Topochemical Synthesis & Characterization of Octahedral Rotation Induced Noncentrosymmetric Layered Perovskites

Open Access
Author:
Sen Gupta, Arnab
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 02, 2016
Committee Members:
  • Venkatraman Gopalan, Dissertation Advisor
  • Thomas E Mallouk, Dissertation Advisor
  • Venkatraman Gopalan, Committee Chair
  • Susan E Trolier Mckinstry, Committee Member
  • Long Qing Chen, Committee Member
  • Zhiwen Liu, Committee Member
Keywords:
  • noncentrosymmetry
  • octahedral rotations
  • perovskites
  • second harmonic generation
Abstract:
A substantial amount of work by the ferroelectric and piezoelectric research community is presently being dedicated to finding new electronic materials for advanced applications in sensors, actuators and transducers. This field of research has been long dominated by ABO3 perovskites. The inversion symmetry in these perovskites are removed by atomic displacements caused by the second order Jahn Teller effect (SOJT). However, there are few SOJT active cations like Ti4+ and Zr4+ with empty 3d and 4d orbitals, and Pb2+ and Bi3+ cations with 6s lone-pair electrons. This confines the selection of chemical species for designing non-centrosymmetric compounds. Also, the most pervasive distortions in ABO3 compounds are the BO6 octahedral rotation and tilting enclosing the B-site cations, which do not break inversion symmetry in these simple perovskites due to the nature of BO6 connectivity. The main motivation of this work is to perform a comprehensive study of new perovskite-base structures in which octahedral rotations and chemical ordering removes inversion symmetry, based on recent predictions. This thesis focuses on n = 1 Ruddlesden-Popper A2[A’n-1BnO2n+1] layered perovskites like NaRTiO4 (R = rare earth), LiRTiO4 and HRTiO4. A combination of tools like density functional theory, synchrotron x-ray diffraction, neutron diffraction, second harmonic generation and piezoresponse force microscopy have been used to understand the mechanism by which inversion symmetry is removed in such layered materials. It has been established that a-boco/boa-co type of OOR in combination with A-site cation ordering make these compounds noncetrosymmetric (space group: P-421m) at room temperature. Rietveld refinement was used to quantify the degree of OOR. First of all, it has been observed that the OOR occurs due to the under-bonded A-sites. In addition, a clear trend of increasing OOR angle with decreasing rare earth ion size has been observed, which indicates that the coordination environment of the rare earth ions determine the degree of rotation. Temperature dependent SHG measurements have shown that the RP compounds undergo phase transition from low temperature noncentrosymmetric (P-421m) to high temperature centrosymmetric (P4/nmm) phase. As the order parameter (displacement of the rare earth ion) decreases with increasing temperature, it is observed that the SHG signal also monotonically decreases with increasing temperature for the noncentrosymmetric members of the RP families. This also means that the degree of noncentrosymmetry decreases with increasing temperature, culminating into a second order like phase transition at high temperature (Tac) to a P4/nmm phase. The Tac is also observed to be higher for the compounds with smaller rare-earth ions, supporting the fact that smaller rare earth ions lead to larger magnitude of OOR and hence need higher temperature to become centrosymmetric. Overall, the family of Ruddlesden Popper layered perovskites presents rich properties and phase transitions, which in single crystals could be useful for applications in the field of piezoelectric transducers and actuators.