Portal de Periódicos da CAPES

Substituted Ge–Sb–Te materials: structure, element distribution and thermoelectric properties

2012; Wiley; Volume: 68; Issue: a1 Linguagem: Inglês

10.1107/s0108767312099047

1600-5724

Simon Welzmiller, Tobias Rosenthal, Thorsten Schröder, Philipp Urban, Felix Fahrnbauer, Christian Stiewe, Oliver Oeckler,

Chalcogenide Semiconductor Thin Films

Oxides with formula AB 2 O 6 , where A is a divalent cation (typically Fe 2+ or Mn 2+ ) and B a pentavalent cation (Nb 5+ or Ta 5+ ), form a family of minerals including columbite (tri-á-PbO 2 structure, orthorhombic Pbcn) and tapiolite (Fe,Ta-rich terms, trirutile structure, tetragonal P4 2 /mnm).Here we present different unmixing processes occurring in such minerals: a columbite sample from Kragero (Norway) and a tapiolite sample from Kimito (Finland).Cation ordering processes at high temperature in columbite hide a complex scenario which could be unravelled by combining structural information at different length.A novel mechanism of microstructure formation associated to cation ordering under non-equilibrium conditions has been found: a fully coherent and regular array of nanosized domains develops in columbite crystals through separation into two discrete phases which have the same composition but different degrees of long-range cation order.Such pattern of microstructure can be created by high temperature annealing of crystals which grew with a metastable state of cation disorder.Size and distribution of such domains remain more or less constant over a prolonged period of annealing, as revealed by synchrotron X-ray powder diffraction and TEM analyses [1].High-temperature treatment of tapiolite above 750 °C causes a significant and progressive contraction of unit-cell parameters and volume.Such phenomenon is interpreted on the basis of a progressive oxidation of Fe 2+ with consequent formation of FeTaO 4 , associated to unmixing of Ta 2 O 5 .FeTaO 4 has the rutile structure and crystallizes in the same space group as FeTa 2 O 6 with similar (but for the tripling of the c-axis in tapiolite due to cation ordering) but slightly shorter lattice parameters.As the transformation proceeds, a significant broadening, but not a clear splitting, of the reflections is observed.Unit-cell parameters values, which after the beginning of the transformation may represent a weighted average of those of the two coexisting phases, were measured also under "cooling-down" conditions.While the diffraction patterns collected down to RT on the recovered crystals are still indexable by a trirutile-type unit-cell, intensity data are not refinable using the tapiolite or the FeTaO 4 structural models.Local probes such as ì-Raman and XANES spectroscopies have then been used and allowed to distinguish the two phases in the HT-treated tapiolite and to determine their relative amounts.