Masao Ohashi

Novel synthesis of the layer structured β-ZrNCl by the direct reactions of zirconium metal or zirconium hydride with ammonium chloride

Abstract The layer structured β-ZrNCl can be synthesized in high yield by the direct reaction of zirconium metal or zirconium hydride with the vapor of ammonium chloride at temperatures ranging from 550 to 700°C. The as-prepared sample was contaminated with a small amount of oxide, and purified by chemical transport in a vacuum-sealed tube with a temperature gradient from 750 to 850°C. The sample transported to the higher temperature zone is highly crystalline and the structure is characterized by a rhombohedral stacking sequence of ZrNCl layers rather than a hexagonal random stacking sequence found in the as-prepared samples.

Chemical vapor transport of layer structured crystal β-ZrNCl

Abstract A layer structured compound β-ZrNCl is transported to a higher temperature zone with the aid of ammonium chloride as the transporting agent in the temperature range of 823–1173 K. The transport mechanism can be explained by the formation of a volatile compound (NH4)2ZrCl6: β− ZrNCl +5 NH 4 Cl →( NH 4 ) 2 ZrCl 6 +4 NH 3 . The measurements of the vapor pressure and the mass spectrum revealed that (NH4)2ZrCl6 decomposed congruently according to the equation: ( NH 4 ) 2 ZrCl 6 ( s )→ ZrCl 4 ( g )+2 NH 3 ( g )+2 HCl ( g ) The enthalpy change for the decomposition was determined to be 533 kJ/mol. By combining the above two equations, a simplified transport equation is derived: β− ZrNCl ( s )+3 HCl ( g )⇌ ZrCl 4 ( g + NH 3 ( g ) .

Preparation and properties of zirconium oxynitrides by the reaction of zirconia with layer structured zirconium nitrochloride

Zirconium oxynitrides were prepared by the reaction of ZrO2 with layer structured β-ZrNCl at temperatures in a range of 900–1000°C in an ammonia stream. In the reaction, β-ZrNCl acted as a nitrogen source having a composition equivalent to ZrN4/3, although without the presence of the oxide it was converted into ZrN. Unlike the oxynitride formation from a conventional mixture of ZrN and ZrO2, the reaction was remarkably fast and completed within 30 min. Two kinds of oxynitride phases, γ and β, were obtained, which have fluorite-related superstructures with a range of composition ZrN4x3O2−2x (0.5⩽x⩽0.8) and an ideal formula Zr7N4O8, respectively. Both of the phases are semiconductors with optical band gap energies of 2.50 to 3.20 eV.

Hydrogen uptake by layer structured β-ZrNCl

Abstract Hydrogen was taken up by β-ZrNCl by heat-treatment with NH 4 Cl in sealed silica tube at temperatures ranging from 500 to 800°C. The maximuum amount of uptake ( x ) was 1.2- for H x ZrNcl. Similar hydrogenation could be done by heating β-ZrNCl in a stream of flowing hydrogen at temperatures ranging from 400 to 800°C. The layered structure of β-ZrNCl is retained on hydrogenation with a very slight decrease in the c dimension of the unit cell. Unlike lithium intercalatd β-ZrNCl, H x ZrNCl is stable even in air, and the electrical and optical properties are essentially unchanged before and after the hydrogenation. The hydrogen taken up is removed by electrochemical oxidation. However, the process is irreversible and re-hydrogenation has not been achieved electrochemically.

Lithium Intercalation in Layer Structured Compound β-ZrNCl

A layer structured crystal β-ZrNCl forms a lithium intercalation compound LixZrNCl. The upper limit for x determined on the compound prepared by the n-butyl lithium technique is ≈0.29. In the electrochemical process, a pressed β-ZrNCl cathode is further reduced up to x = 1 ∼ 1.25 at potentials as low as 0.8 ∼ 0.6 V relative to Li/Li+. The lithium intercalate swells in various polar solvents, increasing the basal spacing. However, in contrast to the salt-like intercalates of transition metal chalcogenides and FeOCl, the lithium intercalated β-ZrNCl does not form hydration phases, but reacts with water, evolving hydrogen. These results can be interpreted in terms of the formation of an alloy-like intercalate like the alkali intercalates of graphite. On interaction, β-ZrNCl is changed from pale yellow-green to black in color, and the electrical conductivity increases by a factor of 106.

Nitridation of calcium stabilized zirconia

Abstract Calcium stabilized zirconia (Ca 0.15 Zr 0.85 O 1.85 ) was nitrided by the reaction with β-ZrNCl at 950 °C in an ammonia stream. β-ZrNCl acted as a nitrogen source having a composition equivalent to ZrN 4 3 . Two kinds of solid solutions having face centered cubic (fcc) and body centered cubic (bcc) lattices were obtained; the formation regions were 0 ≤ x ≤ 0.50 and 2.00 ≤ x ≤ 4.00 for x in Ca 0.15 Zr 0.85+x O 1.85 N ( 4 3 )x , respectively. The XPS spectra showed that the metal ions in the highly nitrided phase (bcc) were in a lower oxidation state than those of the lower nitrided phase (fcc). The two types of the solid solutions were semiconductors with activation energies of 1.44 (fcc) and 0.82 eV (bcc). The ionic conductivities of the nitrided samples were very low, irrespective of their high anion deficient structures.

Preparation and ionic conductivity of alkali metal titanium silicophosphates AxTi3P6Si2O25 (A=Li, Na, K)

Abstract A new series of alkali ion conductors A x Ti 3 P 6 Si 2 O 25 (A=Li, Na, K; 0≤ x 3 P 6 Si 2 O 25 by means of a deintercalation-intercalation technique. K + ions were deintercalated from KTi 3 P 6 Si 2 O 25 by the reaction with Cl 2 gas as a oxidizing agent and new compound Ti 3 P 6 Si 2 O 25 was prepared. Lithium and sodium ions were intercalated into this new compound using n -butyl lithium and Na-naphthalene/THF. Potassium ions were not intercalated into Ti 3 P 6 Si 2 O 25 but into KTi 3 P 6 Si 2 O 25 using K-naphthalene/THF. Lithium intercalation sample Li 1.35 Ti 3 P 6 Si 2 O 25 showed the highest conductivity of 3.4×10 −4 S cm −1 at 400°C with the lowest activation energy of 0.56 eV. A large difference in activation energy for alkali ion conduction in this crystal was explained by comparison of size of bottleneck and that of alkali ions.

Topotactic Conversion of Layer Structured ZrNX (X = Cl, Br, I) to ZrN

The layer structured crystals β-ZrNCl, β-ZrNBr and ZrNI were allowed to react with Na vapor at 450 °C. β- phases have a hexagonal lattice and ZrNI has an orthorhombic one. They were converted into ZrN and the respective sodium halides NaX (X = Cl, Br, I). The transmission electron microscope (TEM) observation and the electron diffraction showed that the shapes of the thin platelets of the layered crystals were kept on the reaction and the products were aggregates of very thin crystalline layers of ZrN. The electron diffraction patterns consisted of simple net patterns of ZrN. From the analysis of the patterns, the following crystallographic relationships were derived. The mechanisms of the reactions were discussed in terms of the structural changes.