John M. Longo

CaMnO2.5 and Ca2MnO3.5: New oxygen-defect perovskite-type oxides

Abstract We found that when the precursors CaMnO 3 and Ca 2 MnO 4 are reduced with any one of a variety of inorganic (H 2 , NH 3 ) or organic (C 2 H 4 , C 3 H 6 ) reducing agents between 300 and 500°C, topotactic reaction occurs to produce the ordered oxygen-defect phases CaMnO 2.5 and Ca 2 MnO 3.5 , respectively. Orthorhombic cell constants for CaMnO 2.5 are a = 5.43(1), b = 10.24(1), and c = 3.74(1) A, and for Ca 2 MnO 3.5 are a = 5.30(1), b = 10.05(1), and c = 12.24(1)A. These compounds were characterized by powder X-ray diffraction, thermogravimetric analysis, magnetic susceptibility, and infrared spectroscopy. The reduced compounds reversibly oxidize to their respective precursor in oxygen at low temperatures.

Promotion of CO2 mass transfer in carbonate solutions

A general, physico-chemical analysis of mass transfer rate promotion in the system CO2-potassium carbonate-water-promoter is presented. Different possible mechanisms of promoter action including homogeneous catalysis, “shuttle” mechanism and surface reactions are discussed and classified. A unified picture of promoter chemistry is presented, showing that differences between inorganic and organic promoters are predominently quantitative, not qualitative. The “shuttle” mechanism is analyzed for absorption. The analysis leads to the prediction that the CO2 mass transfer rate may be influenced by the liquid hold up. This is related to the fact that, although the reaction in the interface region may be fast enough to enhance the mass transfer rate, the different reaction in the bulk may not be fast enough to maintain chemical equlibrium. This complex type of chemical absorption process has not been considered previously in the literature.

Phase relations in the CaMnO system

Abstract The use of solid solution precursors is introduced as an effective low temperature synthesis technique which facilitates solid state reaction in selected mixed metal oxide systems. This synthesis technique has been used to study phase relations in the manganese-rich portion of the CaMnO system at temperatures below 1000°C, and has resulted in the assemblage of a new isobaric (P O 2 = 1.0 atm), subsolidus phase diagram. This diagram contains several low temperature phases having the following compositions: Ca 2 Mn 3 O 8 , CaMn 3 O 6 , CaMn 4 O 8 and CaMn 7 O 12 . The experimental results of this study along with literature data are used to present a proposed isobaric (P O 2 = 0.2 atm) phase diagram for the entire CaMnO system.

New oxide pyrochlores: A2[B2−xAx]O7−y (A = Pb, Bi; B = Ru, Ir)

A new series of mixed metal oxides with the pyrochlore structure has been discovered and can be described by the general formula A2[B2−xAx]O7−y where A = Pb or Bi, B = Ru or Ir, 0 < x ≲ 1 and 0 ≤ y ≤ 0.5. Values of x appreciably greater than zero may only be obtained at relatively low temperatures. It has been observed that when A = Pb and B = Ru, y = 0.5 for all values of x. Linear correlations between lattice parameter and x are presented for both Pb2[Ru2−xPbx]O6.5 and Bi2[Ru2−xBix]O7−y.

Neutron diffraction investigation of ordered oxygen vacancies in the defect pyrochlores, Pb2Ru2O6.5 and PbT1Nb2O6.5

Abstract Powder neutron diffraction has been used to investigate the structure of the defect pyrochlores Pb2Ru2O6.5 and PbT1Nb2O6.5. Both materials show evidence for oxygen vacancy ordering that is accompanied byA-site cation displacement. The diffraction data for Pb2Ru2O6.5 have been successfully fit in the cubic space groupF¯43m. The results confirm half occupancy of the defect site, demonstrate oxygen vacancy ordering, and show that each Pb atom is displaced by 0.040(4)A˚toward its associated vacancy. The diffraction data for PbT1Nb2O6.5 have been fit in the tetragonal space groupP¯4m2 using a model that allows ordering of Pb and T1 on theA site in concert with oxygen vacancy ordering. The fitting results specify a stoichiometry PbT10.90Nb2O6.45 for this material; the slight T1 deficiency is probably associated with some T1 loss to the vessel walls during the sealed tube synthesis at 600°C. The Rietveld structural refinement also shows that Pb is displaced slightly away from its associated vacancy while T1 is displaced significantly toward its associated vacancy with resulting T1 T1 separations similar to those found in T1 metal. It is suggested that theA-site cation ordering observed in each of these materials is a consequence ofA cation-oxygen bonding rather than the formation ofA Abonds through the oxygen vacancy despite the closeA Aseparations observed. These results are believed to provide the first examples of anion/vacancy ordering in a defect pyrochlore of the typeA2B2O7−y.

