Murray Robbins

Magnetic and crystallographic properties of spinels of the type AxB2S4 where A = Al, Ga, and B = Mo, V, Cr

Abstract Spinels with the formula Ga 0.67 Mo 2 S 4 , Al 0.75 Mo 2 S 4 , Ga 0.67 Cr 2 S 4 , and Ga 0.5 V 2 S 4 have been prepared. Since the cation to anion ratio of these spinels is less than 3:4 they must contain cation vacancies. X-ray diffraction studies indicate that the Al, Ga, and vacancies occupy the A (tetrahedral) sites and Mo, V, and Cr the B (octahedral) sites. All of the spinels are semiconductors and exhibit weak ferromagnetism. The magnetic porperties are explained in terms of B site ferro- and antiferromagnetic superexchange interactions. An anomaly observed in the magnetic properties of Ga 0.5 V 2 S 4 at ∼46°K would seem to indicate a possible phase transition.

Thermal analysis of graphite and carbon-phenolic composites by pyrolysis-mass spectrometry

Abstract Dynamic mass spectrometry was used to characterize the pyrolytic degradation of graphite and carbon black-phenolic resin composites. Measurements were obtained on the overall yield, composition and formation rates of the volatile pyrolysis products. Pyrolysis was found to occur by three general processes: 1. (1) low temperature outgassing of free phenol present in the resin material 2. (2) formation of water from post-cure reactions at 150–300°C 3. (3) thermal fragmentation of the polymer structure above 350°C to yield low molecular weight species. Each of these processes was significantly affected by the presence of the carbon black filler material, with substantially lower yields (

A novel synthetic method for the preparation of oxide superconductors: anionic oxidation-reduction

A new chemical method for the bulk synthesis of Ba{sub 2}YCu{sub 3}O{sub 7} powder is described and is applicable to the synthesis of other superconducting oxides. Ionic salt precursors are used which are intimately mixed by spray-drying an aqueous solution of these salts. The anions in the spray-dried mixtures (which in this study are mixtures of nitrate and acetate salts) participate in a low-temperature oxidation-reduction reaction. The exothermic redox reaction and the heat evolved from the reaction completely convert the precursors into their corresponding mixed oxides at temperatures below 300{degree}C in one reaction step. Phase segregation (normally apparent in the decomposition of organic precursors) is not observed in the decomposition of the mixed anion (nitrate/acetate) precursors. The enthalpy of this reaction is strongly dependent upon acetate content in the mixture: {minus}{Delta}H (9:4, acetate:nitrate) = 4016 (70) kJ/mole, whereas {minus}{Delta}H (5:8, acetate:nitrate) = 890 (70) kJ/mole. These precursors can be reacted at 910{degree}C for 10 min to produce single-phase Ba{sub 2}YCu{sub 3}O{sub x}. Sintered disks of Ba{sub 2}YCu{sub 3}O{sub 7}, prepared from Ba{sub 2}YCu{sub 3}O{sub x} powder, show onset T{sub c} of 94-95 K and are fully superconducting by 90.5-91 K.

Epitaxial growth of ferrite

Abstract Several spinel ferrites have been epitaxially grown on 〈111〉 MgO substrates by CVD. Coprecipitated ferrite was used as source, Cl 2 as transport gas and argon as carrier gas. Using coprecipitated material as source provided higher speed of deposition than sintered material. Epitaxial NiFe 2 O 4 and CoFe 2 O 4 films have been grown in a thermal gradient of 1000-900°C for the source and 860–760°C fur the substrate. Thermodynamic calculations show that it is possible to grow ferrites in open systems with chlorine from a ferrite source. Calculations of the vapor pressure of the vapor species involved in the system NiFe 2 O 4 : Cl 2 have been carried out.

Solid solution formation in chalcopyrite systems of the type AgInX2AgMIIIX2 where M = Al, Ga, and X = S,Se

Abstract Previous (1) work on ternary chalcopyrite solid solution formation has shown that the difference in end point axial ratios ( Δc a ) is an important factor in determining the extent of mutual solubility. It was concluded that when Δc a > 0.13 complete solid solubility will not occur. In this work it is shown that complete solid solution formation in the systems AgAlS2AgInS2 ( Δc a = 0.111 ), AgGaS2AgInS2 ( Δc a = 0.11 ) and AgAlSe2AgInSe2 ( Δc a = 0.112 ) does occur. This shows that the value of Δc a = 0.13 as an upper limit for solid solution formation can be approached closely.

Phase relationships and some magnetic properties of spinels in the systems M1−xNixCr2S4 where M = Mn, Fe, Co

Abstract Phase relationships between spinel and defect NiAs structures in the systems M1−xNixCr2S4 (where M = Mn, Fe, Co) were investigated. It was found that the spinel structure is stable between x = 0 and x = 0.3 when M = Mn or Fe. When M = Co the spinel is formed in the region x = 0 to x ∼ 0.4. The apparent stabilization of the defect NiAs phase by Ni2+ may be related to the strong sixfold site preference of Ni2+. Curie temperatures of all three ferrimagnetic systems increases with increasing Ni2+ substitution. This is probably due to higher NiS covalency.

