Aaron Wold

The preparation and properties of some vanadium spinels

In order to study experimentally the properties of outer electrons as a function of inter-atomic separation through the transition region from narrow-band to localized properties, the spinels MV2O4 and M2VO4, where M = Mn, Fe, Mg, Zn and Co and M = Fe and Co, were prepared and characterized by X-ray diffraction and chemical analysis. Conductivity measurements could all be represented by the localized-electron relationship ρ = ρ0 exp(q/kT), but the measured q was sensitive to room-temperature cation-cation separation R, decreasing in the stoichiometric MV2O4 spinels from 0.37 ± 0.01 eV for R = 3.013 A in MnV2O4 to 0.07 ± 0.005 eV for R = 2.972 A in CoV2O4. Attempts to prepare NiV2O4, which might have had a sufficiently small R for a collective-electron relationship, were unsuccessful. Reduction of R by substitution of Al for V in CoAlxV2−xO4 introduced a “foreign” ion into the vanadium sublattice, and q increased. Similarly Co2VO4, with R = 2.965 A and a Co2+ + V4+B-site sublattice; has a q = 0.37 ± 0.01 eV. Further evidence that a small energy is required to create a separated hole-electron pair within the vanadium sublattice comes from maxima at x ~0.11 in the q vs. x and θ vs. x curves for the system Fe1+xV2−xO4, where 0 < x < 1 and θ is the Seebeck voltage. Magnetization measurements indicate noncollinear spin configurations, which indicates strong antiferromagnetic B-B interactions between vanadium cations. The data are compatible with a predicted V3+-V3+ separation in oxides of Rc ∼2.97 A for the transition from collective-electron to localized-electron behavior.

Preparation and crystallographic properties of the systems LaMn1−xMnxO3+λ and LaMn1−xNixO3+λ JA-1106

Abstract Lanthanum manganese oxides containing varying percentages of Mn+4 were prepared and their cell dimensions determined. Lanthanum manganese oxide, prepared under pure nitrogen, contained 0·02 per cent Mn+4. At room temperature it possessed orthorhombic symmetry and was the most distorted member of the series. The orthorhombic-to-rhombohedral transition points for members of this series have been determined, and the shape of the resulting curve interpreted on the basis of an ordering of four, coplanar empty orbitals of the Mn+3 ions and steric effects. The system LaNixMn1−xO3+λ has been prepared and its crystallographic properties studied. La2NiO4, LaNiO3 and an intermediate lanthanum nickel oxide were prepared and their X-ray data are given.

Some Magnetic and Crystallographic Properties of the System LaMn1−xNixO3+λ

The system LaMn1−xNixO3+λ has been prepared under varying conditions for the entire range of compositions 0≤x≤1. The saturation magnetization, lattice parameters, and chemical analysis have been measured for each sample. The origin of discrepancies in the literature on the crystal structure of LaMnO3 is shown to be due to different preparation procedures which result in the presence of differing amounts of Mn4+: the lattice symmetry changes from orthorhombic to rhombohedral as the Mn4+ concentration exceeds 21% of the total manganese. The system is ferromagnetic with magnetization only slightly less than the spin‐only value over the compositional range 0.2≤x≤0.5: a ferromagnetic Ni3+–Mn3+ interaction is indicated. The system remains orthorhombic throughout the range 0≤x<0.5; it shows an increasing distortion to an apparently monoclinic symmetry in the range 0.5<x≤0.8. LaNiO3 is rhombohedral at room temperature. The lattice symmetry is compatible with Ni3+ in a low‐spin state: this is also in accord with t...