CuTa2O6 - crystal growth and characterization

The compound CuTa2O6 has been prepared as crystals from a Cu/O melt and found to be tetragonal (a = 7.510A, c = 7.526A) rather than cubic as reported in the literature. The coefficient of thermal expansion between room temperature and 1000°C was found to be 8.0 × 10−6°C−1. Electrical resistivity measurements on a crystal showed semiconductor behavior between room temperature (ϱ = 2 × 103 Ωcm) and 140°K (ϱ = 7 × 106 Ωcm) with an activation energy of EA = 0.2 eV. Magnetic measurements between 4.2°K and room temperature showed Curie-Weiss behavior with a change in μeff at 120°K. For T>120°K, μeff = 1.76μB and θp = 0°K while for T<120°K μeff = 1.91 μB and θp = −15°K.

The structure of Ba3W2O9; an example of face-shared octahedra with tungsten (VI)

The structure of Ba3W2O9 has been determined from powder x-ray diffraction data and found to be the same as in Cs3Tl2Cl9. Two-thirds of the octahedral sites between hexagonal closest-packed BaO3 layers (ABAB…) are filled by the tungsten atoms. The enneaoxo W2O96− group represents the first crystal chemistry example of tungsten (VI) in a face-sharing arrangement. Infrared and Raman data are consistent with overall 32 (D3) site symmetry. Crystallographic and spectroscopic evidence supports a covalent bonding model with oxygen pπ → metal dπ stabilization of this tungsten (VI) phase.

The electrical properties of A2[Ru2−xAx]O7−y (A= Pb or Bi) pyrochlores as a function of composition and temperature

The variable stoichiometry pyrochlore compounds A/sub 2/(Ru/sub 2-x/A/sub x/)O/sub 7-y/ (A = Pb of Bi) exhibit high electrical conductivity with room temperature values ranging from 10 to 1000 (ohm-cm)/sup -1/, dependent upon composition. For the lead-containing series, resistivity vs temperature data show that there is a smooth variation from a positive to a negative temperature coefficient of resistivity (TCR) as a function of increasing lead content. Substitution of approx. 20% of the ruthenium by lead results in a material with essentially temperature-independent resistivity. In the bismuth-containing series, the essentially zero TCR at zero substitution is made progressively more negative as the extent of bismuth substitution is increased. The electrical conductivity of these pyrochlore compounds is discussed in terms of electron transfer between large conducting segments consisting of connected networks of RuO/sub 6/ octahedra. It is postulated that the thermally activated conductivity behavior exhibited by the negative TCR material is associated with the presence of thin barrier regions resulting from the introduction of Pb or Bi on the Ru site. Results of infrared absorption measurements showing active modes associated with Pb-O but not with Ru-O bonds support this picture.

Magnetic properties of the mixed-valence compounds CaMn3O6 and CaMn4O8

Abstract The preparation of two new mixed-valence compounds, CaMn3O6 and CaMn4O8, are described and their magntic and EPR behavior investigated. The Mn moments in both compounds have nearly spinonly values. CaMn3O6 and CaMn4O8 order ferrimagnetically near 3 and 89 K, respectively. The broad, Lorentzian EPR lines indicate a significant exchange interaction between Mn3+ and Mn4+ ions. The magnetic and EPR results suggest a strong ferromagnetic interaction between Mn3+ and Mn4+ ions and a comparable antiferromagnetic Mn3+Mn3+ and/or Mn+Mn4+ interaction.

The preparation and characterization of Ba3Te2O9; a new oxide structure

The structure of Ba3Te2O9 has been determined from x-ray powder diffraction data and by profile refinement of neutron diffraction data. We find tellurium octahedrally coordinated as expected and the same face-shared [B2O9]6− unit as observed in Ba3W2O9. The phase Ba3Te2O9 has, however, a Cs3Fe2F9 type structure (P63mmc, a=5.8603(1)A, c=14.3037(6)A) rather than the Cs3Tl2Cl9 structure (R3c) found for the tungsten analogue. The two oxide structures have the same BaO3 layer sequence but differ only in the spatial arrangement of the [B2O9] groups in the lattice. Infrared and Raman spectra confirm the different site symmetries associated with the different packing of the tungstate and tellurate anions in their respective structures.