Abstract
Rare-earth-based chromium oxides have attracted substantial research attention over the years because of their unusual properties. These materials' interesting members are rare-earth orthochromites (RCrO3 where R = earth element) with a distorted perovskite (ABO3) structure. This class of compounds has attracted attention from the research community because of the multi-functional physical properties observed in these oxides. RCrO3 materials have canted antiferromagnetic behaviour and a weak ferromagnetic moment in the 113 K to 300 K temperature range because of the Dzyaloshinskii-Moriya interaction (DM) caused by the interaction between broken inversion symmetry and spin-orbit coupling. It has been found that the magnetic behaviour of RCrO3 compounds is strongly influenced by the Cr3+−Cr3+ super-exchange interaction rather than 𝑅3+−Cr3+ and R3+− R3+ super-exchange interactions. The Néel temperature, 𝑇NCr, increase, because of the ordering of Cr3+−Cr3+ with increase in the cationic radius of the R3+ ion. The origin of this observed decrease was scribed to the reduction of the Cr-O covalent bond and dilution of Cr3+−Cr3+interactions. The Cr3+ ions order antiferromagnetically at a much higher temperature than the transition temperature of R3+ ions. The magnetic interaction between 𝑅3+−Cr3+ coupled with the structural distortions determines the magnetization orientation of R and Cr ions. In the formation process of RCrO3, an intermediate RCrO4 phase is formed, having an unstable rare Cr5+ ion. RCrO4 belongs to a family of ABO4-type oxides (where A is a rare-earth and B = P, As, Cr and V). The Cr5+ and R3+ ions contained in these compounds are magnetic, thus, providing an interesting magnetic interaction between the 3d and 4f magnetic moments. Few studies have been done on RCrO4, mainly because of the instability of Cr5+ and its tendency to be reduced to the most stable Cr3+, and the formation of RCrO3. The RCrO4 compounds crystalize in zircon or monazite-type structure depending on the size of the trivalent rare-earth ion and the B-element, pressure, and temperature conditions.
In the present study, the structural, magnetic, and electronic properties of Dy1−xGdxCrO3+y are investigated. The parent RCrO3+y (R = Dy and Gd) were successfully synthesised using sol-gel, sol-gel with cetyltrimethylammonium bromide (CTAB) and co-precipitation chemical route techniques. The structural properties of the synthesized samples were investigated using XRD and further analysed through Rietveld refinement, which confirmed the formation of RCrO4 and RCrO3. TEM analysis showed the formation of micro and nanoparticles, as well as particle size increase with the increase in calcination temperature. The sol-gel techniques resulted in better particle size distribution than that observed for the samples prepared using the co-precipitation technique. Using the sol-gel with CTAB synthesis method resulted in particles showing well-defined particles with a worm-like shape. Therefore, the Dy1−xRxCrO3+y (R = Gd and x = 0.2, 0.5, 0.8) samples were synthesised using only sol-gel techniques. The structure and phase purity of the studied Dy1−xRxCrO4 and Dy1−xRxCrO3 (R = Gd and x = 0.2, 0.5, 0.8) compounds were confirmed through XRD.
To study structural effects on the magnetic behaviour of these materials’ magnetization data under variations in temperature and under the changing magnetic field were gathered. The magnetic behaviour of the parent samples was discovered to be unusual. The change in the crystallographic environment of the ions present in the structure was linked to the origin of this behaviour. More research on these materials is needed to understand their magnetic structure. The phase transition from RCrO4 to RCrO3 also demonstrates that structural changes have an effect on the magnetic properties of the materials.
To change the crystallographic environment of these materials by site doping also affected the magnetic behaviour. The novel Dy0.5R0.5CrO3+y compounds synthesized by sol-gel had the same structure as the mother samples, showing that effective site replacement occurred. The magnetic behaviour shown in these materials was completely unexpected. The magnetocaloric effect potentials of these materials were estimated based on their magnetic behaviour, and the results reveal that all of the materials tested have the potential to be used in the field of magnetic refrigeration.
Finally, XPS was employed to investigate the electronic structure of these materials as well as the impact of site replacement on the electronic environment. Cr5+ is preferable at low calcination temperatures, however, because of its unstable nature, when the sample was heat treated, the additional thermal energy transformed from Cr5+ to Cr3+. GdCrO4, DyCrO4, and Dy0.5Gd0.5CrO4 all exhibit similar characteristics. This not only causes the crystal structure to change, but it also affects the magnetic orderings associated with the parent structures. The current work establishes a relationship between the production of RCrO4 and RCrO3 phases and displays the accompanying magnetic and structural changes.
This dissertation is a first step in understanding the relationship between the structural, magnetic and electronic properties of Dy1−xGdxCrO3+y with the effect of the synthesis method.