M. J. Sienko

Effect of oxygen-deficiency on electrical transport properties of tungsten trioxide crystals☆

Abstract Single crystals of various substoichiometric tungsten trioxide crystals have been prepared by vapor phase transport and annealed under varying conditions. Electric resistivity, Hall voltage and Seebeck coefficient have been investigated as functions of temperature in the range 100–500°K and as functions of composition down to the compound WO 2.90 . For stoichiometric WO 3 , the results are in general agreement with previous reports. The jump in resistivity at the monoclinic-triclinic transition is due mainly to a decrease in electron mobility. The change at the triclinic-(low temperature) transition is almost entirely the result of a large decrease in the carrier concentration. Changes in thermoelectric power cannot be explained on the basis of the ordinary electronic component; a lattice contribution must be present. Removal of oxygen pushes the transitions to lower temperatures, increases the carrier concentration, decreases the Hall mobility, and changes the sign of its temperature dependence. Segregation of oxygen defects apparently occurs even for x 3− x , and it is believed that the basic defect unit is not the oxygen vacancy but the Anderson-Hyde disk of shear. The upper bound for this type of defect structure appears to be at x = 0.013. The increase of mobility with increasing temperature is attributed to hopping of charge carriers in place of band processes in the defect regions. Decreases in the optical phonon drag and transport energy terms are needed to fit the thermoelectric power behavior of the monoclinic phase. X-ray studies show essentially no change in the cell parameters for the reduced crystals. There is strong optical absorption in the red and near infrared associated with the oxygen deficiency, but only negligible paramagnetism. DTA studies show that the monoclinic-triclinic and triclinic-(low temperature) phase transitions are of the first-order and displacive type as in BaTiO 3 ; they are accompanied by transition heats of about 20 and 39 cal/mole, respectively. Measurements of resistivity and Seebeck coefficient on a W 20 O 58 crystal suggest metallic-like behavior.

Examination of substoichiometric WO3−x crystals by electron microscopy☆

The defect structure of high purity crystals of tungsten oxides with compositions between WO3.00 and WO2.99 has been investigated by transmission electron microscopy. The oxygen deficiency appears to be accommodated by Wadsley defects, disordered crystallographic shear planes, parallel to {120}R (R = ReO3-type parent lattice). The planes tend to collect into bands which cause some regions of the crystal to have compositions far removed from the bulk. An observed maximum in carrier mobility is attributed to the destruction of conduction band states in the defect-rich regions. A modified form of the dislocation model for shear plane formation is proposed.

Anomalous electrical and magnetic properties of vanadium diselenide

Abstract Powder samples of VSe2 have been made by direct synthesis from the elements. Single-crystal specimens have been made by vapor phase transport in the presence of iodine. X-Ray characterization indicated only the 1T polytype. Behavior was metallic over the range 1.5 to 300°K, but there were discontinuities at about 100°K in resistivity, magnetic susceptibility, and Hall voltage. These are apparently due to the onset of charge density waves. Below 50°K, there was a clear quadratic dependence of resistivity on temperature which combined with relatively high Pauli paramagnetism to indicate Baber-type electron-electron scattering. No superconductivity was observed down to 1.5°K.

X-ray and low-temperature magnetic studies of the compounds hexaammineeuropium (o), hexaammineytterbium (o), and their solid solutions

Abstract The compounds Eu(NH 3 ) 6 , Yb(NH 3 ) 6 , and their solid solutions have been synthesized by direct reaction of the metals with liquid ammonia. Low-temperature X-ray studies at 200K indicate that the unit-cell parameters are very close to those of Sr(NH 3 ) 6 , corresponding to a body-centered cubic array of octahedral molecules. A Wigner-Seitz metallic model involving delocalization of electrons through 7 s 7 p band formation is proposed. Magnetic studies have been carried out by a Faraday method over the range 2.2–300 K and up to 22 kilo-oersted. Eu(NH 3 ) 6 is found to be ferromagnetic with a Curie temperature of 5.5K, which does not change with dilution. Above 5.5K, the paramagnetic moment per europium is 10.15 BM; it changes to 7.9 BM above 47K. As expected, Yb(NH 3 ) 6 is found to be diamagnetic, but it crosses over to paramagnetic below 31K. A model for Eu(NH 3 ) 6 is proposed in which valence electrons, delocalized in a 7 s 7 p conduction band at higher temperature are progressively frozen out into localized 5 d orbitals below 47K. The extremely large distances over which the ferromagnetic coupling occurs suggests an indirect exchange mechanism via nondegenerate-gas electrons. At higher temperatures, the exchange appears to be of the degenerate-gas type; at very low temperatures, perhaps of the excitonic-insulator type.

