Characterization of spinel-type Cd1−x CoxCr₂O₄ nanocrystals by a microwave-combustion synthesis

Spinel nanocrystals of Cd 1− x Co x Cr 2 O 4 (0 ≤ x ≤ 1) solid solution are successfully synthesized by a microwave combustion method and their properties are systematically studied. Single phase spinel structure is determined for all the synthesized compositions by x-ray diffraction (XRD) and the two vibrational bands of metal-oxide bonds are assigned by Fourier transform infrared spectroscopy (FTIR). Energy dispersive x-ray spectroscopy (EDX) indicates almost stoichiometric compositions in the whole range of x of the synthesized Cd 1− x Co x Cr 2 O 4 nanoparticles. Crystallite sizes of 6 – 10 nm estimated from XRD patterns are in consistence with sizes of cubic nanocrystals observed by transmission electron microscopy (TEM). The lattice constant behavior with x shows a deviation from Vegard’s law with a kink anomaly at x ∼ 0.5 that may be attributed to different site-preferences of divalent cations in the two spinel sublattices. Optical absorbance measurements reveal a wide energy gap of 3.5 eV for CdCr 2 O 4 nanoparticles that is smoothly narrowed by Co-substitution for Cd to become 3 eV in pure CoCr 2 O 4 with particular optical characteristics. Magnetization isotherms measured at 300 K confirm the parmagnetic behavior, while those measured at 2 K show hysteresis evolution from a suppressed antiferromagnetic state to a ferrimagnetic state with maximum coercivity for Cd 0.7 Co 0.3 Cr 2 O 4 .