61 research outputs found
Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: Isovalent substitution effect
In the Fe-Mo based B-site ordered double-perovskite, A2FeMoO6.0, with iron in
the mixed-valence II/III state, the valence value of Fe is not precisely fixed
at 2.5 but may be fine-tuned by means of applying chemical pressure at the
A-cation site. This is shown through a systematic 57Fe Mossbauer spectroscopy
study using a series of A2FeMoO6.0 [A = (Ba,Sr) or (Sr,Ca)] samples with high
degree of Fe/Mo order, the same stoichiometric oxygen content and also almost
the same grain size. The isomer shift values and other hyperfine parameters
obtained from the Mossbauer spectra confirm that Fe remains in the
mixed-valence state within the whole range of A constituents. However, upon
increasing the average cation size at the A site the precise valence of Fe is
found to decrease such that within the A = (Ba,Sr) regime the valence of Fe is
closer to II, while within the A = (Sr,Ca) regime it is closer to the actual
mixed-valence II/III state. As the valence of Fe approaches II, the difference
in charges between Fe and Mo increases, and parallel with this the degree of
Fe/Mo order increases. Additionally, for the less-ordered samples an increased
tendency of clustering of the anti-site Fe atoms is deduced from the Mossbauer
data.Comment: 19 pages, 6 figures, submitted to Phys. Rev.
The Current NHL Lockouts: For Better or Worse?
Since 1967, the National Labor Relation Board (NLRB), an independent government agency that governs labor issues and unfair practices, has demanded the professional athletes forming their player union to negotiate with team owners concerning all labor issues under the collective bargaining agreements (CBAs). Under the CBAs, players and owners must faithfully negotiate mandatory issues such as length of contract, work hours, wages and working conditions. Despite the great intent of the establishment of CBAs, players and owners can fail to reach agreement and eventually lead their dispute to either a strike or lockout
- …