by the reaction of MnO 2 with hydrothermal NaOH solution. The crystal of Na 0.70 MnO 2.25 has the layer structure in which sodium ions are located between the sheets composed by MnO 6 octahedra. The oxygen fugacity under hydrothermal conditions was found to influence the valence of manganese ion in a grown crystal, determining the kind of crystalline phase in the NaMnO system. The chemical and physical properties of Na 0.70 MnO 2.25 were measured.

Diamond formation from glassy carbon under high pressure and temperature conditions

The process of the formation of diamond from the glassy carbon with its characteristic bond nature was investigated in the diamond stable region at high pressures (up to 10 GPa) and temperatures (up to 3000° C), without any intentional addition of metals as solvent. The process of diamond formation was found to obey Ostwaldss step rule as follows: amorphous glassy carbon crystallized to form fully well-crystallized graphite prior to diamond formation and then the graphite crystals were converted to diamond by further heat treatment at pressures above 9 GPa. The many trigons formed are considered to be essentially a record of growth failure in the growth period. As a result of heat treatment for a longer time and/or at a higher temperature close to the diamond—graphite stability boundary, the diamond tended to grow with the (111)-face composed of the thin growth layers.