Manganese diborate: Crystal structure, magnetization, and thermal extinction dependence

MnB4O7 is paramagnetic at room temperature, with effective paramagnetic moment of 5.84 μB, and orders antiferromagnetically at about 3°K. The lattice constants of orthorhombic MnB4O7 are a =8.62354±0.00003, b =14.0071±0.0001, c =8.07237±0.00002 A at 298°K, with space group Pbca and eight formulas in the unit cell. A total of 11 317 reflections were measured with PEXRAD resulting in 1060 symmetry‐independent structure factors. The structure was solved by a combination of Patterson and Fourier series and refined by the method of least squares. The final R factor for a model with all atoms undergoing anisotropic motion is 0.046. The structure is shown by half‐normal probability plot analysis to be accurately isomorphic to that of CdB4O7. The B4O7 group consists of two BO4 tetrahedra with one oxygen atom in common, each sharing two other oxygen atoms with two triangular BO3 groups. Each triangular group contains an oxygen atom from both tetrahedra. The dihedral angle between planes containing the two tetrahed...

Synthesis of Ba2YCu3O7 by the SCD method using amino acid salt reducing agents

The anionic oxidation-reduction or SCD method relies on an internal oxidation-reduction reaction which converts spray-dried precursors into intimately mixed and highly reactive metal oxide powders. Earlier studies focused on precursor mixtures containing oxidizing components such as NO{sub 3}{sup {minus}} salts with reducing salts (RCOO{sup {minus}}, where R is H, CH{sub 3} or CH{sub 2}CH{sub 3}) which react to form Ba{sub 2}YCu{sub 3}O{sub 7} powder. In this work, amino acid reducing agents are used. The impact of the functional group change, in the reducing agent of the NO{sub 3}{sup {minus}}/amino acid precursors, is shown by DSC to result in sharp (and therefore rapid) oxidation-reduction reactions which occur at lower temperatures. The trend in reaction temperature correlates inversely with the expected basicity of the amino group in the series (numbers in parentheses represent mole ratios): CH{sub 3}CH(NH{sub 3})COO{sup {minus}}/NO{sub 3}{sup {minus}} (4/9) = 206{degree}C > NH{sub 2}CH{sub 2}COO{sup {minus}}/NO{sub 3}{sup {minus}} (4/9) = 195{degree}C > NH{sub 2}CH{sub 2}CH{sub 2}COO{sup {minus}}/NO{sub 3}{sup {minus}} (4/9) = 191{degree}C. The mixed oxide product is highly reactive and can be converted to single-phase Ba{sub 2}YCu{sub 3}O{sub x} when fired at 900-910{degree}C for 10 min in flowing oxygen.

EuFCl—A new mixed-halide rare-earth compound

Abstract This paper reports the synthesis and bulk magnetic properties of a new rare-earth compound, EuFCl, isostructural with tetragonal PbFCl, and solid solutions of it with several LnOCl compounds, where Ln = Eu, Sm, Nd, Gd.

Synthesis of Ba2YCu3O7 powder by anionic oxidation (NO−3)-reduction (RCOO−, where R is H, CH3, and CH3CH2)

Abstract A new method for synthesizing homogeneous oxide powder, reported earlier, is studied. This technique involves spray drying a solution containing an oxidizing and reducing salt into an intimately mixed powder. The spray-dried powder undergoes an internal oxidation-reduction reaction which completely converts it into the mixed oxide at 95%) Ba 2 YCu 3 O 7− x is formed in 10 min at 910°C. Previously, mixtures of nitrate (oxidizing) and acetate (reducing) salts were studied. This is now extended to include other organic reducing agents such as formates and propionates and comparisons are made for R COO − , where R = H, CH 3 , and CH 2 CH 3 . The formate/nitrate and propionate/nitrate precursors are completely converted into their oxides at temperatures as low as 210° and 260°C, respectively (as opposed to 270°C for the acetate/nitrate precursors). The enthalpy of the low-temperature reaction is dependent on the organic fraction in the mixture. In addition, the reaction enthalpies of the formate/nitrate precursors are less than that of the propionate/nitrate or acetate/nitrate precursors: − ΔH (7:6, propionate:nitrate) = 3184, − ΔH (7:6, acetate:nitrate) = 3473, and − ΔH (8:5, formate:nitrate) = 958 kj/mole. Hence, control over both the reaction enthalpy and the decomposition temperature can be achieved by the choice of organic ligand reducing agent and the nitrate/reducing agent ratios.