Factors influencing phase formation and band gap studies of a novel multicomponent high entropy (Co,Cu,Mg,Ni,Zn)2TiO4 orthotitanate spinel

Abstract

High entropy oxides (HEOs) with 5 or more cations in a single sublattice have recently emerged as a novel class of ceramic materials with interesting properties in the fields of energy and electronics. On the other hand, Ti-based oxides are well-known for their applications in the field of energy harvesting. In this study, Ti was added to a transition metal high entropy oxide, TM-HEO (Co,Cu,Mg,Ni,Zn)O and, a phase-pure orthotitanate inverse spinel structure (tetragonal lattice – P4122 space group) was obtained as (Co,Cu,Mg,Ni,Zn)2TiO4 and confirmed by X-ray diffraction (XRD) studies. The presence of tetragonal distortion was confirmed from Fourier transform infra-red (FTIR) spectroscopy and attributed to the smaller ionic radii of Ti. The degree of inversion in the spinel structure was studied by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The stoichiometry, distribution of cations and the defect states in the compound were also quantified from XPS. The effect of the individual cations in the phase formation was analysed through the systematic elimination of each cation from the (Co,Cu,Mg,Ni,Zn)2TiO4 system, which revealed that formation of a phase-pure spinel structure was achieved when a 1:2 ratio of cations occupying the tetrahedral to octahedral sites was maintained. The band gap energies of the (Co,Cu,Mg,Ni,Zn)2TiO4 system as well as those of the phase-pure lower combination spinels were found to be in the infrared range of 1.02–1.54 eV which varied with the composition and the fact that it was lower than any of the individual oxides could be attributed to the band alignment of the individual cations. Band diagram for (Co,Cu,Mg,Ni,Zn)2TiO4 was constructed from the valence band spectrum obtained from the XPS studies. (Co,Cu,Mg,Ni,Zn)2TiO4 could, therefore, be a potential material for light harvesting applications as the band gap lies in the infrared regime.