Investigation of structure-property relationship in some low dimensional nickelates

Abstract

The realm of spin 1 quantum systems remains relatively uncharted compared to their spin ½ counterparts. They exist at the delicate borderland between quantum and classical physics, displaying moderate quantum fluctuations that influence the behaviour of the systems in unexpected ways. This thesis delves into the experimental study of spin 1 quantum systems comprising two principal low-dimensional lattices, namely, the square and the triangular. The thesis deals with an exhaustive investigation of structure-property relationship in the Ruddlesden-Popper (RP) nickelates R4Ni3O10 (R = La, Pr, and Nd) where Ni ion is located in an octahedral environment, the infinite layered T’ nickelates R4Ni3O8 where Ni ions form a square-planar lattice, and the triple perovskite Ca3NiNb2O9 where the Ni ions form well-isolated triangular layers resulting in a triangular lattice antiferromagnet. The n = 3 members of the Ruddlesden-Popper nickelates (R4Ni3O10) with mixed-valent Ni2+/Ni3+ states are scrutinized for their structural, transport, magnetic, and thermodynamic properties, with a focus on understanding the metal-to-metal transition (MMT) associated with the Ni 3d electrons. This MMT was earlier said to arise from a charge density wave (CDW) driven instability, we therefore used high resolution, low temperature synchrotron powder x-ray diffraction to study any subtle change in structural parameters or crystalline symmetry across this transition. The magnetic ground state of the rare-earth sublattice is also thoroughly examined, revealing novel insights not explored in detail in previous literature. Further, we focus our attention on the oxygen reduced counterparts of these RP nickelates commonly referred to as the infinite layered T' nickelates, R4Ni3O8. These T’ nickelates are known to be structurally and electronically analogues of the high TC superconducting cuprates. But owing to the difficulty involved in their synthesis, not much progress has been made in understanding their ground state properties. In this thesis, the mixed-valent triple layer T’ nickelates, having Ni1+ and Ni2+ in the ratio of 2:1, are investigated. The evolution of the ground state from a charge/spin stripe insulator in La4Ni3O8 to a correlated metal in (Pr/Nd)4Ni3O8 is probed systematically across these trilayer nickelates using transport, magnetization, low temperature synchrotron x-ray diffraction and specific heat probes. This study elucidates the effect of chemical pressure on the metal-insulator transition (MIT) seen for the La4Ni3O8 compound at 105 K. In the last part of the thesis, we study the triangular lattice antiferromagnet Ca3NiNb2O9 and its non-magnetic counterpart Ca3MgNb2O9. Here, the S = 1 Ni2+ ions are arranged on a triangular motif that are well separated from each other by non-magnetic buffer layers, which renders the system as quasi two dimensional. We show that this compound exhibits a 120o ordered ground state that gives way to field-induced quantum phase transitions in the form of magnetization plateaux. We grew single crystals of Ca3NiNb2O9 using the optical floating-zone method. Depending on the growth condition, we obtained two structural variants of Ca3NiNb2O9 – a low-symmetry 1:2 ordered (Ni/Nb order) phase, and a high-symmetry, disordered CaMO3-type perovskite structure with M-site occupied randomly by Ni and Nb. The ground state properties of these two variants are studied in detail. The ordered specimen is shown to undergo a two-step transition to the 120 degree spin structure, confirmed using the neutron scattering experiments. On the other hand, the disordered sample Ca(Nb,Ni)O3 exhibit a complex glassy ground state with no long-range order.