  Nanoparticles of Cr3+-substituted Ni-Zn ferrites with a general formula
Ni0.4Zn0.6-xCrxFe2O4 (x = 0.0 - 0.6) have been synthesized via a facile
microwave combustion route. The crystalline phase has been characterized by
XRD, TEM, FT-I and XPS revealing the spinel ferrite structure without extra
phases. Crystallite sizes of 23 - 32 nm as estimated by XRD analyses, after
corrections for crystal stains by Williamson-Hall method, are comparable to the
average particle sizes observed by TEM which indicates successfully synthesized
nanocrystals. Rietveld refinement analyses of the XRD patterns have inferred a
monotonic decrease behavior of the lattice parameter with Cr doping in
agreement with Vegard's law of solid solution series. Furthermore, cations
distribution with an increased inversion factor indicate the B-site preference
of Cr3+ ions. The oxidation states and cations distribution indicated by XPS
results imply the Cr3+ doping on the account of Zn2+ ions and a partial
reduction of Fe3+ to Fe2+ to keep the charge balance in a composition series of
(Ni2+)0.4(Zn2+, Cr3+)0.6(Fe2+, Fe3+)2(O2-)4. The optical properties were
explored by optical UV-Vis spectroscopy indicating allowed direct transitions
with band gap energy that decreases from 3.9 eV to 3.7 eV with Cr doping.
Furthermore, the photocatalytic activity for the degradation of methyl orange
(MO) dye was investigated showing largely enhanced photodecomposition up to 30%
of MO dye over Ni0.4Cr0.6Fe2O4 for 6 hours. A vibrating sample magnetometry
(VSM) measurements at room temperature show further enhancement in the
saturation magnetization of Ni0.4Zn0.6Fe2O4 , the highest in Ni-Zn ferrites,
from about 60 to 70 emu/g with the increase of Cr concentration up to x = 0.1,
while the coercivity shows a general increase in the whole range of Cr doping.
