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Introduction
The perovskite like cobaltite Pr0.5Ca0.5Co03-δ exhibits an interesting properties such as physical phenomena of spin-state transition, metal-insulator transition and magnetoresistance effect [1-3]. In last years, an important number of researches have been studying the ceramic materials based on this cobaltite using doping for replacement the position of the atoms inside in the crystalline symmetry [4]; in this work, the influence of the ionic-radio will be analyzed for the structural system response of Pr0.48TR0.02Ca0.5Co03-δ where TR=Eu, Sm and Dy by means of the X-rays diffraction patterns (XRD) and Rietveld's refinement analysis in GSAS software.
Experimental details
From the precursors Pr6O11, Co3O4, CaO, Sm2O3, Eu2O3 y Dy2O3 of high purity (>99.99%), three polycrystalline samples were prepared Pr0.48TR0.02Ca0.5Co03-δ (TR=Eu,Sm,Dy) by the solid state reaction method [5]. Oxides were dried at 200°C for 12h and subsequently weighed in appropriate quantities and macerated for 3h in an agate mortar, then they press tablets at a pressure of 2MPa. They surrendered to thermal calcination treatments to 750°C and sinterization to 830°C. The Structural characterization of the samples was carried out through diffraction (XRD) technique in a Diffractometer panalytical x'pert PRO-MPD, using Cu radiation (λ = 1.54186 Å) in intervals of 10° and 90° with steps of 0.02° [6]. The patterns were analyzed by Rietveld refinement. In addition were measurements of magnetization as a function of temperature with the vibrating sample magnetometer VSM from Quantum Design in the temperature range from 50K to 300K using the ZFC-FC (zero-Field Cooled) method. They were measurements of magnetization as a function of the field applied, in a range between - 30kOe to 30kOe to 50K.
Results and discussion
The theorical model of the crystal structure of the system Pr0.48TR0.02Ca0.5Co03-δ is exposed in the Fig 1 whit (TR=Eu,Sm,Dy). This was built from Crystallographic databases, software SPUDS and models proposed by other authors [1,7].
The three samples present peaks of secondary phases, with the symbol ♦ the corresponding reflections are represented to Co3O4 and with the symbol ♣ the reflections corresponding to CoO are represented. The respective XRD bosses allow to infer that the whole system Pr0.48TR0.02Ca0.5Co03-δ it crystallizes in the same spatial group. The results suggest that the size of the ionic radius affects the presence of the second phases. The results by means of analysis for method Rietveld indicate that the samples crystallize in the spatial group orthorhombic Pnma (#62) as the mentioned one in the Fig 1. In the Table 1 the structural parameters are recorded of structure obtained by the Rietveld for every rare earth refining process; these are shown in Fig 3, shown that volume increases in proportion to the ionic radius of rare earth. By comparing the values obtained for these parameters with other authors shows that they are similar [1].
Table 1 Lattice parameters for the system Pr0.48TR0.02Ca0.5Co03-δ obtained from the Rietveld refinement of the XRD patterns.
The figure 4 shows the magnetization curve in function the temperature mode ZFC and FC for a field of 1000 Oe in a range from 50K to 300K, is a behavior observed paramagnetic type. Around 180K the value of the magnetization is similar in rare-earth Eu and Sm, while it is higher in Dy, the curves ZFC and FC are separated near the 70K, 60K y 150K, for the Sm, Eu and Dy respectively; indicating a possible order weak ferromagnetic due to the presence of ferromagnetic clusters.
The Figure 5 shows the magnetization in function of the field to 50K, is perceived as more radio ionic value of magnetization is greater without reaching a saturation value. In the inset shows a behavior ferromagnetic weak which is in accordance with the temperature of the branch FC and ZFC. In the Table 2 summarizes the fields of coercion (HC) and the magnetization remnant (Mr) of each one of the three systems. [8]
Figure 5 Magnetization as a function of the applied field (Oe) curves for system Pr0.48TR0.02Ca0.5Co03-δ.
The Figure 6 shows the inverse of the susceptibility X-1(T) in function of the temperature for the branch ZFC, showing a linear behavior. In the temperature range between 100K to 200K, an adjustment was made type Curie-Weiss (1/X=1/C T- θ/C) from which was calculated the effective magneton (μelf), obtaining values of 3.30 (4) μB, 3.48 (6) μB and 3.95 (6) μB for Sm, Eu and Dy respectively.
Conclusions
The new system Pr0.48TR0.02Ca0.5Co03-δ was produced by the solid state reaction method. According to the results of Rietveld structure refinement (XRD), the system crystallizes in an orthorhombic system for Pnma space group (#62); the results show that the volume increases with the ionic radius of the Rare Earth doping; however, it preserves the space group. The precedent shows that the employment of CaO as precursor is equally effective in comparison with another samples made with calcium-carbonate precursor (CaCO3) used by other authors [1,4]. By another hand, the magnetization measures have a paramagnetic behavior with a weak ferromagnetic component in low field, and temperatures and the magnetic date interpreted lead to the value between μ = 3.30(4) μB and μ = 3.95(6) μB Bhor magnetons. The Conductivity and magnetization analyses below 50K studies are necessaries, in order to verify the possible existence of metal-insulator transition (MIT) reported on similar systems.
