Mott insulating behaviour in double perovskite Ruthenates A2BRuO6 (A = Ba, Sr and B = Gd, Sm, Dy)

Abstract

Perovskite materials, characterized by their distinctive ABX3 crystal structure, have emerged as a prominent class of compounds with versatile properties and a wide range of applications. Originating from the discovery of the perovskite mineral in 1839, the exploration of perovskite materials has evolved significantly, especially in the latter half of the 20th century and beyond. Perovskites exhibit diverse functionalities such as fer roelectricity, piezoelectricity, and superconductivity. Double perovskite materials, also known as A2BB′O6 perovskites, are a subclass of perovskite structures where two dif ferent B-site cations occupy alternating layers. This structural modification introduces unique properties and advantages over single perovskites, making them appealing for var ious applications. In recent years, double perovskite compounds have attracted attention due to their enhanced stability compared to their single perovskite counterparts, as well as their tunable electronic properties, multiferroic characteristics, and potential applica tions in photovoltaics. In this thesis, our objective has been to investigate novel double perovskite compounds based on ruthenium and their significance in diverse applications. The long-range interactions within these compounds and the oxidation states of metal ions remain unresolved. We are addressing the magnetic interactions and determining the oxidation states of metal ions using various techniques. Additionally, we highlight the use of thin films of double perovskite compounds and investigate their magnetic properties and other characteristics in thin-film form. Chapter 1 is an introduction to the thesis, where we begin with the history of per ovskite materials and their application in real life. We have discussed various types of perovskites along with their potential properties, such as superconductivity, ferroelectric ity, piezoelectricity, etc. Further, we have explored the crystal structure of perovskites and examined the factors that influence the crystal structure of materials within the perovskite family. We have also detailed the crystal structure of double perovskite compounds and discussed the magnetic interactions within these structures. We have also given a concise overview of thin films of double perovskites and their applications, including uses in solar cells, photovoltaic applications, and more. Finally, a concise discussion on the theoretical studies of double perovskites and the primary challenges encountered in DFT calculations for these strongly correlated systems. In Chapter 2, the synthesis methods employed for preparing the compounds and the experimental techniques utilized throughout the study are detailed. To prepare the polycrystalline samples, we employed the solid-state reaction method, and Pulsed Laser Deposition (PLD) was utilized for the preparation of thin films. For their characterization, we used different techniques including powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), magnetization measurements, Raman spectroscopy, atomic force microscopy (AFM), UV-VIS spectroscopy, and Fourier transform infrared spectroscopy (FTIR). In Chapter 3, we provide a detailed exploration of the theoretical background. In this chapter, we have covered the topic of many electron systems and the process of solving the wave function for complex structures. Firstly, the Hartree-Fock (HF) approximation method is employed to estimate the wave function of a many-electron system, commonly applied in the study of atoms and molecules. The HF method neglects electron correlation effects, which can be significant in systems with strong electron-electron interactions. Electron correlation beyond the HF approximation is often addressed using post-Hartree Fock methods. Following that, we explored the Density Functional Theory (DFT) and the reasons behind its replacement with the HF method. DFT has become a widely used method in the computational community; it can be seen as an extension of the Hartree Fock approach. DFT replaces the many-electron wave function with the electron density, simplifying the description of electron-electron interactions. We also provided detailed explanations of the Hohenberg and Kohn (H-K) theorems, as well as the Kohn-Sham (K-S) theorems for electronic structure calculations. Finally, we provided an in-depth discussion on the computational tools utilized in this thesis, including Quantum Espresso and CASTEP. In Chapter 4, the outcomes of an experimental and theoretical examination of the structural, electronic, and vibrational attributes of Ba2GdRuO6 (BGRO) and Sr2GdRuO6 (SGRO) double perovskite ruthenates are presented. The XRD patterns, fitted using Ri etveld analysis, indicate that BGRO possesses a single-phase cubic structure with the Fm¯ 3m space group, while SGRO exhibits a monoclinic structure with the P21/n space group. Consequently, there is an observed tilting of the octahedra in SGRO, attributed to the effective Jahn-Teller distortion in the monoclinic structure. We have investigated the vibrational properties of both BGRO and SGRO compounds, both experimentally (Raman spectroscopy) and