الفهرس 
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Abstract
Polycrystalline samples of the chemical formula Li0.5Crx
Fe2.5−xO4 (x = 1.15, 1.20, 1.25, 1.30 and 1.35) were prepared by
using the standard ceramic method. X-ray diffraction analysis
confirmed the formation of a single phase of a face centered cubic
spinel structure for all prepared samples. The lattice parameter of
each sample was accurately determined by using Nelson-Riley
function. The lattice parameter and bulk density decreased, while
the x-ray density and porosity increased with increasing Cr content.
Magnetic measurements were carried out by using the
electromagnetic induction method in the temperature range from
−100 to 350 ºC. The temperature variation for all samples exhibited
the N type anomalous dependence in which a compensation point
appears. The magnetization below the compensation temperatures
decreased, while that above them increased with the increase in the
Cr content. Meanwhile, both compensation and Curie temperatures
manifested a decreasing trend. The magnetic results were explained
using Néel two-sublattice model of ferrimagnetism.
Electrical and dielectric measurements were performed by
using the two probe method in the temperature range from −70 to
390 ºC and frequency range from 102 to 106 Hz. All specimens
showed semiconducting nature. The conduction mechanism
changed once at the compensation temperature, where the
activation energy increased, and another at Curie temperature. The
values of both the dielectric constant and dielectric loss in addition
to their dependency on temperature and the frequency increased
with increasing temperature. The increase in the dielectric constant
was accompanied by the appearance of a hump. Such a hump
shifted towards higher temperatures with increasing the frequency
as well as it grew from nothing to a peak as the Cr concentration
increased. The temperature variation of the frequency exponent
factor revealed that the conduction mechanism around the
compensation temperature in all compositions is attributed to the
polaron hopping Fe2+↔Fe3+. Also, it showed that the polaron
effective mass increases at the compensation temperature and
decreases with increasing Cr content. The composition dependence
of the dc resistivity and dc activation energy exhibited maxima at x
= 1.25, while that of the total resistivity and total activation energy,
in the paramagnetic region, exhibited an increasing trend.
Meanwhile, the total activation energy in the ferrimagnetic state,
below and above compensation, decreased with increasing the Cr
concentration. It is the same compositional dependence of both the
dielectric constant and dielectric loss. The total resistivity,
dielectric constant and dielectric loss showed the well known
dispersion of ferrites. The resistivity dispersion decreased and
shifted towards higher frequencies with increasing temperature.
The electrical and dielectric results were explained in light of
Koops model and the excess formation of Fe3+ clusters ith
increasing Cr content. Also, it was proposed that the spin polaron is
formed on B sublattice at the compensation temperature.