Effect of the Degree of Inversion on the Electrical Conductivity of Spinel ZnFe₂O₄

Abstract

Spinel ferrites (MFe 2 O 4 , where M is a metal ion) in general, and zinc ferrite (ZnFe 2 O 4 , ZFO) in particular, have recently received considerable attention due to promising results in the field of photocatalysis and, especially, photoelectrochemistry. ZFO exhibits a variable crystal structure in which the distribution of the cations between octahedral and tetrahedral sites in the cubic close-packed oxygen lattice, i. e., the degree of inversion, is known to depend closely on the synthetic conditions. Although the effect on the magnetic and optical properties is well known, fundamental research is still missing to fully understand the impact of the cation distribution on the photoelectrochemical activity. In this context, an utmost important physicochemical property is the electrical conductivity. In this work, the dielectric properties of ZFO samples with degrees of inversion ranging from 0.07 to 0.20 were investigated. The conductivity of the ZFO samples was found to increase as the degree of inversion increases. At an onset value of the degree of inversion between 0.13 and 0.16, the bulk conductivity increases by two orders of magnitude. This increase is a direct evidence of the high impact of the cation arrangement in the crystal lattice on the electronic properties of ZFO.

Effect of the Degree of Inversion on the Photoelectrochemical Activity of Spinel ZnFe2O4

“…It becomes obvious from Figure 1 that an increase of the cation distribution results in an increase in the photoelectrochemical activity of ZFO. The electrical conductivity of the pellets used in the present work was reported in a previous study [20]. An increase in the conductivity from 9.28 × 10 −9 to 1.82 × 10 −5 S cm −1 was observed as the degree of inversion increased from x ≈ 0.07 to x ≈ 0.20 [20].…”

“…Recently, we reported the preparation of ZFO by a solid-state reaction and its further processing into pellets with varying degrees of inversion [14,20]. The elemental analysis of the ZFO pellets revealed a Fe to Zn ratio of 2:1 within the limit of the experimental error, as expected for ZFO [14].…”

“…Mössbauer spectroscopy confirmed the absence of Fe 2+ and, hence, of oxygen vacancies for all the ZFO pellets [14]. The crystallite size values deduced from the Rietveld refinements and the particle size distribution obtained from the SEM confirmed that the pellets exhibited similar crystallite and particle sizes independently of the degree of inversion [14,20]. Thus, the degree of inversion was found to be the only independent variable between the different ZFO pellets.…”

“…The electrical conductivity of the pellets used in the present work was reported in a previous study [20]. An increase in the conductivity from 9.28 × 10 −9 to 1.82 × 10 −5 S cm −1 was observed as the degree of inversion increased from x ≈ 0.07 to x ≈ 0.20 [20]. Interestingly, a significant increase of two orders of magnitude was observed in the conductivity by increasing the degree of inversion from x ≈ 0.13 to x ≈ 0.16.…”

Impact of inversion and non-stoichiometry on the transport properties of mixed zinc-cobalt ferrites

“…For the activation energy of ZnFe 2 O 4+l , values in the range between 0.05 and 0.55 eV are reported in literature. 11,40,43,64,[66][67][68][69][70] The large variation in E A is mainly related to the concentration of Fe 2+ B , which depends on nonstoichiometry, on Zn content, and on the degree of inversion. In ideal ZnFe 2 O 4 , the antiferromagnetic SE coupling between Fe 3+ B impedes electron hopping between B sites.…”