STRUCTURE AND PROPERTIES OF (La_2-xSr_x)MnO₄ COMPOUNDS

Polycrystalline single phase (La_2-xSr_x)MnO₄ (1.10≤x≤1.40) samples were prepared by a citrate precursor method. They have tetragonal K₂NiF₄ structure with a space group of 14/mmm, increasing x, unit-cell a-axis increases and c-axis decreases, which is a result of decreasing of the Jahn-Teller effect due to the increase of the Mn(IV) ion. Spin glass state exists at low temperature and a hump observed from the susceptibility (χ) measurement is probably caused by the interactions of several magnetic phases. Among them, (La_0.6Sr_1.4)MnO₄ has the largest magnetoresistance ratio ([ρ(H)-ρ(0)]/ρ(0)), which reaches to 78.4% at 48 K. It is not possible to obtain single phase materials with 0.60≤x≤1.00 in the heat treatment temperature range of 1000-1600°C. Impurities were easily observed under SEM and X-ray diffraction patterns. Increasing the heating period and decreasing the temperature, the amount of ABO₃ perovskite phase increases. In order to extend the solubility range, of the SrO in the (La_2-xSr_x)MnO₄, it probably needs a higher temperature (>1600°C) and a shorter heat treatment period (<1 h).