theoretically (Density functional perturbation theory). The structures predicted from XRD were studied theoretically using an accurate plane wave pseudo-potential method based on density functional theory (DFT) with Hubbard pa rameter, U (DFT+U). The outcome of this study comes out to be, that the compound BGROis found to be metallic with the difference in density of states for spin-up and spin down electrons; whereas SGRO is insulating with a band gap of 4.0 eV. The vibrational analysis of perovskites shows A1g, Eg and 2F2g, Raman active modes for BGRO; while for SGRO the observed active modes were Ag and Bg, respectively. It is noteworthy that the experimentally measured values correspond to those predicted by our DFT calculations. In Chapter 5, we present the results of combined experimental and theoretical in vestigations into the oxidation states of Ru ions in the double perovskite ruthenates: Ba2SmRuO6 (BSRO) and Sr2SmRuO6 (SSRO). Based on the earlier study, the assump tion was that Ru exists in the +5 oxidation state, but this is not consistently the case. The oxidation state of Ru is observed to be in a mixed state, combining both +4 and +5 states. In this chapter, we explore the oxidation state of Ru in the two structurally distinct double perovskites, specifically BSRO and SSRO. For this, we utilized the Magnetization study, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) tech niques. We conducted an analysis of the M2,3 edge of Ru for both compounds, i.e., BSRO and SSRO. The XAS spectra, when compared with that of RuO2 (RO), suggest that Ru ions in these ruthenates exist in an oxidation state greater than +4. To gain deeper in sights into the experimental data, calculations for core-level spectroscopy were conducted within DFT using CASTEP software to compute the M2,3 edges. The calculated Ru-M2,3 edge spectra agree very well with our experimental XAS spectra. This suggests that Ru ions in SSRO are present in a mixed oxidation state (+4 or +5) and Ru ions in BSRO are present in a +5 oxidation state. The objective of this study is to grasp the oxidation state of Ru ions, a factor that holds considerable sway over the magnetic properties. In Chapter 6, the focus is on the thin films of double perovskite ruthenates, specif ically Ba2DyRuO6 (BDRO) and Sr2DyRuO6 (SDRO). We have successfully grown the thin films on a SrTiO3 (STO) substrate using the pulsed laser deposition technique. We performed characterization of the thin films using X-ray diffraction (XRD), Atomic force microscopy (AFM), ultraviolet-visible spectroscopy (UV-Vis), and magnetization mea surements. The BDRO samples crystallize in a cubic structure, whereas SDRO exhibits a monoclinic structure, as revealed in their X-ray diffraction analysis. The AFM analyses indicate a smooth growth of the thin films on the STO substrate. The UV-visible mea surements for both samples demonstrate a direct impact of the A-site element (Sr/Ba) on their band gaps, with values of 3.66 eV and 2.59 eV for BDRO and SDRO samples, respectively, indicating their insulating nature. Temperature-dependent magnetization measurements indicate the existence of ferromagnetism in BDRO, while paramagnetism is observed in the SDRO thin film. Interestingly, both films exhibit canted antiferro magnetism at approximately T = 5 K, as indicated by their isothermal magnetization curves. In summary, when comparing the magnetic properties of both thin films, BDRO and SDRO, it becomes evident that there is a suppression of bulk magnetic ordering in comparison to their bulk counterparts. The absence of magnetic ordering in these thin films may be attributed to potential modifications in superexchange interactions, the presence of exchange bias, stress-strain effects, or uncompensated spins in such thin film structures. We have conducted first-principles calculations using the CASTEP software to obtain further insights into the experimental data. Chapter 7 presents an analysis of the electronic structure of double perovskite com pounds, namely Ba2SmRuO6 (BSRO) and Sr2SmRuO6 (SSRO), through X-ray photo electron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), and X-ray absorption near edge spectroscopy (XANES) measurements. In this chapter, we explore the exact nature of the double perovskite ruthenates, which were initially predicted to be metallic using the GGA approach in DFT. To address this ambiguity, we incorporate a Mott-Hubbard model within the framework of the GGA approach. The Mott-Hubbard type interactions are confirmed to arise from strong correlations in Sm-4f and Ru-4d states. In the SSRO compound, Hubbard interactions also dominate, facilitated by the presence of large crystal field distortions. The density of state calculations reveals strong hybridization between O-2p, Ru-4d, and Sm-4f electrons. The outcomes reveal strong hy bridization between the 4d electrons of Ru and O-2p states near the Fermi level, resulting in an insulating state with a Mott gap of ∼ 1.05 eV for BSRO and ∼ 0.82 eV for SSRO. These findings contribute to our understanding of strongly correlated phenomena near the Fermi level, providing valuable insights for the design of optoelectronic devices. Chapter 8 provides a summary and conclusion of the current studies.