Structure and transport properties of La0.75-xRExCa0.25MnO3 (RE = Gd, Dy, Ho; 0 ≤ x ≤ 0.75) manganites

Structure and transport properties have been studied for a series of La0.75-xRExCa0.25MnO3 manganites with heavy rare earth ions of Gd, Dy, Ho substituting La with X = 0, 0.10, 0.25, 0.50 and 0.75. Polycrystalline samples were prepared by the carbonate precipitation route. The oxygen content was determined by the iodometric tritiation. The x-ray investigations carried out by the powder method show that the unit cell volume gradually decreases and orthorhombic distortion of the lattice increases with rising RE content. Below the room temperature the electrical resistivity is of the semiconducting type for all the samples studied. Electrical resistivity vs. temperature dependences were analysed within models: simple thermal activation, Mott's variable range hopping, adiabatic, nonadiabatic and bipolaron. The Curie temperatures of Gd, Dy, and Ho substituted manganites determined from magnetization measurements show that at 280 K all the samples are in the paramagnetic phase. The increasing RE fraction reduces magnetization at 4 K as compared to La0.75Ca0.25MnO3. (author)