Oxide electronics

Abstract:

Magnetite is a classic example of a mixed-valent transition metal oxide, in which electronic conductivity and ferromagnetism result from electron hopping between octahedrally coordinated Fe²⁺ and Fe³⁺ states. Below the 122 K Verwey transition, the conductivity falls by a factor of ∼100 and a complex monoclinic (or triclinic) superstructure of the high temperature cubic spinel arrangement is adopted. This is assumed to be the result of Fe²⁺/Fe³⁺ charge ordering on the octahedral sites, but this has not been confirmed crystallographically, as single crystal refinements have been hampered by the extensive twinning that accompanies the Verwey transition. We have used very highly resolved powder diffraction data to attempt Rietveld refinements of the low temperature structure. The powder sample was prepared by grinding a single crystal of stoichiometric magnetite. Data were collected at 90 K on instruments HRPD at the ISIS neutron source, UK, and BM16 at the European Synchrotron Radiation Facility, France. The very high resolution of these data enables the monoclinic distortion to be observed, and the structure has been refined on the supercell proposed by Iizumi et al (Acta Cryst. B38, 2121 (1982)) with Pmca pseudosymmetry, giving parameters a = 5.94443(1), b = 5.92470(2), c = 16.77518(4) Å, β = 90.236(1)°. The mean octahedral site Fe-O distances differ from each other significantly, but the maximum difference between values is only 20% of that expected for ideal Fe²⁺/Fe³⁺ charge ordering.

Abstract:

We report the synthesis under pressure, the critical temperature and the structural characteristics of YSr2Cu3Ow. The compound does not form at normal pressure, but, once formed, can be retained at normal pressure in a metastable state. The synthesis was carried out in a multi-anvil apparatus at 30 kBars and 1050°C. The as-prepared sample was found to become superconductor with a Tc of ≈ 63K. Tc increased up to ≈ 70 K after annealing in oxygen at 290°C. The refinement of the neutron diffraction data indicated that the as prepared YSr2Cu3Ow is tetragonal (a=3.7855 Å and c=11.386 Å) and the oxygen content, w, is ≈ 6.84.

Abstract:

High real-space resolution atomic pair distribution functions (PDFs) have been obtained from ZnSe(1-x)Te(x) using neutron powder diffraction. Distinct Zn-Se and Zn-Te nearest neighbor (nn) bonds, differing in length by delta_r= 0.14Angstroms, are resolved in the measured PDF allowing the evolution with composition of the individual bond-lengths to be studied. The local bond-lengths change much more slowly with doping than the average bond-length obtained crystallographically. The nn bond-length distributions are constant with doping but higher-neighbor pair distributions broaden significantly indicating that most of the strain from the alloying is accommodated by bond-bending forces in the alloy. The PDFs of alloys across the whole doping range are well fit using a model based on the Kirkwood potential. The resulting PDFs give excellent agreement with the measured PDFs over the entire alloy range with no adjustable parameters.