Summerfield scaling model and electrical conductivity study for understanding transport mechanisms of a Cr³⁺...

Solid-state and sol-gel procedures were used to prepare ZnAl_1.95Cr_0.05O₄ nanocrystal spinels. From the results obtained by X-ray diffraction (XRD) and transmission electron microscopy (TEM), it can be concluded that the samples prepared by sol-gel synthesis are better crystallized than the ones resulting from the solid-state method. Studies by spectroscopy of impedance were done in function of frequency (40-10⁷ Hz) and temperature (540-680 K) in the sample prepared by sol-gel synthesis. The electrical conductivity spectra obey Jonscher's law and two models were observed studying the variation of the exponent 's' as a function of temperature, Correlated Barrier Hopping (CBH) and Non-overlapping Small Polaron Tunnelling (NSPT). The predominant conduction mechanism is bipolaron hopping. The scaling behavior of conductivity spectra was checked by Summerfield scaling laws. The time-temperature superposition principle (TTSP) points to a common transport mechanism working for the low and middle frequency ranges. The scaling mechanism fails in the high-frequency ranges suggesting that conduction dynamics, and the usual hopping distance of mobile species, have changed. The values obtained for the activation energy from the hopping frequency, conductivity σ _dc, bulk resistance R _gb, and relaxation (f _max), in the temperature range of 540-680 K, are very close. A higher and negative temperature coefficient of resistivity (TCR coefficient) equal to -2.7% K^-1 is found at 560 K. This result shows that our compound is suitable for uncooled infrared bolometric applications and infrared detectors.