Crystal Growth and Exploration of Structure-Property Correlation in Perovskite and Pyrochlore Oxides

Abstract

The intimate correlation between crystal structure and physical properties continues to be a central topic in condensed matter physics. Subtle structure modifications often lead to pro- found changes in electronic, magnetic, and transport behaviour. Perovskites and pyrochlores based oxides provide versatile structural frameworks in which competing interactions such as electron correlation, spin-orbit coupling, lattice distortions and and geometric frustration coexist, giving rise to a rich spectrum of ground states such as quantum spin liquid, spin ice, random singlets, spinons, Majorana fermions, spin glass, spin dimer, etc. In perovskite oxides (ABO3 and their ordered derivatives), variations in A-site ionic size, B-site electronic configuration, and oxygen coordination govern the octahedral tilts, rotations, and superex- change paths. These structural degrees of freedom significantly influence the subsequent physical properties. On the other hand, pyrochlore oxides (A2B2O7) host a three-dimensional network of corner-sharing tetrahedra that intrinsically promotes geometric frustration. Here, the interplay between the lattice symmetry, local distortions, and strong spin-orbit coupling, particularly in 4d and 5d transition metal systems, stabilises unconventional magnetic states such as spin liquid, spin ice, and non-collinear antiferromagnets. The research work in this thesis can be broadly categorised into three parts depending upon the experimental platform in which the structure property correlation is explored. In the first part, a series of B-site disordered perovskites with the general formula ACu1/3M2/3O3 ( A = Ba, Sr and M = Nb, Ta) are studied. In these compounds, the random but uniformly dis- tributed Cu2+ (S = 1/2) exhibits a novel dimerized state known as random singlets (RS) due to the structural disorder. Synchrotron X-ray diffraction and X-ray absorption spectroscopy establish the long-range and local structure of the compounds. The specific heat and magneti- zation do not show the presence of any long-range magnetic ordering or spin freezing, though the spins are strongly coupled and the site occupancy is random which should have favoured a glassy phase or an inhomogeneous ordering. The absence of power law dependence and single coefficient data scaling behavior (required for a singular power law distribution of exchanges P(J) ∼ J−γ ) in the specific heat measured down to 0.1 K, motivated us to reformu- late the existing model with a non-singular probability distribution known as the data-driven dimer-monomer model. Our model could reproduce our experimental results successfully, presenting a new scenario where the quenched disorder does not result in a random singlet ground state expected from the real-space renormalization group theory, extended to the case of a frustrated three-dimensional system. In the subsequent chapter, we studied two poly- morphs (orthorhombic and tetragonal) of SrCu1/3Ru2/3O3 (SCRO), which are also B-site disordered perovskites. However, the presence of two magnetic ions in a correlated structure with structural disorder now leads to an even more complex magnetic ground state. While the orthorhombic SCRO exhibits a ferromagnetic ground state, its tetragonal counterpart shows antiferromagnetic ordering. Although both polymorphs are insulating, the tetragonal one exhibits a simple Arrhenius-type conductivity, whereas the orthorhombic analog dis- plays Mott-like insulating behavior with variable range hopping. The contrasting magnetic and electric properties of the two polymorphs could be qualitatively explained on the basis of their different crystal structure. Further, we focused on a family of ordered ruthenium-based triple perovskites with the general formula Ba3MRu2O9 (M = Co, Cu, Ni, Zn). The substitution at the M site leads to differences in the structure and physical properties. The compounds show long-range mag- netic ordering when the M-site ions are magnetic, i.e Co, Ni, Cu. However, a spin-dimerized state comprising S = 3/2 Ru-Ru dimers is obtained when the M-site is occupied with a non- magnetic Zn2+ ion, which was previously claimed to be a novel spin liquid candidate. The last experimental work involves the growth of single crystals of a family of zirconium-based pyrochlores with the general formula A2Zr2O7, with A = La, Nd, Pr. Here, we studied the crystal growth of the spin ice candidate Pr2Zr2O7 using the flux method and demonstrated the usefulness of the same flux to grow the La and Nd alloys. Though the crystals of Pr2Zr2O7 were grown previously using the floating zone method, which involves melting at very high temperatures (> 2200◦), leading to inherent structural disorder and off-stoichiometry. In our method using flux, we could grow the crystals below the order-disorder transition due to (a) the evaporation of rare-earth oxide from the molten zone, and (b) the order-disorder transi- tion above 1500◦. The grown crystals were then well-characterized using various structural analysis tools, such as XRD, SEM, HRTEM, Raman spectroscopy, and Laue diffraction. etc. Finally, the magnetic measurements were carried out on crystals grown under various conditions to benchmark them against the previously reported crystals grown by the optical floating-zone method.