Authors
Abstract
Results from a detailed investigation on the structural, optical, electrical and magnetic properties of polycrystalline bulk samples of La1-xCaxMnO3 (x=0, 0.3, 0.5 and 0.7) synthesized by solid state reaction method are presented. The Rietveld analysis of the X-Ray diffraction (XRD) profiles clearly indicated that the XRD patterns are well fitted with orthorhombic structure. Raman spectral features revealed their finger print modes and irreducible representations at the brillouin zone center as per the group theory. It is also observed that as doping is increased, these compounds tend towards a cubic form. Ooptical band gap ‘Eg’ study reveals that the Eg decreases with Ca doping resulting in increase in conductivity. This is consistent with the resistivity measurements. In all the samples, except when x = 0.0 and 0.7, the resistivity at the highest temperature measured (ρ300 K) is less than that at 5 K (ρ5K), although for temperature T<Tp, the material shows a metal variation of ρ with temperature (d ρ/dT > 0) whereas the value of ρ300 K decreases as x increases. Magnetization study revealed that temperatures corresponding to magnetic transitions Tc increases with doping. It is observed that the composition x = 0.5 show both a paramagnetic to ferromagnetic transition and an antiferromagnetic transition. An effort has been made to relate above observed results in the compound with the structural changes brought about by Ca doping.Possible mechanisms such as activated transport and Zener double exchange are used to understand the phase diagram of these materials.
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