La0.7Sr0.3
xCaxMnO3 (x¼0, 0.1, 0.2 and 0.3) nanoparticles, prepared by sol–gel method, were studied by means of X-ray diffraction, transmission electron microscopy, thermal expansion and magnetostriction measurements. It was revealed that samples of low Ca concentration (xr0.1) crystallize in rhombohedral perovskite structure with R-3c space group, and by increasing x, smaller Ca radius causes the lattice to convert to orthorhombic structure (space group of Pbnm). The results exhibit an anomalous behavior in the thermal expansion behavior of all samples, as well as temperature dependence of their volume magnetostriction being related to the presence of a conductive magnetic two-phase state at temperatures higher than TC. Moreover, the magnitude of this magnetovolume effect increases with x due to smaller Ca size, which softens the crystal lattice. The results show that Ca substitution results in weakening the double-exchange interaction and consequently reducing TC, as well as decreasing the magnetic anisotropy. Furthermore, our nanocrystalline samples have a relatively lower TC, compared with the reported values for similar compounds with larger particle size. Among them, the sample with x¼0.2 possess TC value of about room temperature; to say by partial substitution of Ca for Sr we succeeded in adjusting the transition temperature of this series of manganites which was a main motivation for this research.

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