Rietveld refinement of the semiconducting system Bi2−xFexTe3 from X-ray powder diffraction
Introduction
Thermoelectric materials (TE) useful for thermoelectric cooling at room temperature and below have attracted much attention recently because of their potential applications in superconductors and electronic apparatus. V2-VI3 compounds such as Bi2Te3 are good materials for thermoelectric refrigeration and power generation at room temperature [1], [2], [3], [4], [5]. Excellent TE materials require a perfect combination of electrical (σ) and thermal (κ) conductivity which define a figure of merit (ZT) [6], [7], [8] given by ZT = S2σT/κ, where S is the Seebeck coefficient and T is the temperature. The S and κ parameters are known to be related with the number of carriers, a possibility is expected that ZT could be controlled by the concentration of anti-site defects (AS). It is well known that (AS) defect occurs in the TE materials Bi2Te3 [9]. It originates from the non-stoichiometry of this material and the Bi and Te ions are easily replaced for the regular Te and Bi sites, respectively. Recently, semiconductors containing transition or rare earth ions in an amount excluding direct exchange interaction between their magnetic moments are referred to as diluted magnetic semiconductors (DMS) [10]. Earlier, partial ferromagnetism was observed at low temperatures [11] in uniform III–V (In, Mn) As films grown on a GaAs substrate [12]. Ferromagnetism was also observed in (Ga, Mn) As films [13] and in group IV–VI PbSnMnTe DMS [14]. Contrary to the case of III–V host matrices, Mn does not stimulate ferromagnetic order in the family of bulk layered Sb2−xMnxTe3 DMS materials at least in the range of magnetic impurity and carrier concentrations studied [15]. Single crystal of Bi2−xFexTe3 was studied through magnetic, Hall effect and resistance measurements [16]. The transition into a ferromagnetic state with easy axis parallel to the C3 axis was observed at temperature Tc increasing with in concentration and reaching 12k at x = 0.08.
The aim of the present work is firstly to determine the unknown structure and the microstructure changes induced to the Bi2Te3 system as Fe is incorporated. Characterization of the Bi2−xFexTe3 system as a new diluted magnetic semiconductor is in course and will be given in a next work.
Section snippets
Experiments
A series of V2−xMxVI3 samples in the system Bi2−xFexTe3 (x = 0.0, 0.02, 0.04 and 0.08) have been prepared. Pure starting materials, Bismuth (99.999%), Te (99.999%) and Fe (99.999%) supplied by Aldrich Chem. Coop. were used. Appropriate amount of Bi, Te and Fe according to their atomic weight were used to obtain 5 g of each composition. The ingots were prepared by the direct fusion and cooling cycle of the constituent elements in vacuum-sealed silica tubes. The synthesis process was carried out at
Characterization by SEM
Fig. 1 gives, as example, the EDX patterns of the virgin and the heavily doped samples. The determined chemical composition is within ±0.2% of the prepared compositions.
The SEM images of Bi2−xFexTe3 samples are shown in Fig. 2(a–f). We observe that, for the binary Bi2Te3 sample there is a mixture of sheets and undefined shape particles in irregulars morphology. As the Fe is added with x = 0.02, the morphology of the crystallites changes to be dominant by sheet like-texture with no obvious basal
Conclusion
The scanning electron microscope of the Bi2−xFexTe2 system shows the tendency of the system to form more aligned sheet structure. The size and the thickness of the flacks increase, with increase in the Fe content. The Rietveld refinement was used to analyze the crystalline structure of the system under study. The refinement of instrumental and structural parameters led to reliable values of RB, RF and Chi2. Better residual factors were obtained when the Fe atoms accommodated in the 0, 0, Z
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