SYNTHESIS, CHARACTERIZATION AND ab-initio STUDY OF SOME RARE EARTH TRANSITION METAL OXIDES

Abstract

In the present thesis, the magnetic, structural, thermal, and electrical transport properties of the compounds belonging to the RCrO3 and RVO3 families have been studied. Along with the experimental studies, theoretical calculations for some systems were done. In the RCrO3 compounds, spin reorientation, long range ordering of the rare-earth and transition metal sublattices occur at different temperatures. Some of them are multiferroics, undergo temperature-dependent structural variations, coupling of spins with lattice, exchange bias, magnetocaloric effect, etc. These physical properties vary with substitution and can be tuned. Therefore, substitution can be used as a control parameter for required applications. Several of the physical properties which are observed in rare-earth transition metal oxides await to be utilized for device applications. On the other hand, the RVO3 family of compounds shows diverse phenomena such as orbital ordering, re-entrant magneto-structural transitions, negative magnetization, metal-insulator transitions, etc. Several members of this family have the potential for photovoltaic and spintronic device applications. Here, some of the oxides belonging to these two families have been explored from the perspective of fundamental understanding and potential applications. The 50% substituted compound Nd0.5Dy0.5CrO3 was synthesized in pure phase, the effect of substitution on the physical properties was studied. The dc magnetization was measured to explore the magnetic transitions in the system. We observed antiferromagnetic ordering of the chromium sublattice (near 180 K; Néel temperature), spin reorientation of the chromium spins (near 60 K), and negative magnetization at low temperature (below 5 K). The negative magnetization vanished for higher magnetic fields and was found to have switching behaviour. The presence of rare-earth with large magnetic moments in this compound, as well as multiple magnetic transitions at low temperature led us to explore magnetocaloric i effect in the compound in different temperature regimes. The magnetic entropy change (-ΔSM) values were estimated below 60 K and near the ordering temperature of the Cr-sublattice. The maximum value of magnetic entropy change below 10 K is 8.7 Jkg−1K−1 for applied magnetic field 10 T. The corresponding change near 180 K was much lesser. The −ΔSM values were also calculated from the mean field approximation and Monte Carlo simulation using an Ising type Hamiltonian. The exchange parameters were calculated by fitting the theoretically obtained −ΔSM values with the experimentally calculated values. The magnetic structure of Nd0.5Dy0.5CrO3 was also explored using powder neutron diffraction; it was found that the chromium sublattice orders in G-type antiferromagnetic structure below 180 K in Γ2 (Fx, Gz) arrangement. The temperature dependence of the magnetic structure revealed a Γ1 (Gy) magnetic structure of Cr3+ spins below 60 K; this magnetic structure for chromium sublattice persists in the entire low-temperature range. At 1.5 K, ordering of the rare earth moments was seen in the powder neutron diffraction data. A signature of the ordering of Cr3+ moments was observed near 180 K in the temperature variation of the heat capacity data, and Schottky effect was observed at low temperatures. No signature of spin reorientation was observed in the heat capacity data. The theoretical calculations done using Density Functional Theory also suggested the G-type antiferromagnetic state as the ground state for the Cr3+ moments and A-type antiferromagnetic state for the rare-earth (Nd3+/Dy3+). Non-collinear theoretical calculations confirmed the Γ2 magnetic structure as a lower energy state above the spin reorientation temperature and Γ1 magnetic arrangement of chromium sublattice as the lower energy state below the spin reorientation temperature. A deviation in the temperature variation of the unit cell parameters (volume and lattice parameters) was seen near 260 K, which is above the Néel temperature, similar to DyCrO3. The Raman spectroscopy experiments indicated that some phonon modes deviate from the anharmonic approximation above Néel temperature in Nd0.5Dy0.5CrO3. The RVO3 compounds are very sensitive to the synthesis conditions, and a significant impact on the physical properties due to changes in synthesis conditions can be seen. Their synthesis requires a chemical reaction of the constituents in an inert gas atmosphere or reduction of oxygen-rich phases (RVO4) in an Ar/H2 gas mix at

high temperatures. The compound LaVO3 is known to exhibit anomalous magnetic properties (negative magnetization). A unit cell larger than the conventional unit cell was found in our study, which led to different magnetic properties, viz., the absence of negative magnetization below Néel temperature in the field-cooled magnetization data. Although several experiments, such as temperature variation of electrical transport, thermogravimetric analysis, x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, etc., suggested a stoichiometric LaVO3 sample, we found a correlation among the synthesis conditions, unit cell, and dc magnetization. The spin canting of V3+ moments appeared to be absent in the present compound, which in turn affected the negative magnetization in this compound. The magnetic structure from powder neutron diffraction experiments was found to be collinear Ctype antiferromagnetic. The theoretical calculations using the Density Functional Theory (DFT) were performed to study the ground state magnetic structure. The RVO3 compounds show various physical properties; this family is relatively less explored due to their non-stability at high temperatures in air or oxygen atmosphere. For instance, this family lacks behind in the exploration of magnetocaloric effect applications. The compound DyVO3 is an important candidate due to the large magnetic moment of Dy3+; therefore, the magnetocaloric effect in DyVO3 was studied. A significant value of magnetic entropy change (-ΔSM) of 12.9 Jkg−1K−1 was found for applied magnetic field 5 T; larger than several Dysprosium based perovskite rare-earth transition metal oxides. The effect of doping Sr2+ on the electrical transport properties of DyVO3 (Dy1−xSrxVO3; x=0.1, 0.25, and 0.5) was studied. The materials have an orthorhombic crystal structure (space group Pbnm) at room temperature. For x=0.5, we observed impurities of Sr6V2O11 in the compound. With the increase in doping, a consistent decrease in resistivity and activation energy was observed. However, metal-insulator transition with doping was not found in our study. The structural and magnetic properties of Ho0.5Dy0.5VO3 and Ho0.5Y0.5VO3 compounds were also studied. Magnetic ordering of vanadium moments was observed in both the compounds (near 114 K and 112 K), along with the signature of multiple magneto-structural transitions with decreasing temperature. The structural distortion in both compounds is not drastic at room temperature as compared to iii the parent compounds (viz. DyVO3, HoVO3, and YVO3). A structural transition (from orthorhombic to monoclinic) associated with G-type orbital ordering was also observed below 200 K in both the compounds. It was observed that coexisting orthorhombic and monoclinic phases (and, therefore, coexisting magnetic phases) are present in both the compounds below Néel temperature. As the temperature decreases below Néel temperature, the monoclinic phase decreases, and the orthorhombic phase grows; below 50 K, only the orthorhombic phase is present in Ho0.5Y0.5VO3, while the phase coexistence was observed till 30 K in Ho0.5Dy0.5VO3. The signatures of magnetic ordering of vanadium moments was also seen in the heat capacity data; the magnetic contribution to the heat capacity exhibited a lambda peak near the Néel temperature and also a hump near the orbital ordering temperature. A Schottky anomaly was seen in the low-temperature heat capacity data of both compounds, which was analyzed to determine the splitting of the energy levels.