Magnetic Susceptibility of Tetraamminelithium(Zero) in the Range 1.5–194°K

The magnetic susceptibility of the compound tetraamminelithium (zero), Li(NH3)4, has been measured by the Faraday method from 1.5 to 194°K. In the liquid the susceptibility is paramagnetic and shows a small positive temperature coefficient that can be attributed to variation of bandwidth with temperature. At the freezing point, 88.8°K, the susceptibility undergoes a 6% decrease, also attributable to a change in bandwidth. At 82.2°K, where there is a change from cubic to hexagonal structure, the susceptibility shows an abrupt 25% drop and Curie‐Weiss behavior down to about 15°K. Below 15°K, the susceptibility flattens out. Two models are presented to account for the observed behavior. In one, Li(NH3)4 is considered to be an antiferromagnetic metal with a Neel temperature of about 10°K; in the other, a nearly degenerate electron gas. Hysteresis at the 88.8°K and 82.2°K phase transitions has also been investigated. Fermi‐surface topologies for the cubic and hexagonal phases are discussed on the basis of the ...

Magnetic properties of iron-intercalated titanium diselenide

Abstract Powder samples of Fe x TiSe 2 (0.00 ≤ x ≤ 0.50) have been prepared by direct reaction of high purity elements. Crystal parameters were determined by the Debye-Scherrer powder diffraction technique. Magnetic susceptibility measurements were made by the Faraday method in the temperature range 1.5–300 K and indicated magnetic odering for compositions with x > 0.03. The magnetic ordering temperature, T m , was observed to move to higher temperature with increasing iron concentration. These results have been interpreted in terms of a spin-glass model for low iron concentrations ( x x > 0.2).

An X-ray study of the alkaline earth hexaammines at 77°K☆

Abstract X-Ray diffraction patterns taken at 77°K of the compounds M(NH3)60, M = Ca, Sr, Ba, correspond to bcc structures with a0 values of 9.20, 9.45, and 9.77 A, respectively. No sign of a gross crystal structure change was observed.

Conduction‐ and localized‐electron spin resonance in the lithium–methylamine system: Inferences for the existence of the metallic compound tetramethylaminelithium(zero), Li(CH3NH2)4

Electron spin resonance (ESR) studies of fluid and frozen solutions of lithium in anhydrous methylamine are reported. The composition and temperature dependence of the ESR properties (electron spin relaxation times, electronic ge factor) are indicative of metallic compound formation in the lithium–methylamine system at low temperatures. Marked changes in both the electron spin relaxation properties and the ESR line shapes in the temperature interval 156–158 K are consistent with those expected for a metallic system moving through the melting point. Electron spin relaxation characteristics of the corresponding metal–ammonia compounds Li(NH3)4 and Ca(NH3)6 have been adequately interpreted in terms of a nearly free‐electron (NFE) picture for these low electron density materials. In contrast, electronic properties of the compound tetramethylaminelithium(zero), although nominally metallic (σ∼400 Ω−1 cm−1 for a 22 mole% metal fluid solution), cannot be described in the context of a NFE material. In particular, ...

Electron spin resonance study of metallic lithium–ammonia systems

Electron spin resonance (ESR) in metallic lithium−ammonia systems has been investigated in the temperature range 12−296°K. In the liquid solutions and in the cubic phase of Li(NH3)4 the conduction−electron−spin resonance (CESR) line shapes are in excellent agreement with Dyson’s theory. The g shift, which is independent of concentration and temperature, is equal to 7.9±0.3×10−4. The spin−orbit coupling constant derived from the g shift is about 10 cm−1. The diffusion time TD is nearly independent of temperature in the liquid state, but it is strongly dependent on concentration and undergoes a fourfold decrease at the freezing point of Li(NH3)4. The spin−lattice relaxation time T1 is weakly dependent on concentration and temperature in the liquid state and undergoes a twofold increase at the freezing point of Li(NH3)4. The data are consistent with the spin−orbit relaxation mechanism and indicate that to a good approximation the solvated ions are the only spin scatterers in the liquid state. The paramagneti...

Low temperature magnetic susceptibility of uranium and rare earth tungsten oxide bronzes

Magnetic susceptibilities of uranium tungsten bronzes, UxWO3, and their thorium‐diluted counterparts, UxTh0.10−xWO3, have been measured by the Faraday method over the range 1.6–300°K. The sharp, dilution‐independent antiferromagnetic behavior previously reported was not confirmed. Weiss constants observed showed a weak decline with dilution. Susceptibilities of light rare earth bronzes (RE=Pr, Nd) deviated at low temperatures from Curie‐Weiss behavior, probably because of changes in population of intra‐atomic levels. For the heavy rare earth bronzes (RE=Gd, Tb, Dy, Ho, Er), very small Weiss constants were observed. Near independence of the Weiss constants of conduction electron density and rare earth magnetic content suggests that the apparent Curie‐Weiss behavior is mainly a manifestation of crystal field interactions, not an indication of magnetic exchange effects.