Síntese e caracterização de pirocloros do tipo (GdxBi1-x)2FeTaO7 com 0,0 ? x ? 1,0

Abstract

(GdxBi1-x)2FeTaO7-type pyrochlores with 0 &#8804; x &#8804; 1.0 were synthesized by high energy milling, followed by treatment in a free atmosphere. The prepared samples were characterized structurally and magnetically by X-ray diffractometry and Mössbauer spectroscopy on the 57Fe and 155Gd isotopes. The results showed that the samples crystallized in one or even two structures, depending on x: for 0.0 &#8804; x &#8804; 0.6 the stable structure is cubic (Fd-3m), while for 0.8 < x &#8804; 1.0, the rhombohedral structure (R-3) prevails; at the interval 0.66 &#8804; x &#8804; 0.8, both the cubic and rhombohedral structures coexist. The parameter of the cubic structure decreases linearly with gadolinium concentration, x, up to approximately x = 0.6. In the region where the two phases coexist the lattice parameter remains approximately constant. For the rhombohedral phase both lattice parameters, a e c, decrease with x. At room temperature, the Mössbauer spectroscopy measurements on57Fe showed a paramagnetic component (dublet) for the cubic structure and two for the rhombohedral phase. For some concentrations, magnetic components were observed in the spectra. It is discussed whether the sextets indicate that there was a break in the geometric magnetic frustration by substituting gadolinium with bismuth in the triangular nets of sites A or whether they belong to secondary phases. However, the subspectral areas supplied by the fit, when the two structures coexist, presented good agreement - that is valid for Mössbauer measurements for both the nuclear probes - in relation to the molar concentration data given by the Rietveld refinement.