Effect of Co doping at the B-site on the structure and tranport properties in La 2/3 Ca 1/3 MnO 3

Summary form only given. Polycrystalline samples La2/3Caι/3(Mnι_xCox)θ3 with x=0.1, 0.2, 0.3, 0.4, and 0.5 were synthesized by standard solid-state reaction. X-ray diffraction results show that they are of single phase for the specimens with x<;0.3. Rietveld refinement of X-ray diffraction data shows Mn-O-Mn angle decreases from 178.8° for x=0.1 to 128.8° for x=0.3. A metal-to-insulator transition was only observed in sample with x=0.1. The different transport properties of these samples were attributed to the different Jahn-Teller distortion. The temperature dependence of reisitivity at low and high temperature can be described by the small polaron model and Kondo-like effect. In the present work, effect of Co doping at B-site of La2/3Caι/3Mnθ3 (LCMO) is discussed. The spin configuration of Co is very complex. The Co ions are in their low-spin (LS) tg6e2g0 (S=0) state at the low temperature (T<;35 K) and high-spin (HS) tg4e2g2 (S=2) state at high temperature (300 K<;T<;600 K). There maybe a intermediate-spin (IS) tg5e2gl (S=l) state or a mixture of HS and LS of Co3+ in the interval 35 K<;T<;100 K. The structure and properties are close dependent on the Co dopant as discussed below. Polycrystalline samples La,/3Ca2/3(Mnι_xCox)θ3 (LCMCO) with x=0.1, 0.2, 0.3, 0.4, and 0.5 were synthesized by standard solid-state reaction. The analytical purity materials La203, CaC03, Mn02 and Co203 were well mixed in stoichiometric proportions and pestle for 30 minutes before calcined at 1000 °C for 6 hours. The calcined powder was pressed into slices and then sintered at 1400°C for 32 hours in air. All the specimens were slowly-cooled to room temperature in air for about 15 hours. The structure of the samples was characterized by X-ray diffraction (XRD), which was performed at a Rigaku Dmax-γB diffractometer at room temperature with step scanning model. The structural parameters were obtained from a standard Rietveld program GSAS. The resistivity with/without magnetic field were measured by the standard four-probe method at a Quantum Design PPMS. Measurements of x-ray diffraction at room temperature show that samples within x<;0.3 are of single phase which can be well refined in orthorhombic symmetry with the space group of Prima (Rwp<;15%). The lattice parameters of a, b, and c are almost independent of the Co dopant which indicates the unit cell is unchanged after Co was doped in LCMO. The bond angle of Mn-Ol-Mn decreases largely from 178.8° for x=0.1 to 128.8° for x=0.3. It will induce the large change of electric transport properties as discussed below.