Charge Ordering In Univalent Alkali Element Half Doped Manganites

Pr_0.75Na_0.25MnO₃, (Pr_1-xLa_x)_0.75Na_0.25MnO₃ ( 0≤x≤1.0) and Nd_0.75Na_0.25Mn_1-xCr_xO₃ ( 0≤x≤0.1) manganites were prepared successfully by sol-gel method. The structural, magnetic, electrical and thermal transport properties were investigated systematically. X-ray diffraction results show that Pr_0.75Na_0.25MnO₃ is orthorhombic structure with Pnma space group. At room temperature, the compound shows paramagnetic characteristic. There is a peak appears in the magnetization versus temperature curve around 220 K. To clarify whether this peak is associated with the ordering arrangement of Mn³⁺ and Mn4+ ions, electron diffraction experiments were carried out below and above 220 K, respectively. Only basic Brag diffraction spots can be observed at high temperatures, however, superlattice diffraction appears below 220 K. This provides direct evidence for the existence of charge ordering in Pr_0.75Na_0.25MnO₃. We find the Mn³⁺ and Mn4+ cations form zigzag chains in a-c plane by analyzing the diffraction patterns. Combining with the magnetization measurements and the results of electron spin resonance, we confirm the antiferromagnetic phase and ferromagnetic components coexist in Pr_0.75Na_0.25MnO₃ below 120 K. In addition, the compound shows insulator behavior, and charge ordering can be destroyed by strong field, and only a paramagneitc to ferromagnetic transition which concomitant with insulator to metal transition is observed under high field. Thermal conductivity and thermalpower measurements suggest that the transport properties of the metallic phase under high field can not be explained by free electron gas model.When Pr³⁺ ions of Pr_0.75Na_0.25MnO₃ are substituted by La³⁺ ions, the structure of (Pr_1-xLa_x)_0.75Na_0.25MnO₃ converts from orthorhombic structure (for x≤0.5) to rhombohedral structure (for x≥0.7), which is arose by the substitution of La with large ionic radius. The Curie temperature increases with increasing La contents for x≥0.05. While the charge ordering transition temperature decreases with increasing La doping when A>≤1.197A, there is no charge ordering features on magnetization curves for A>≥1.203A. The compounds also show large magnetic entropy change under low field near or above room temperature for x≥0.5. They are strongly recommended to use as suitable substances for magnetic refrigeration.