Electron ordering transitions in several chromium spinel systems

Abstract Several systems of chromite spinels have been prepared and studied crystallographically from liquid nitrogen to room temperature in order to investigate the relative influence of Jahn-Teller and spin-orbit effects. In the system CoxFe1−xCr2O4 there is a cubic to tetragonal ( c a ) transition for x ⩽ 0·3. For the system NixFe1−xCr2O4 there is a cubic to tetragonal ( c a ) transition for x ⩽ 0.2 and a cubic to tetragonal ( c a > 1 ) transition for x ⩾ 0·3. It appears that A-site Co2+ is more effective than A-site Ni2+ in maintaining a tetragonal ( c a ) distortion at A-site Fe2+. This implies that more A-site Fe2+ is needed to change the sign of a tetrahedral Ni2+ from c a > 1 to c a than is needed to cause a cooperative distortion of the structure with non-Jahn-Teller Co2+ sharing the A-sites. The system NiFexCr2−xO4 shows a cubic to tetragonal ( c a > 1 ) distortion for x ⪅ 0.15 and a cubic to tetragonal distortion ( c a ) for x ⪆ 0.28. At 0.2 c a It has also been shown that a Jahn-Teller distortion of tetrahedral Fe2+ may not only stabilize tetragonal ( c a > 1 or c a ) .distortions but also give rise to an orthorhombic distortion.

The use of CO-Co2 atmospheres for the preparation and free energy determinations of several oxide systems

Abstract A number of transition metal oxide systems have been prepared that require controlled CO-CO 2 atmospheres for attaining certain desired valencies. In addition standard free energies for numerous mixed oxides are determined by a direct evaluation of the equilibrium atmospheres.

Crystallographic transitions in several chromium spinel systems

The systems NixFe1−xCr2O4, CuxNi1−xCr2O4, and CuxCo1−xCr2O4 have been prepared and their crystallographic properties studied. From consideration of Jahn‐Teller effects, it was shown that A‐site Cu2+ is more effective than A‐site Fe2+ in stabilizing a tetragonal (c/a 1 to c/a<1.

Antiferromagnetism in NiTiO3

The effective internal field in NiTiO3 as a function of temperature has been determined from zero‐field resonance absorption in a powder sample at millimeter wavelengths. Assuming a g value of 2, the resonant frequency (187 kMc) leads to an effective field at 4.2°K of approximately 67 kG. The temperature dependence fits within experimental error a Brillouin function for S = 1, with a Neel temperature of 22°K. From dc susceptibility data, a Neel temperature of 23°K is obtained and an exchange field of approximately 270 kG is computed. These data yield an out‐of‐plane anisotropy of about 8.3 kG.

Magnetic Transitions in Cubic Spinels

Magnetization curves of the cubic spinels MnCr2O4, CoCr2O4, MnV2O4, and CoV2O4 have been obtained between 4.2°K and their Curie points. With the possible exception of MnV2O4, all the curves indicate the presence of a magnetic transition within this temperature range. Although all these materials have been reported to be normal spinels, inconsistencies between our results and the recent theory of spin configurations in normal cubic spinels led to further experiments which established that CoV2O4 is not, in general, normal.Theory suggests the presence of two types of transition in MnCr2O4 and CoCr2O4, but only one has been observed in each case. The temperatures at which the observed break in the magnetization curves of these materials occurs can be interpreted by ascribing them to different types of transitions. The interpretation of the MnCr2O4 magnetization curve is compatible with the neutron diffraction results of Corliss and Hastings.

Cobalt Ferrite Single Crystals

A crystal of cobalt ferrite was grown from the melt at 1600°C under an oxygen pressure of 790 psi. Chemical analysis of a portion of the crystal gave a ferrous ion content of 1.3%.

Magnetic Properties of Cr5S6 in Chromium Sulfides

Several samples of CrSx with 1.145<x<1.200 were carefully prepared and chemically analyzed. The expected crystal structures were verified by x‐ray analysis, which failed to indicate the presence of any impurities. Conductivity measurements indicated the existence of metallic conductivity in these samples between 77° and 300°K. Magnetic measurements supported the suggestion that the ferrimagnetism of CrSx materials arises from the presence of the Cr5S6 phase.The magnetization curves for x=1.194 demonstrated the following salient properties of the Cr5S6 phase: a ferrimagnetic Curie point of 305°K, a peak magnetization of 6.9 emu/gm in an external field of 8500 oe, a ferrimagnetic‐antiferromagnetic transition over a 5° temperature interval, a transition temperature in the neighborhood of 158°K which is subject both to thermal hysteresis and to shifting by an external magnetic field, and an antiferromagnetic susceptibility below this transition. The transition temperature, as well as the peak magnetization, d...