R. J. Arnott

Preparation, semiconduction and low-temperature magnetization of the system Ni1−xMn12+xO4

Abstract The preparation of a complete range of solid solutions between the cubic spinel nickel manganite, NiMn 2 O 4 , and the tetragonal spinel, Mn 3 O 4 , is described. Studies of the crystal symmetry, the semiconductor behavior and low-temperature magnetization are reported each as a function of composition. Analysis of the crystal-chemical and thermoelectric properties leads to a tentative chemical formula for the system which provides an explanation for the electrical conductivity and magnetization. The conductivity involves the “hopping” of electrons from octahedral-site Mn 3+ ions to octahedral-site Mn 4+ ions. The unpredictable magnetic moments are also accounted for by the properties of octahedral-site Mn 3+ and Mn 4+ ions.

Ferromagnetism in CdCr2Se4 and CdCr2S4

A study has been made of the magnetic properties of the spinels ACr2X4, where A is diamagnetic zinc or cadmium, and X is oxygen, sulfur, or selenium. CdCr2Se4 and CdCr2S4 are apparently ferromagnetic with moments somewhat below the predicted value of 6.0 μB/molecule. By contrast, ZnCr2Se4 and ZnCr2S4 are antiferromagnetic at low temperature despite having positive asymptotic Curie points (TA). We found very little thermal variation of cell size in these materials, which further corroborates Lotgerings conclusion that the antiferromagnetism is due to next‐nearest‐neighbor interactions.Disagreement between experimental values of TA/TC and the theoretical value calculated assuming nearest‐neighbor interactions indicate that next‐nearest‐neighbor interactions are playing a role in the ferromagnetic materials as well.Comparisons of TA and TN in these materials indicate that the magnetic interactions are strongly dependent upon the diamagnetic A‐site ion. However, the data cannot uniquely attribute this effect...

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...

Single‐Crystal Growth and Properties of the Perovskites LaVO3 and YVO3

LaVO3 and YVO3 are intermediate between LaTiO3 (metallic and Pauli paramagnetic) and LaCrO3 (an insulator and antiferromagnetic below TN=320°K). Single crystals of these vanadites were grown from the melt by a modified Bridgman technique. The temperature dependences of electrical resistivity indicate that both materials are semiconductors in the temperature range 120°–670°K. However, the activation energies for semiconduction are considerably lower than those reported in the literature for powder specimens. Magnetic susceptibilities reveal antiferromagnetic ordering at 137° and 110°K for LaVO3 and YVO3, respectively. LaVO3 undergoes a structural transition at the Neel point that is suggestive of spin‐orbit‐coupling electron ordering. These results clearly support a localized‐electron model at low temperatures. However, the low activation energies for semiconduction at temperatures above TN probably reflect a covalent‐mixing parameter λ near the critical value λc for localized vs collective electron behavior.

Effect of Trivalent Manganese on the Crystal Chemistry of some Lithium Spinels

Crystallographic properties of the spinel systems Li0.5Ga2.5−xMnxO4 (0≤x≤0.7), Li0.5Fe2.5−xMnxO4 (0≤x≤0.5), and Li0.5Al2.5−xMnxO4 (0≤x≤0.5) have been investigated in order to determine the effects of trivalent manganese substitution on the crystal chemistry of the ordered spinel hosts. In each system relatively small concentrations of Mn3+ removed long‐range ordering of lithium ions on the octahedral sites. This loss of order was accompanied by anomalies in the dependence of lattice parameters on composition. These observations can be qualitatively interpreted by assuming that Mn3+ ions tend to cluster in order to reduce the elastic energies associated with Jahn‐Teller stabilization. These effects may provide indirect evidence for lattice imperfections such as have been invoked to account for square B‐H hysteresis loops in ferrite materials containing Jahn‐Teller ions.

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.

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.

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%.

Single crystal growth of transition metal oxides from polytungstate fluxes

Abstract Single crystals of M +2 Cr 2 O 4 , M +2 Fe 2 O 4 , where M +2 is a transition (or alkaline earth) ion, and the garnet Y 3 Fe 5 O 12 have been grown from the flux system (1− x )Na 2 W 2 O 7 − x Na 2 W O 4 , where 1 ≧ x ≧ 0 . In addition, single crystals of the system Co 1+δ V 2−δ O 4 ( where 1 ≧ δ ≧ 0) have been prepared by electrolysis of CoO - V 2 O 5 mixtures dissolved in a suitable sodium polytungstate flux. The phase boundaries present in the CoCr 2 O 4 -Na2 W 2 O 7 -Na 2 W O 4 system were determined and a general discussion is made of a solution process for transition metal oxides in a sodium pyrotungstate flux that is formulated upon Lewis acid-base theory.