The half-metallic ferromagnet K(2)Cr(8)O(16) with the hollandite structure has been chemically modified using soft chemistry methods to increase the average oxidation state of chromium. The synthesis of the parent material has been performed under high pressure/high temperature conditions. Following this, different redox reactions have been carried out on K(2)Cr(8)O(16). Oxidation to obtain potassium-de-inserted derivatives, K(2-x)Cr(8)O(16) (0 ≤x≤ 1), has been investigated with electrochemical methods, while the synthesis of sizeable amounts was achieved chemically by using nitrosonium tetrafluoroborate as a highly oxidizing agent. The maximum amount of extracted K ions corresponds to x = 0.8. Upon oxidation the hollandite structure is maintained and the products keep high crystallinity. The de-insertion of potassium changes the Cr(3+)/Cr(4+) ratio, and therefore the magnetic properties. Interestingly, the Curie temperature increases from ca. 175 K to 250 K, getting therefore closer to room temperature.
Perovskite. -The Bi 0.5Pb0.5CrO3 double perovskite is prepared by high pressure reaction of stoichiometric amounts of PbO, Bi 2O3, Cr2O3, and CrO2 (Pt capsule in a belt-type press, 70 kbar, 1000 C, 15 min). The distorted orthorhombic perovskite crystallizes in the space group Pnma (powder XRD) and undergoes a structural phase transition to a monoclinic P2 1/m distorted perovskite phase at <150 K. The structural transition coincides with the onset of magnetic interactions. A weak ferromagnetic structure is observed at 40 K which is related to antiferromagnetic Cr-spin canting and a spin-glass transition. The small semiconducting-type electric resistivity is associated with Cr 3+ /Cr 4+ electron hopping and shows considerable magneto-resistance (15%). Due to the low resistivity, a dielectric permittivity of 300 is measured at <80 K which does not show any strong temperature-dependence. Neither ferroelectricity nor magnetocapacitance effects are detected in the investigated temperature range. -(PIRROTTA, I.; SCHMIDT, R.; DOS SANTOS-GARCIA, A. J.; GARCIA-HERNANDEZ, M.; MORAN*, E.; ALARIO-FRANCO, M. A.; J. Solid State Chem. 225 (2015) 321-329, http://dx.
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