NaCl-type Crystal Structure Research Articles

Magnetic susceptibility of CfN, CfAs and CfSb

Magnetic susceptibility measurements are reported for the compounds CfN, CfAs and CfSb. These semimetallic compounds evhibit the NaCl-type cubic crystal structure. For these magnetic measurements the temperature was varied from 4.2 K to 340 K and the magnetic field from 1 to 50 kG. The paramagnetic effective moments derived from Curie-Weiss fits to the data agree, within evperimental error, with a theoretical value of 10.2 μ B calculated for the 5f 9 electronic state of Cf 3+ assuming an intermediate-coupling model. Deviations of the susceptibility from Curie-Weiss behavior at lower temperatures are indicative of magnetic ordering and crystal-field splitting of the ground state. The ordering temperatures of these californium compounds are considerably lower than those observed previously for pnictides of curium and berkelium. This decrease in ordering temperature with increasing atomic number is consistent with the trend observed for the lanthanide monopnictides.

High T c superconducting NbN films deposited at room temperature

Thin films of niobium nitride with superconducting transition temperature (Tc ) of 15.7 K have been deposited on a variety of amorphous as well as crystalline substrates including glass, glazed ceramic, fused quartz, and sapphire, maintained at room temperature, by dc reactive magnetron sputtering in a mixture of Ar and N2 gases. The effects of the deposition conditions, particularly the carrier gas pressure and composition, on the film crystal structure, orientation, and resistivity have been studied in an effort to maximize the superconducting transition temperature. A study of the variation of nitrogen consumption with nitrogen injection pressures for constant background argon pressures is conducted and found to be an absolute indicator of the NbN formation systematics. Initially, the consumption increases linearly with the injection pressure but beyond a certain threshold, it shows a distinct drop. The desired high Tc NbN with B1 crystal structure is formed in the vicinity of this turning point of the reactive gas consumption-injection characteristic. High Tc films possess B1 (fcc, NaCl-type) crystal structure as revealed by their x-ray diffraction patterns. An initial increase in the injection pressure of the reactive gas (N2) results in a remarkable increase in the (111) diffraction line intensity along with an increase in the film Tc. This trend continues up to the turning point of consumption-injection characteristic, beyond which the crystal structure distorts into the substoichiometric tetragonal phase with a consequent reduction in the transition temperature. A general protocol for studying the formation systematics of transition metal nitrides has thus emerged.

Preparation of vanadium carbide single crystals by a floating zone technique

Vanadium carbide has a wide range of nonstoichiometric compositions where the atomic ratio of carbon to vanadium C/V varies between 0.71 and 0.88. Within this range, V 8C 7 and V 6C 5 phases exist due to carbon vacancy ordering in addition to the compound VC x ( x<0.76) which has the NaCl-type crystal structure. In this paper, the solidus-liquidus relationship, the composition change due to evaporation and the composition range where the phases exist as single phases were determined under the growth conditions. On the basis of these results, the single phase crystals of V 8C 7, V 6C 5 and VC x were prepared by a floating zone technique. The crystal rods obtained were about 6 cm long and 0.9 cm in diameter. The homogeneity of the crystal composition was confirmed to be within about 1 at%. Inpurities in the crystal, starting material, evaporation product and feed rod were examined using X-ray fluorescence spectroscopy. The refining effect due to evaporation was discussed.

Magnetic disorder in uranium compounds due to fission damage

Ferromagnetic US ( T c = 180 K) and antiferromagnetic UN ( T N = 52 K), both with NaCl-type crystal structure, were examined with respect to the magnetic properties after neutron irradiation. Shifts of the magnetic transition point in both cases and a drastic reduction of the magnetization in US were accompanied with an increase of the lattice parameters due to fission damage.

Sign of the Nearest-Neighbor Exchange Interaction and its Derivative in GdAs

The sign of the nearest-neighbor exchange interaction ${J}_{1}$ and its logarithmic derivative $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ has been determined for the intermetallic type-II antiferromagnet GdAs. The ${\mathrm{As}}^{75}$ nuclear-magnetic-resonance Knight-shift data in the paramagnetic state of GdAs were used to yield a value for the Curie-Weiss temperature $\ensuremath{\theta}\ensuremath{\simeq}\ensuremath{-}35\ifmmode^\circ\else\textdegree\fi{}$K. Combining this value with the N\'eel temperature ${T}_{N}\ensuremath{\simeq}25\ifmmode^\circ\else\textdegree\fi{}$K and using the molecular field theory gives ${J}_{1}=\ensuremath{-}0.08\ifmmode^\circ\else\textdegree\fi{}$K and ${J}_{2}=\ensuremath{-}0.40\ifmmode^\circ\else\textdegree\fi{}$K. The measurement of the sign of $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ in GdAs was performed using low-temperature x-ray powder-diffraction techniques in order to study the exchange-striction-induced trigonal lattice distortion for $T&lt;{T}_{N}$. The observed trigonal distortion was found to be a compression along the [111] direction of the NaCl-type GdAs crystal structure. From the sign of the trigonal distortion, it is concluded that $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ is negative. Similar data for GdP suggest that $\frac{\ensuremath{\partial}\mathrm{ln}{J}_{1}}{\ensuremath{\partial}r}$ is also negative for this compound.

On the Resistivity and Crystal Structure of Binary Oxide of Nickel and Cobalt

The solid solution MeO (Me=Ni 1 Co 2 ) was prepared by quenching the specimens derived from mixture of nickel and cobalt carbonates from a temperature higher than 850°C. The resistivity measurement during the heat treatment provided a convenient measure of the phase change. MeO shows a high resistivity at room temperature and has a NaCl-type crystal structure with lattice parameter of 4.23 5 Å. We could lower its resistivity at our disposal through the definite heat treatment. The lowering of resistivity accompanied the shrinkage of lattice parameter and increase in oxygen content without appreciable change in crystal form. The conduction mechanism in MeO having low resistivity may be considered to be quite the same as in the case of black nickel oxide, except the degree of oxygen excess can amount to several percent in former case. The phase change in MeO, caused by the heat treatment was also investigated, in comparison with the case of pure cobalt and nickel oxides and the effect of mixing was discussed.