Utilization of metallic Ag and Ag+ ions to optimize room-temperature TCR and MR of La0.7(Ca0.205Sr0.095)MnO3:xAg2O composites

Abstract

Herein, high-density La_0.7(Ca_0.205Sr_0.095)MnO₃:xAg₂O composites (LCSMO:xA, 0.0 ≤ x ≤ 0.5) are successfully fabricated by using a combination of sol–gel and solid-state methods. The electromagnetic properties are investigated using a phenomenological percolation (PP) model, double exchange (DE) mechanism and Jahn–Teller (JT) effect. Moreover, X-ray diffraction (XRD) and scanning electron microscopy (SEM) are employed to analyze the crystal structure and morphology of LCSMO:xA composites. The distortion of MnO₆ octahedron is confirmed via Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Also, the valence states and ionic stoichiometry are assessed by using X-ray photoemission spectroscopy (XPS). The results reveal that Ag⁺ ions, occupying the A-sites, significantly influence the crystal structure, formation of Mn³⁺/Mn⁴⁺ pairs, and distortion of MnO₆ octahedron. Furthermore, the presence of metallic Ag at grain boundaries (GBs) improves the conductivity of LCSMO:xA composites by providing the desired conductive channels. At the Ag₂O content of x = 0.2, the maximum temperature coefficient of resistance (TCR) of the resistivity is found to be 17.26% K⁻¹ at 299.9 K, whereas the low-field magnetoresistance (MR, 0.8 T magnetic field) is found to be 30.34% at 300.93 K. This room-temperature TCR value is the highest among those for the studied perovskite-structured manganese oxide composites. These results confirm that the room-temperature TCR and MR values of LCSMO:xA composites can be tuned by optimizing the content of metallic Ag and Ag⁺ ions.