K.D. Becker

A chemically driven insulator-metal transition in non-stoichiometric and amorphous gallium oxide.

Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.

Enhanced magnetisation in nanocrystalline high-energy milled MgFe2O4

The changes in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by means of Mossbauer spectroscopy, magnetisation measurements, and electron microscopy. The observed enhancement of the magnetisation in nanoscale milled MgFe2O4 is discussed with respect to the mechanically induced cation redistribution and spin canting.

Structural disorder in the high-energy milled magnesium ferrite

The structural and magnetic evolution in magnesium ferrite (MgFe2O4) caused by high-energy milling are investigated by Mossbauer spectroscopy. It is found that the nanostructural state of the milled MgFe2O4 is characterized by a mechanically induced cation redistribution between tetrahedral (A) and octahedral [B] sites. The reduced concentration of iron ions at (A) sites in the mechanically treated samples leads to the variation in the number of magnetic and nonmagnetic (A)-site ions as nearest neighbors of the Fe3+[B] ions. This results in a broad distribution of magnetic hyperfine fields at the [B] sites. In addition to the local magnetic fields B(6), B(5), and B(4) characteristic of nonactivated ferrite and corresponding to Fe3+[B] ions with n=6, 5, and 4 nearest (A)-site iron neighbors, respectively, the distribution curves of mechanically treated samples show additional components at smaller magnetic fields. The weight of the B(6) field decreases with increasing milling time, and the B(5) field becom...

Electrocoloration and oxygen vacancy mobility of BaTiO3

The electrical-insulation degradation of BaTiO3 is now of growing interest as the BaTiO3-based dielectric layers of multilayer ceramic capacitors are getting thinner to submicron thicknesses. The degradation is understood to be due to the electrotransport of oxygen vacancies and may be monitored by the colors emanating from the cathode and/or anode. In the case of single crystal BaTiO3, a brown color emanates from the anode and a blue color from the cathode. We will experimentally review the generation of the colors in BaTiO3 in electric fields, and discuss their origins and kinetics of color front migration. From the latter the oxygen vacancy mobility against temperature in the range of 150–500°C is subsequently determined and compared with all the literature data that have normally been estimated by other means at elevated temperatures.

Mechanically induced cation redistribution in ZnFe2O4 and its thermal stability

The changes in zinc ferrite caused by high-energy ball-milling are investigated. Formation of spin arrangement in the ball-milled ZnFe2O4 is caused by the onset of the exchange interaction of the Fe3+ (A)O2−Fe3+[B] type, taking place due to the mechanically induced inversion as well as by the onset of the interaction of the Fe3+[B]O2−Fe3+[B] type with deformed bond angle. Structural metastability of the milled ZnFe2O4 is manifested by the gradual recrystallization terminating at 900 K by a total recovery of the structure.

Mechanochemical reduction of nickel ferrite

Abstract The changes in nickel ferrite (NiFe2O4) due to high-energy milling in a stainless steel vial have been investigated by Mossbauer spectroscopy, X-ray diffraction (XRD), and thermal analysis. The milling process reduces the average crystallite size of NiFe2O4 to the nanometer range. The Mossbauer spectra show the formation of metallic iron and of a phase containing Fe2+ ions. The fraction of the reduced products increases with increasing milling time. The range of thermal stability of the metastable milled reduction products has been determined by studying their response to changes in temperature.

Nonstoichiometry and defect structure of Mn-doped BaTiO3−δ

Abstract Oxygen nonstoichiometry (δ) of 1 m/o Mn-doped polycrystalline BaTiO3−δ has been measured as a function of oxygen partial pressure in the range of 10−16≤PO2/atm≤0.1 at elevated temperatures (900≤T/°C≤1100) by a solid-state coulometric titration technique. The extent of nonstoichiometry of Mn-doped BaTiO3 is much larger than that of undoped BaTiO3, which is attributed to defect-chemical role of Mn as acceptors. The nonstoichiometry isotherms indicate that Mn-ions change their valence from +4 (or MnTix) to +3 (or MnTi′) to +2 (or MnTi″) with decreasing PO2.

Mechanically induced cation redistribution in magnesium ferrite and its thermal stability

Abstract The changes in magnesium ferrite (MgFe 2 O 4 ) caused by high-energy milling are investigated. Mechanical treatment reduces the average crystallite size of MgFe 2 O 4 to the nanometer range and induces cation redistribution between tetrahedral and octahedral sites. The degree of inversion of the mechanically treated ferrite is compared with that of MgFe 2 O 4 quenched from high temperatures. The range of thermal stability of mechanically induced metastable states is determined by studying the response of the metastable mechanically activated MgFe 2 O 4 to changes in temperature.

Mechanically Induced Cation Redistribution and Spin Canting in Nickel Ferrite

The structural and magnetic evolution in nickel ferrite (NiFe2O4) caused by high-energy milling are investigated by Mössbauer spectroscopy. It is found that the nanostructural state of the milled NiFe2O4 is characterized by a reduced concentration of iron ions on tetrahedral sites. The degree of inversion in NiFe2O4 is calculated from the subspectral area ratio of both high- and zero-field Mössbauer spectra. Several interesting features are involved in the work, e.g., superparamagnetic relaxation, mechanically induced cation redistribution, and spin-canting effect.

A temperature-dependent Mössbauer study of mechanically activated and non-activated zinc ferrite

The changes of ZnFe2O4 caused by mechanical activation as well as the structural evolution of nonactivated and mechanically activated zinc ferrite occurring during heating, have been investigated by in situ Mössbauer spectroscopy. Attention is directed to mechanically induced changes in magnetic properties of zinc ferrite, the variation of nuclear quadrupolar interactions, the thermally induced changes of the Mössbauer shift, and also to the structural response of mechanically activated zinc ferrite to changes in temperature.