MAGNETIC, ELECTRICAL-CONDUCTIVITY, AND EPR INVESTIGATIONS OF A LOW-SPIN D(5) SYSTEM IN FE8V10W16O85

The temperature dependence of the magnetic, electrical conductivity, a nd electron paramagnetic resonance (EPR) properties of Fe8V10W16O85 ha s been investigated. The magnetic susceptibility measurements revealed an almost Curie-Weiss law behavior above room temperature and an addi tional magnetic interaction at low temperature, causing a steep rise o f magnetization as the liquid helium temperature was approached. The v alue of the magnetic moment at high temperature, mu(eff) = 1.80 mu(B), suggests a predominance of trivalent iron ions in a low-spin state, I n the 300-4.2 K temperature range a difference between the zero field cooling (ZFC) and the field cooling (FC) modes was recorded. This irre versible behavior might be related to the presence of weakly coupled c lusters. The EPR measurements revealed a broad, temperature-dependent resonance line at high temperature and two weaker lines at low tempera ture. The two low-temperature lines were attributed to antiferromagnet ically coupled high-spin Fe3+ ion clusters and to high-spin iron ions placed at sites with low symmetry of the crystal field. The broad line at high temperature was separated into two Lorentzian lines. These co mponent lines were attributed to the two paramagnetic centers connecte d with the Fe3+ ions involved in the magnetic structure of Fe8V10W16O8 5: dominant low-spin centers and a small admixture (<15%) of the high- spin centers. The line broadening and shift of the resonance field of the two component lines with decreasing temperature were studied and a nalyzed using a model of the EPR lines of antiferromagnets, The temper ature dependence of the electrical conductivity showed a typical semic onducting-type behavior with an activation energy of 0.40 eV. The hopp ing mechanism of small polarons was proposed to explain the transport properties of the sample. (C) 1998 Academic Press.