Spectroscopic studies of Co2TiO4 and Co3O4 two-phase composites

Abstract

In this article, we report a comparative analysis of various spectroscopic studies including low-temperature (25T300K) Raman spectroscopy of cobalt-orthotitanate (Co2TiO4) and tricobalt-tetraoxide (Co3O4), and their solid solutions (1-x) Co3O4+x of Co2TiO4 (0x1 (100wt.%)). For all the lower and intermediate compositions, five Raman-active modes were recognized at 689,618,518,480, and 195cm-1 that are associated with A1g, Eg, and 3F2g phonon symmetries. Conversely, pure Co2TiO4 exhibits a broad spectrum of width approximate to 93.3cm-1 without any signatures of F2g(3) mode. At low-temperatures (down to 25K) the A1g and F2gpeaks of both Co2TiO4 and Co3O4 shift toward the high-frequency side with anomalies across the ferrimagnetic Neel temperature (TN approximate to 48 +/- 5K) and antiferromagnetic Neel temperature (TN approximate to 30 +/- 10 K), respectively. All the investigated samples exhibit two distinct bands at 576cm-1 (B1) and 665cm-1 (B2) in the Fourier transform infrared spectra recorded at 300 +/- 10 K, associated with the vibrational stretching of the metal-oxygen bonds of length approximate to 195.8pm (B-O) and approximate to 185.4pm (A-O), respectively. The intensity of these sharp bands gradually decreases as the crystal structure transforms from normal-spinel (a=8.07 angstrom) to inverse-spinel structure (a=8.45 angstrom). The X-ray photoelectron spectroscopy (XPS) studies revealed that the Ti was incorporated into the octahedral B-sites of inverse-spinel structure of Co2TiO4. Interestingly, the XPS spectra of Co2TiO4 provide evidence of the trivalent character of Ti instead of tetravalent cationic configuration together with a weak Co3+ character at the octahedral sites. These results are discussed in terms of the binding-energy (BE) difference between the O-1s and Ti-2p3/2 ([O-Ti-2p3/2]=BE(O-1s) -BE(Ti-2p3/2)) and the mean chemical bond length l[Ti-O]. The peculiarities of all these results in consonance with the crystal-structure (bond angles and bond lengths) and electron-spin-resonance studies are discussed in detail.