Improved temperature coefficient of resistance and transport properties of Nd 0.7 Sr 0.3− x A x MnO 3 (A = Ag, Na, K: x = 0 & 0.1) manganites

We have investigated the structural, magneto-resistive, electrical, and thermal transport properties of monovalent substituted polycrystalline Nd 0.7 Sr 0.3− x A x MnO 3 ( x = 0, 0.1 and A = Na, Ag, K) compounds. The Rietveld refinement of the X-ray diffraction patterns reveals that all samples crystallize in orthorhombic structure with Pnma space group. The scanning electron microscopy micrographs evidenced that the grains acquire uniform morphology, nearly spherical, and unequal grains in all the studied samples. The decrease in metal-insulator transition temperature on the monovalent substitution is a result of reduced double-exchange interactions. Further, above the metal-insulator transition, electrical transport properties are analyzed through variable range hopping and small polaron hopping models. The substitution of monovalent cations enhanced the experimental parameters like hopping distance and activation energy, while the density of state at the Fermi level decreased significantly. On the monovalent doping, the samples showed very high magnetoresistance (MR%), and the temperature coefficient of resistance (TCR%) makes them prospective candidates for advanced uncooled infrared bolometers. The thermoelectric power ( S ) demonstrates a positive to a negative crossover, and the value of S increased in doped samples. At the higher temperatures, the small polaron is responsible for the conduction process.