Magnetic/Crystalline Disorder and Transport Phenomena in La_0.54Ho_0.11((Sr, Ca)(K, Na))_0.35MnO₃ Manganites

La1-x(Sr, Ca)xMnO3 manganites present a special interest due their high Curie and metal-insulator transition temperatures and the possibility to obtain magnetoresistive sensors able to work near room temperature. We have synthesized two rows of manganites, with La0.54Ho0.11Sr0.35-xKxMnO3 and La0.54Ho0.11Ca0.35-xNaxMnO3 chemical compositions. XRD analysis, performed with a Huber diffractometer at room temperature, indicated that the manganites contain perovskite phases, with Pnma structure. The atomic positions, lattice parameters and some microstructure parameters, as average size of coherent blocks and microstrains, were obtained by using GSAS program. The dependence of the resistivity with temperature and the applied magnetic field was determined by means of four probes method, between 77 and 300 K. The influence of K/Na concentration on the disorder degree (at atomic and crystallite level) and on the transport mechanisms was discussed.