DECREASE OF FE MAGNETIC-ORDERING TEMPERATURE IN YSR2CU2FEO6+X THROUGH SYNTHESIS

The effect of synthesis on the structural and magnetic properties of YSr2Cu2FeO6+x has been investigated. Materials were prepared by bulk solid-state oxidizing (method [O]) and reduction (method [NO]) reactions, and by sol-gel solution (method [S]) synthesis. Materials were characterized by X-ray powder diffraction, iodometric titration, magnetic susceptibility, and Fe-57 Mossbauer absorption spectroscopy. We observe a decrease in the magnetic-ordering temperature T(m) from T(m) almost-equal-to 65 K (method [O]) to T(m) almost-equal-to 61 K (method [S]) and T(m) almost-equal-to 44 K (method [NO]) even though the Fe occupancy of the Cu(2) site increases in going from method [O] to [S] and [NO]. We interpret the decrease of T(m) to be due to an increase in hole transfer from the Cu(1)O(x) planes to Cu(2)O2 sheets. The degree of hole transfer is increased by a decrease in the crystallographic a-axis and in the ratio 3a/c of the lattice parameters, and is modulated by the local Fe-oxygen coordination and oxygen disorder in the Cu(1)O(x) planes.